Limecrete Slab

After many delays, the limecrete slab was laid by contractors The Limecrete Company on October 8th.

I had hoped that this would take place in late August or September. This would have allowed time this year, before the risk of frost, to build the stem walls. Because of the delay, I will have to build the stem walls in April next year, and will not be ready for wall-raising util the beginning of June at the earliest.

Preparation: approx. 250mm of recycled foamglass, Glapor brand from Ty Mawr, is laid onsite with a geotextile membrane below and then compacted with a plate compactor.

Recycled Foamglass: looks like a Crunchy bar, airy and very lightweight.

                                                                           

Delivery in bulk bags                                                

The Limectrete Company, Norfolk                     

Compacting                                             

250mm Average Depth                                       

Geotextile Membrane over Glapor           

 The Big Green Limecrete Machine                               

                Mixing 6 Cubic Metres in about half a day!              

Laying the slab                                                                 Floating                                          

And by the end of the day................................

And here it is a week later, now hard enough to walk on

The care advice for a limecrete slab is that it needs to harden for around 90 days before it can withstand frost, so I am going to have to do my best to protect it from winter conditions. Exposing it to air is helpful, and moisture is helpful at first.

October 24th. It's a warm and sunny day. In fact, it has been exceptionally mild since the slab was laid on October 8th, and no cold weather is on the forecast yet. However, it's November next week, and it's time to protect the new slab from frost. 

This is not exactly high-tech. I've spent £20 on tarpaulin, £15 on some bales of straw (not construction standard), and the method is best explained in pictures:

That took 3 hours, including trips up the road for straw and tarps. The idea is to leave the covers and the straw on the slab over the winter, and uncover it when I am ready to start building in the spring. It's dry and inviting under the tarps at present, hope it stays that way. There's a storm forecast for the weekend.

Guess what shape my temporary straw storage is going to be!

I hope my straw has been cut and baled by now. I am buying 120 bales from Longhay Ltd, who come recommended as construction-standard bale brokers. The plan is that it will stay in the farmer's barn over the winter and then be delivered here in May. At which point I will need dry storage in the garden for 20 cubic metres of straw.

You can buy a tarp garage to cover a small truck from Machine Mart for £360. Then there are Party Tents, available in a large range of sizes, and standard, heavy duty and extra heavy duty gauges. A 6 x 4 metre tent would be around £400, perhaps £650 for the heavy duty.

Then I saw this rather beautiful structure at a music festival, and thought: I could make one of these instead.

It's a 2V dome, very simple to make. What I like about this one is the way it's been covered with one large tarp and one small one. The large tarp has extra eyelets along one edge, and these are attached to struts along the opening. Then the whole sheet is pulled taught from the back and tucked under.

The dome pictured is around 4.5 metres diameter. I will make mine a little larger - perhaps 6.5 or 7 metres. This will mean that the "door" (as shown above) will be an equilateral triangle with 2.1 metre sides.

I thought about making it from hazel rods with water pipe strut endings, but had some anxieties on a number of fronts: would I be able to find a source of straight hazel rods, 2.1 metres long? Would any knobbly bits on the hazel rub holes in the tarp? Would it be strong enough?

I have decided to go with "Cundy poles". These are softwood poles, possibly forestry thinnings, that have had the bark removed my machine. The diameter is variable, and they are not absolutely straight, but they look strong, durable (they are generally used as stakes in the ground) and smooth enough to support a tarp without damaging it. They are also much cheaper than machine-round poles.

I've bought 80 poles of 2.4 metre length, now stacked up wigwam-style under a tree:

Straw Bale Workshop 2: Groundwork

This will become obvious quickly enough, so I shall tell you now: I am not a builder. I've made a few structures out of wood before, re-tiled a roof very badly, done some wiring, plumbing and small patches of plastering, but I'm about to embark on the first serious bricklaying I've ever attempted. 

First, I had to get the hole straight.  I had given my digger-driving neighbour some poor instructions, and needed several days with a spade and a wheelbarrow to get the hole to the right dimensions and depth. Here it is:

I hope that's deep enough. It's definitely subsoil, and straw bale buildings are relative light-weights.

Next job (according to the book, which is: Building with Straw Bales; A practical Guide for the UK and Ireland by Barbara Jones) is to lay a strip of limecrete around the edge 50mm thick and 150mm wide (2 x 6 inch). I did as much research as I could on materials and method, and this is what I came up with:

Buy 4 bags of NHL 5 from a conservation builder's merchant. Walk past the ballast at the regular builder's merchant, because I wasn't sure it was well-graded. (It looked like 20mm gravel mixed with sharp sand.) Luckily we have sand and gravel pits just down the road, so I bought half a ton direct from there. Hire a regular concrete mixer. Batches start with 1.5 buckets ballast, then 1 bucket lime, mix for a while, add another 1.5 buckets ballast, mix for 5 minutes, add enough water to make a dry mix (4 - 5 litres), mix for 10 minutes, then add another litre or 2 of water to make a sticky mix in a further 5 - 10 minutes. Wear dust mask and eye protection when there is dust around. Once the mix turned sticky, it needed a certain amount of poking with a piece of 2x2 to stop it from sticking to the mixer drum.

It's not a great photo-point of the project:

According to the photo data, this was on the 20th June. Limecrete takes a while longer than concrete to harden, so I started bricklaying in the second week of July.

The advice I was given was to use engineering bricks below ground, so I bought 600 very cheap, new ones. I have a pile of recycled bricks and local flint that I am planning to use above ground.

Mortar mix for this level was 3 buckets sharp sand to 1 bucket NHL3.5 with around 8 litres of water. The method was very similar to that for limecrete, i.e holding back the last couple of litres of water (and the stickiness) until the last few minutes of mixing. It's beautiful mortar to work with: no plasticizer needed, and nice and sticky for dressing the ends.

This section ends with a rather impressive-looking photo of my foundation with 5 courses of below-ground brickwork. Before we get there, I will just confess to some problems encountered along the way. The limecrete levels went wrong on the foundation strip, and I had to gradually get back to level by varying the mortar thickness. More seriously, I managed to get the brick bonds wrong, with 37 whole bricks along one side and 36.5 bricks opposite. Luckily, it's all below ground and no-one will ever see it. Unless you look closely at the photo below:

There are plenty of books and internet resources on how to do this right. This is more of guide to muddling through.

Here's the finished foundation:

It's not perfect, but I'm reasonable happy with it. More or less level, about the right size, reasonably square. It's July 12th 2013. My wall needs 4 weeks to harden, and I then have a contractor coming for the next stage: 250,mm recycled foamglass insulation and 150mm limecrete base.

Straw Bale Workshop 1: The Point of No Return

This must be the same for every building that has ever been constructed. First, it is built in the imagination and only then can it be built on the ground. So, through the winter I have been building, in my mind, a straw bale workshop.

I already have a garage. Brick construction, uninsulated, ice-cold in winter, and jammed full of stuff. There's a large workbench in the centre made of recycled chipboard from some built-in wardrobes that we took out of the house; some tea-chests full of wood pieces and scraps, shelves of paint, hardware, electrical spares, glass, garden tools, bicycle parts, scrap metal, old carpet, canoe paddles, camping gear, drain rods, tools, theatrical props, fencing and piles of rubbish for recycling. It does function as my workshop. In fact, on my earlier blog, the DIY dome, there is video of work actually taking place there. But it's usually a struggle to find space to work in there, and for three or four months each winter, it's like working in a meat store.

The workshop (in my mind) will be large and spacious, easy to heat and within my capabilities to build. Here's where I would like it to go:

It's next to the dome, outlined with blue rope around four stakes. There's an existing shed in the way, but I can move that. There's also a leylandii hedge which I've already taken a saw to. (Leylandii! Never liked it anyway!) I can still move the stakes: thinner, longer, more or less angled, left a bit, right a bit. Nothing is absolutely settled yet. 

The plan is to build a structure 8.4 metres by 4.75 metres. Taking account of the thickness of straw bale walls, this should produce a building with a floor area of just under 30 square metres. Subject to a few other conditions, this means that it will not require planning permission or building regulations approval from the local council.

Next step was to move the shed. Here we go:

That didn't take long to get the roof off and the walls down. It took another week or two to reconstruct it on the other side of the garden. 

At this point, I'm not fully committed to the time and expense of building. I'd just chopped a hedge down and moved a shed. I could still back out, build something else, build somewhere else, put it off for a year or two. 

Things really started to get serious a couple of weeks later, in early May, when our neighbour John came round, with  his digger. He had come to pull the hedge stumps out, but that only took about 15 minutes. He said "Shall I dig your foundations while I'm here?" You can't really say no to an offer like that, can you?

(BONUS: I got to drive the dump truck!)

So you might now say that I had passed the point of no return.

DIY Dome. A Geodesic Dome Greenhouse

Introduction   

In early 2011, I built a geodesic dome greenhouse in my garden in Norfolk. I recorded the process of designing and building it in a diary-style as I went along (on another blogging site), as much for my own record as for publication to others. I am now reposting the blog on Blogspot with a few changes, including URL addresses and video. During the winter of 2012-3 the original blog was receiving over 1000 visitors per month, and I would now like to update the look and the content of the blog.

The new content here of most probable interest to dome builders is the inclusion of some video.

I will also be posting about my latest project: the building of a straw bale workshop next to the greenhouse.

Feel free to leave a comment.

18th January 2011

The beginning: One of the first things to say is this: if you want to find out about building one of these structures, you really should look elsewhere. You should, in fact, have a look at www.geodome.co.uk or perhaps www.domerama.com  where you can find more information about self-build domes than you could read in a week. There you will find calculator tools that take care of the maths.  This is a personal account of what I should probably call my first attempt at building a dome,

So: here's the site:

The picture shows a 6-metre circle on the grass which is the proposed greenhouse size. Yes, it is quite a large one; more the size of a small polytunnel. It's outlined with a piece of scrap pipe, and the hole in the centre recently held a large tree stump. The ground is very sandy, and I am wondering how substantial to make the foundation.

Meanwhile, indoors, I have started making the hubs - the connection points for all the struts that make up the dome. I have decided to make them out of 3-inch diameter pressure water pipe, cut into 2-inch cylinders. Finding a short length of 75mm hdpe pipe was not easy; it usually comes in 50m coils (and I only needed 2 metres). I found an ebay supplier ("SupplyingDirect") eventually and bought 6 metres for a not-unreasonable price.

This is a cutting jig for slicing the pipe up 2 inches at a time. Using a jig saves a lot of time, and makes it easy to get the hubs square-cut and uniform size.

19th January 2011

Of course, it has struck me that I might be building a geodesic pile of broken sticks and polythene. The PR for these structures is all about the spreading of the weight and stress, but I've just put in an order for a large pile of wood, and I'm wondering whether these little rings of plastic I've been making are man enough for the job. A set of purpose-made hubs would be very nice, but I haven't seen any for sale at less than £500. (That's a lot more than I'm planning to spend in total.) Another potential weakness is the method of fixing the wooden struts to the plastic hubs: I am planning to use M8 machine thread to woodscrew dowels, that will screw into the ends of the wooden struts and provide a threaded bolt that can be attached to the hubs with a nut. I think there is a danger that the fixings will split the end of the wooden struts. At one point, I was looking into using Western Red Cedar for the struts, but this is a timber that is very easy to split, so possibly not ideal for this project. I'm going to use European Redwood, treated, 38 x 38mm. I'll drill pilot holes for the dowels, try to get them very accurately centred, and for good measure I will lash up each strut end with a cable tie.

On the subject of cost, which I was a minute ago, the timber is going to cost around £150. I think the polythene might be around £100, and I will try and keep all other costs to £100 - though this depends on the foundations, could be £150. £400 max - What do you reckon?  (See notes on costs at the bottom of this page)

Not included in these costs is my new toy. I have bought myself a chop saw, a sliding, mitre power saw. There is a lot of accurate cutting of struts with some fancy angles on the ends coming up.

20th January

I have been marking up the bits of pipe to make dome hubs today. It's not as straightforward as you might think from a casual glance at a dome. Mine has 46 hubs, and I am putting my faith in online dome resources as usual for the information about the angles needed between struts. I have 6 pentagons (72 degrees), 5 regular hexagons (60 degrees), 5 part-hexagons, regular (4 @ 60 degrees), 20 irregular hexagons (56 and 62 degrees) and 10 irregular part-hexagons (56 and 62 degrees).

23rd January

I've finished all the hubs now, all drilled with the required number of holes in what I believe to be the right position. Here's a jig for accurate drilling of holes that I put together from garage scrap:

A box of hardware arrived this week: woodscrew to M8 dowels, nuts, washers and cable ties. 

Screwfix sell these dowels (item 11850-12); however, I bought from Warehouse Direct, where they are much cheaper. (Note, 2013: Warehouse Direct has now ceased trading.)

I think the foundation is just going to be a 9-inch trench, 6 inches deep and filled with gravel. I'll rest the structure on 6-inch timber bearers and hold it down with rebar stakes. I'd be surprised if 4 or 6 men couldn't lift the finished structure off the ground, so I think concrete foundations would be over the top. The site is sloping slightly, (6 inch fall), so I might put a course of concrete or cinder blocks in as well.

I am making a 4/9th 3v dome. The 4/9th means that my dome will be less than a hemisphere; this will give the structure a low profile, and should reduce the risk of wind damage. (As you can see from the site photo, the site is also well sheltered from wind in two directions). A 5/9th dome would be larger than a hemisphere. A 3v dome that is exactly hemispherical would result in a row of truncated triangles along the bottom, which would offend the eye, and would be something of a rarity.

30th January

I had a struggle for a couple of days trying to level the site "The Easy Way". I mean by this, trying to do it with a spade, a few levelling pegs and an inexperienced eye. Getting nowhere, I decided to invest a bit of time in planning and equipment and came up with the radial arm level beam shown below

Not exactly rocket science, but it's the first time I have had to prepare a site of this size, and I was impressed how easy it made the job. So, all's flat and much more perfectly circular than it used to be. I have started to dig a trench 6 inches deep and 9 inches wide around the circumference, still using the radial arm to keep the trench bottom level. I'm going to fill this with gravel.

Meanwhile, I have collected the timber from the woodyard. It looks quite good quality with no large knots, and cut to a very regular 38mm square. I think if I have any problems, it is likely to be warping and sagging over time.

I have started cutting. The chop saw does it in a flash; after about an hour, I had 50 (of 120) struts cut to 1124 mm with 12 degree angles on each end.

1st February

You might imagine that building this structure was all geometry and woodwork. For my project, so far, it's been mainly spades and wheelbarrows. For every hour I've spent on construction, I must have spent two or three on site levelling and foundations. The good news is that the site is now flat, and the foundation is about three-quarter dug.

Here's what it looks like today:

Just another 4 metres of trench to dig, and a half ton of gravel to fill it up. In terms of supporting the structure, I think this work may be over the top. However, I think the gravel trench will help with drainage. There's no way to put a gutter on a dome, so there could be a lot of rainwater running down the surface onto the surrounding soil.

I have been cutting and drilling the struts. Geodesic dome struts are not all the same length. In the version I am making, there are three different strut lengths,  "A" are 925mm, "B" are 1094mm and "C" are 1124mm. They all have 78 degree cuts on the ends except the "A" struts, which have 80 degree ends.

Once again, I found it very useful to build a cutting jig so that the struts come out with accurate angles and lengths. It takes a little while to set up, but you can then cut all your struts of the same length in a few minutes:

I have cut all the "C"s (x 50) and all the "B"s (x40). I have used all the best timber, so the shorter "A" struts are going to contain a few knots.

 I have also drilled all the "C" strut ends, and screwed in the dowels. The dowels go in at 90 degrees to the strut ends, but the strut ends are cut at 78 and 80 degrees. Here's my jig for accurate drilling of the dowel holes:

So I now have 50 struts with a short length of M8 machine thread sticking out the end for attaching to the hubs.

February 28th.

Both the building and the blogging have been interrupted by weather, work and travel this month. However, the preparation has continued in fits and starts. The foundation is complete to ground level, with a levelled circle of gravel. I have made a wooden riser wall with 6 x 2 treated timber and am trying to figure out what the next step might be.

The photo shows the riser wall in fourteen sections. The fifteenth section is void for the door. Before the dome is erected on the riser wall, I will need to join the sections together, find a way of anchoring the hubs to the ground, and adjust the top edge of the riser wall sections to accommodate a dome that does not have a straight horizontal lower edge. I have an idea of how these edges will need to cut, but to be safe, I plan to erect the dome and check the adjustments needed before I take the saw to them.

I have now completed the struts, all with the machine head end of the dowels protruding from the ends awaiting assembly.

I have put a cable clip around each strut end to try and ensure that the timber ends do not split. I have also done a little pre-assembly of the struts, making up the five pentagons that will comprise parts of the first and second layers of the dome. A sixth pentagon will crown the structure, but it may be simpler not to pre-assemble this

Work this month has also included the purchase of 77 square metres of horticultural polythene. I needed 30 triangular panels for the six pentagons, and 45 triangular panels of different size for the hexagon and part-hexagon areas. I made a pattern for both triangles with a piece of vinyl flooring, and I used the patterns to cut out triangles of polythene four or five at a time. The polythene has an inner and outer surface, so I have marked each panel with a letter "e", which reads correctly when viewed from the outside only.

So, all the parts are made. I now have a complete dome "kit", a finished site and foundation, a riser wall in progress awaiting inspiration about how to complete it, and a need to start assembly before I can sort out the riser wall and ground anchor details. The preparation stage is complete; it's time to start construction.

After six weeks of sourcing, digging, levelling, measuring, cutting, drilling and screwing, it's time to assemble the parts and find out if it comes out dome-shaped. If it does, I would once again like to thank the online dome-wonks at (not permitted to say -  search for "geo-dome" for online advice) for all their guidance and their dome calculator tools.

I started work at around 9.00 this morning, moving materials into place and completing preparations

I prefabricated 5 x 12 strut shapes, each a pentagon with a triangular extension. This photo was taken at around 10.00 this morning.

I propped them roughly in place with 5 long pieces of wood. I added 5 x 3 struts in a "Z" pattern to complete the lower course of triangles. Then I completed the second course by adding 5 x 4 struts on 6 x 60 degree hubs.

The time was now 12.30, and the dome is assembled to two courses. It is also self-supporting.

I had thought that I might need assistance with the assembly. However, with a little planning, it was quite straight-forward to support the sections with props whilst linking them together with more struts.

I added 5 x 3 strut "Y"s to the top of each pentagon, and reached a stage where I was just 10 struts from completion. Unfortunately, I looked in my strut pile, and found that, although I had 10 struts remaining, they were not the lengths I was expecting. Somewhere in the built structure, I had used 2 x "B" struts where I should have used "C" struts. An easy mistake: each strut is over a metre long, and there is only 30mm difference in length between these two types. However, this is quite sufficient to cause distortion, or even weakness in the structure, so I had to take around an hour out of assembly to measure every strut again, and substitute a couple of "C"s for "B"s. As a result, it was nearly 4.00 when I tightened the final nuts.

This is what it looks like fully assembled. It is by no means finished, but visually it looks pretty much what a finished dome will look like.

A detail of the 75 mm plastic waterpipe hubs. The first picture shows the five-way hub somewhat distorted under pressure. The second picture shows the there is plenty of room in these hubs to use a ratchet head socket spanner, provided no space is wasted with protruding dowels.

Jobs remaining:
-     Cut the riser wall to accommodate unlevel base of dome, with semi circular cutouts for hubs.
-     Fix riser wall sections (14 of them) to gravel boards underneath
-     Join riser wall sections at edges for stability
-     Make / Find / Improvise 3 or more ground anchors to hold everything down in case of high winds.
-     To achieve this, I think it will be easiest to jack the dome up and assemble these parts underneath.
-     Lower dome onto riser wall and fix
-     Create rectangular door opening in east-facing pentagon.
-     Create opening window frame in 2 or more upper triangles
-     Cover the whole structure with pre-cut polythene shapes

3rd March

I've spent a day making the riser wall for the dome, shown above. There are 14 sections of 6 x 2 timber, and I already had them cut to length with 12 degree angle ends to make a quindecagon wall. (One section of wall is "missing" - for the door.) Yesterday, I had to notch the corners to accommodate the hubs, and also trim the wall height to allow for the unlevel base of the dome. The pentagon bottoms are horizontal, and the walls need to rise either side to the half-hexagon hubs: about 35mm. I had already got this profile in mind, but I wanted to check before cutting the timber.

This was the hard way to do it. It meant I had to lift the dome onto the wall to check the profile, and this took a couple of hours of careful jacking and propping. There was also an issue with the hubs dropping; I had to hold them in shape in the horizontal plane to see what their vertical profile would be. A bit of a slog.

Here's what it looks like today

and a close up of the hub recesses

It's all just standing loose at present; tomorrow I need to screw everything down, screw those riser wall sections together. Constructing the door is the next job.

I don't think I'm going to make ground anchors. It's a low-profile dome in a sheltered position, and it will weigh around 400Kg with the riser wall attached. Shouldn't be going anywhere.

6th March

I've spent a morning doing "invisible" but essential work, screwing the riser wall sections together and screwing the lower dome struts to the riser wall. Everything feels much more stable and rigid now, and it's no longer possible to lift the dome or adjust it's position.

In the afternoon I created a doorframe.

I took out the five radial struts from one of the pentagons.

Added "vertical" door frames. They appear vertical when viewed from outside, head on, but they are lying in the plane of the pentagon, so are tilted upwards about 15 degrees.

Note the fancy short bracing struts, making effective use of the vacant strut bolt hole on the side hubs of the pentagon.

A bit of dome theory. This is a 3V 4/9ths dome. Geodesic domes are based on the icosahedron, a platonic solid with 20 triangular faces and 12 5-way vertices.

In geodesic domes, the straight edges of these triangles are divided into 2, 3, 4 or 5 etc equal lengths to make a more spherical shape; the more divisions are made, the more the dome will approximate to a Hemisphere, however the complexity of construction will also increase.

In my dome, each straight edge is divided into 3, hence the term "3V".

In an icosahedron, opposite vertices are separated by 3 triangles. If geodesic spheres were created, the separation of opposite vertices would be:

2V 6 triangles
3V 9 triangles
4V  12 triangles
5V 15 triangles
6V 18 triangles

Hemi-spheres - i.e. domes - are easy to create in 2V, 4V and 6V domes; you just build half a sphere and you obtain a dome shape with vertical sides (at the ground) and a nice flat bottom. With a 3V dome, a semi-sphere would be 4.5 triangles. This could be done, but it is more usual to construct a dome that is less than the hemisphere, 4 triangles from apex to base (4/9th) or one that is more than the hemisphere with 5 triangles from apex to base (5/9th). Either way, you end up with a shape that is made up of just two sizes of triangles, but which has a slightly non-planar base.

I have gone for a 6 metre diameter 3V, 4/9th dome. For smaller diameter domes, the headroom in a 4/9 type might be a problem, and so the 5/9 (more than a hemisphere) is a good choice. For my dome, 5/9th construction would have created a different sort of headroom problem; a 5/9th dome of 6 metres diameter would have been well over 3 metres in height, and would have required planning permission. It is also less work - just 75 triangles for a 4/9th instead of 105 for a 5/9th.

So far, so logical. However, I have made myself a bit of a problem. Geodesic domes are made up of hexagonal and pentagonal arrangements of triangles.  In a 5/9th dome,  there are five hexagons among the bottom two courses of triangles, and hexagons can be easily adapted to take a rectangular door-frame, having a good, wide horizontal bar as an upper edge of the hexagon. But in my 4/9th dome, there are five pentagons in the bottom 2 courses, and the only place to put a door is in one of the pentagons. Here it is again:

This opening is just high enough to walk under with a small duck of the head. If I squared off the door frame with a horizontal bar, then I would have to crawl under it every time. What I'm trying to explain is this: it's not going to be a square door frame, it's going to be the shape shown in the photo

Monday 7th March

I have bought 77 square metres of:

"XL Horticulture's Sterilite Polythene, a High Diffused, Thermic, Disease Reducing, Anti Fog, 600g/150mu Polythene Film. Designed especially for Polytunnels.

To cut out the shapes I needed, I assembled one of the triangles from the dome hexagons and one from the dome pentagons and transferred the shape to two patterns made from vinyl flooring offcuts. Then I used the vinyl patterns to cut the polythene with a craft knife, 4 or 5 layers at a time. The triangles tesselate nicely for economical cutting.

I also cut some 3-triangle, half-hexagon shapes. The theory is that this is easier to fix three at once and saves time.

I started at the bottom, so that upper layers overlap those below, tiled roof style. My photo shows how far I got on the first day with a hand staple gun, before my wrist started to ache.

I was out to Machine Mart next morning for an electric stapler and a big box of staples.

It was a cloudless morning, bright sunshine all day, and I had my triangles of polythene cut and my new electric stapler.

First course completed, second course under way:-

Detail of pentagon centre hub with polythene cover complete:-

Two courses complete, and a very atmospheric pattern of internal shadows. (Once the upper courses are covered, there will be no more shadow patterns like this, as the sterilite diffuses incoming light.)

And, just as the shadows were lengthening, a near-complete dome. I've got 4 out-of-sight triangles yet to do, having managed to count the triangles incorrectly. I've also got the door, which I haven't made yet, and two opening windows, also unmade. In the photo above, you can also see four large paving stones in the doorway providing a level surface against which the door can lean.

Saturday 12th March

So, I've cut a bit more polythene and finished all the fixed triangles, which just leaves the windows and door.

For the two windows, I just used the vinyl pattern from polythene cutting as a template for making a couple of wooden triangle frames. I've used tiling battens to keep the weight and profile down. Covered with polythene and fixed with two brass hinges.

It's the upper triangle of this hexagon, hinged at the top. I don't have any window stays yet; I'm thinking about getting some automatic window openers, think it might prevent the plants from over-heating when we are out.

As for the door......

It's a double door, made of 38 x 38mm like the dome struts. I could have squared the top, but would have lost another 9 inches headroom, and I'm already needing to stoop to enter as it is. So ..... pointed top doors. The door panels needed a little angled bracing, so I have echoed the pattern of the pentagon that has been removed for the door opening.

The doors open upwards, as you can see above. Whilst this is slightly inelegant, the only way to get hinged doors to vertical in this structure would be to have a canopy / tunnel projecting out to vertical, and this would be sticking out all the time, not just when the doors are open.

I like the no-fuss door stays I've come up with: 2 posts stop the doors from opening further than 90 degrees, and a swinging arm is used to jam both doors in this position. The swinging arm is fixed to the top corner of the left door, and clips into place on the inside of the door when not in use.

 

At this point, when asked by friends how the greenhouse is coming along, I say "It's finished. Ready for some plants." So this is a suitable time to stop recording the building process. There are some outstanding jobs to do: the window stays, or automatic openers; some staging; I need to block the holes in the ground-level hubs to stop mice or rabbits getting in; perhaps stick down some of the polythene corners that flap in the wind. It's had quite a good wind test this week, without any damage.

I intend to leave this blog in place for my own record, and as a reference point for anyone who expresses a lot of interest in this structure or in building something similar. I may add some more photos once it's full of plants, staging, colour and interest. But for now, that's it!

A note on the (approximate) costs of this project:

75mm hdpe pressure water pipe.......................................£25
38mm x 38mm timber , 156 metres................................£168
20mm shingle, 0.5 ton.....................................................£12
Machine thread to wood screw dowels, nuts, washers etc..£46
Timber for riser wall.........................................................£54
Sterilite Polythene 11m x 7.3m........................................£94
Staples, screws, hinges..................................................£25

Total £424

July Update

I have added a little hardware to the dome: some rodent fencing over the lower hubs and some automatic vent openers for the windows.
 
The vent openers are a must-have, even in our very temperate climate. As soon as the sun comes out and the temperature rises, the upper windows rise like flower petals and allow the heat to escape. I've no idea if the rodent barriers are necessary or not.

And here's what the greenhouse looks like in mid-July:

 
  
So far, so good with the tomatoes. We also had a lot of early salad leaves and radishes before the outdoor crops got going. I might need to invest in some better plant supports next year. I wasn't expecting quite such a heavy crop.

2012 Update. Despite a poor summer, the sweet peppers have cropped well this year:

Here are some things I would have done differently;

          1.  Used solid brass hinges on the windows. I've had to replace the (brass-plated) ones already

          2. Sited my greenhouse somewhere else, and not under the power lines. (The pigeons crap on it.)

          3. Used stainless steel staples, if I could find some. They are rusting quickly, but should last for 5 years before I need to recover the dome.