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Posted on 2024-03-06 by Nick Thomas
This is a followup to previous posts about straw bales and garden rooms:
Six months on from taking the straw bale course, I've managed to make it to "first fix" stage on the garden room - which is to say, it's watertight, there is a roof, and I'm ready for internal fit out. Have a picture, in case the words are too... wordy.
In the end, I went with a plinth approach. The first priority was to get a good bearing surface, and the second was to avoid having water wick up into the organic parts of the structure. After obsessing over a range of different "raised plinth full of loose gravel" options, I realised I was losing too much time on this stage and made progress by digging a series of pits down to the clay subsoil (about 700mm deep), filling them with the gravel, lining them with geotextile membrane and standing bog-standard concrete blocks on top of the gravel.
My first few attempts at soil investigation just didn't go deep enough - the clay layer I finally hit was rock solid by comparison, and more bearing capacity calculations confirmed I was going to be fine, weight-wise. Digging the holes was back-breaking, and my friend managed to break my shovel(!) when he came up to help. A bit of mechanical assistance in the form of a mini digger would have made short work of this job, but it wasn't available to me. Equally back-breaking was disposing of the spoil - it went into bags which went into the car and on to the local HWRS, all by hand.
One downside to this approach is that the building itself isn't tied into the foundation - I strapped the whole thing to the concrete blocks, but those are just sat on top of the pea gravel. I did some wind loading calculations to reassure myself that it wasn't going to just roll off away into the sunset. It was complicated by wind *uplift* factors, as well as lateral force - the wind is effectively making the building lighter by pushing against the roof, at the same time it's pushing laterally against the wall. I don't recall the maths - the friction was not overcome when assuming a windspeed of 100 miles/hour, but it wasn't order-of-magnitude safe, so definitely worth the calculation.
After digging all the holes (~2T of soil out) and filling them with pea gravel (~2T in), and levelling the piers, I put together a timber box beam from 150x50mm C16 timber. This is 450mm wide (same as the bales), sits on top of the concrete blocks, and acts to bear the weight of the wall over the piers. I used the Timber Beam Calculator to work out what depth and strength class was sufficient for the span.
I was planning to clad both top and bottom of the box beam, but got a bit ahead of myself and built it as a single piece - which then became too heavy to lift up by myself to clad the underside. So I only clad the top. The top cladding and the noggins should act to prevent the timbers from spreading, so it's not the end of the world, but it's a (relatively minor) weak point when it comes to insulation and could potentially give a space for critters to nest. Really not ideal, but I just couldn't face disassembling it again once it was together - those 100mm screws are hard to drive.
Doing it all again, I think I'd get a mini-digger and a skip for the excavated material, cut out a whole trench (rather than just piers), fill it with pea gravel and top it with something cementitious - maybe regular concrete, maybe limecrete if budget allowed, then try the plinth wall approach. What I ended up with should be fine, but the timber wasn't much of a labour or money-saver in the end, and now I have a set of 1M wide voids to cover or fill below the box beam.
Phew. This is the easy bit, right? I did a week-long course on exactly this! How hard could it be to replicate it? Really hard, as it turns out.
First, I needed a baseplate - building this was only briefly covered in the course. It gives somewhere for the initial, upward-pointing set of 350mm stakes to fix to - these hold the bottom course of bales in position - and also lifts the bales a bit above the expected floor level, in case of water spills. I begrudged having this in addition to the box beam, but couldn't figure out how to do without. It's made from 100x50mm C16 (laid flat) in a ladder pattern, screwed together with 150mm screws and fixed to the box beam with 100mm screws. It was important to remember to put the strapping (6M long pieces of it, 19mm wide, at 1.2M intervals) in place before fixing the baseplate - this loops over the top of the wall and is used to hold compression - and I did. More on that later.
Once the prep was done, I was able to start putting bales down. Advice was to put together a wallplate at this stage, using the baseplate as a template, but I didn't have the wood for that yet and was anxious to play with the straw, so I just noted down some measurements and left it for later. That was a mistake, but look at how distracting the straw was:
Those along the back wall needed a coat of lime render, following the "french dip" approach. I'd bought some hydrated lime (powder); mixing it up with water and leaving it to stand for a week or so gets lime putty, and mixing *that* with sand gets lime render. It didn't have to be great, and indeed it wasn't - plenty would slide off the bale if it was handled roughly at any point after application. The longer the lime putty sits, the fattier - stickier - it gets, but I hadn't left time for that, and struggled to source small volumes of ready-mixed stuff. It seems to have worked and set hard, though, and it's mostly there for fire resistance.
Between the render and the wall, I also placed some vapour-permeable membrane, to protect the straw and render from rain. The wall and roof provide plenty of screening, but I figured this was worth it as defence in depth - and it would have been impossible to retro-fit. The membrane is 1M wide, so every two bales (700mm) I put a strip of it in and pulled it taut against the bales. Once tightened, the strapping helps to hold this in place, and the edges will be embedded in lime render at the corners.
I built up the north and west walls to four high first, and definitely experienced some bale frenzy in doing so. Each bale needs to be dressed and sized, then you just hoist it up and there's half a square meter of wall... done. Absolutely hypnotic. I eventually tore myself away from that for long enough to do the upright posts for the door and window, which unlocked filling in the east and south walls:
These uprights are 2x50x100mm C16, nailed together to create 100x100 posts (apparently this is stronger than a single piece of wood). I cut holes into the OSB and fixed them into place with a combination of screws, nails, and blocking (extra noggins placed to constrain the wood) but they were pretty easy to flex around, so I put some temporary supports up for them until they could be supported by the rest of the wall.
Up until now, I hadn't had any notches to cut, for wrapping the bale around the posts, and I was getting by dressing the bales with a hedge trimmer (although did cut through the cord once, oops). It's supposed to be possible to cut notches with a hand saw, but I wasn't having any luck, so decided to shell out on a second-hand alligator saw. It's got a tiny 350mm blade, but made cutting notches and dressing bales much easier - looking at the walls, I can still tell when I switched from one tool to the other from the quality of the dressing. I was quickly up to five or six (the target) high all around. I drove 1M long stakes down into the wall at four bales high - two stakes per bale.
So, yes. Bale frenzy. Once I'd gotten six high, I pulled out my baseplate measurements, built a piece of wallplate, and hoisted it onto the west wall... only to discover that it really wasn't level. It was actually very obvious that the line was wandering just by eye, but I was rushing and not paying attention to such things. Next time, I'll have a string line up, but this time I had to take the corners down, resize a bunch of bales (it was especially soul-destroying to resize a bale I'd already resized a week earlier) and get them back up - all while the weather deteriorated and hopes of getting a roof on before winter receded. I got there eventually, though, put the wallplate (100x50mm timber, upright, OSB glued and nailed onto the bottom, lots of noggins) up in sections, and screwed them together.
Even this effort didn't make the walls perfectly level, but at least the wallplate sat on top of the bales all the way around, rather than being out in space. I found a reference in the Canadian Straw Bale Association code that suggested load-bearing straw walls shouldn't be more than a couple of inches out, and as far as I could tell I was within that, anyway, so I decided to press on with compression.
I'd decided to use the in-situ strapping for this, since it was going to be very difficult to use anything else along the back wall, and if it'd work there, it'd work everywhere else too. With hindsight, this was a bad decision - I couldn't get much on the back wall as the straps would break when I tried! Examining the architectural details belatedly revealed that 29mm strapping is recommended, rather than 19mm - but the main cause of the breakage was repeated use of the tensioning tool on the straps, as I moved between them in an attempt to keep the pressure even. It has a grabby metal "foot" which digs into the strap, and that would cause a bit of damage every time it was used; disentangling the foot from the strap under tension, in particular, would just cut it. I ended up using truckers straps everywhere else and just accepting minimal tension on the back wall, which at least gave me a nice fall on the roof by default.
This wasn't the end, though - the wallplate needed finishing. After getting the compression on with trucker's straps to squish the walls, tightening up the strapping to match, I needed to cut holes in the bottom of the wallplate for the 1M hazel stakes to go down through, using a hole saw - again, two stakes per bale. That done, I had to add noggins for the roof rafters to rest on top of (600mm centres, more on that shortly), then fill the wallplate with straw for insulation, *then* put the OSB lid on top. That last bit was especially important as every time it rained, water was filling the tarp-lined voids in the top of the wallplate, then slowly seeping through if I didn't bail them out. And it was raining a *lot*.
I got all that done for the north section of wallplate, but didn't have time for the rest - we were going into a prolonged period of bad weather, and I had work, christmas, and a vacation in Finland to juggle - so I decided to pop some OSB sheeting on top of the unfinished wallplate to address the pooling issue temporarily, tarp it all up, and leave it for the new year.
For various reasons this ended up being a two-month break. I'd imagined the walls were safe beneath the tarps, but over this period some water did get into the north and west walls - due to how I'd put the compression on (a single trucker's strap with the buckle on the inside of the wall), the top of the wallplate was angled inwards and water was running constantly down the surface of the tarps, soaking through in some places. I ended up having to dig out rotten straw to a ⅓rd depth in several patches. Fortunately, it's not critical - the voids can become niches and shelves - but it's definitely not ideal. That's the price of not getting the roof on in time, I guess.
I also had to revisit the compression - I'd actually compressed *too much* along the south wall, and the tilt introduced by the uneven compression meant the roof rafters would mostly be out of touch with the wallplate. I ended up cutting the straps there so I could finish the bottom of the wallplate along the door opening, and the whole thing *relaxed* and straightened in front of my eyes.
No pics of it, but previously, the lintel above the door was intruding perhaps 20mm into the void formed by the wallplate, meaning I couldn't clad the underside. Great fun.
With that sorted, it was time to complete the wallplate along all three sides, including fixing the upright posts into place with noggins and cutting them to length, then going through the hole - stake - insulate - close procedure.
With that done, finally, I was ready to put the roof onto the wallplate.
When I got started on the foundations, I had no idea what kind of roof I'd have space for. Permitted development gives me 2.5M to work with, but how much of that the walls were going to take up, and where I needed to measure from, was unclear. A vaulted hip roof would minimise wind uplift forces, and give me more head height internally, so I was aiming for that... but the lack of compression along the north wall, coupled with a decision to be conservative about planning law interpretation, meant that wasn't an option. Instead, I decided to go for a regular flat roof, to minimise height.
The buildup is pretty simple - Timber Beam Calculator told me that 200x50mm C24 rafters were enough to span the distance at 600mm centres, while also being strong enough to convert the flat roof into a green roof if I decided to. 18mm thick T&G OSB3 on top of that, and a single-ply EPDM membrane stuck onto the OSB for waterproofing.
I'd decided I needed to get it done fast, so stuck with standard timber sizes as much as possible for the joists and edge pieces - 3.6M and 4.8M. I wanted a bit more overhang on the east and west than the 4.8M gave me (600mm of overhang in the end), which required me to extend the north and south (4.8M) pieces a bit, but nothing time-consuming - I just sistered the timber behind the join. I'd also helped out future me by marking the noggins I'd put down in November, so I knew where the joists had to go. I laid it all out, used a string line to get things square (I'd learned my lesson!), put some screws in to fix the joists to the wallplate, then added joist hangers for a really secure fixing - wind uplift really has me paranoid.
I got all this done in a single day - 28th Jan 2024. It was very intense, and the T&G wasn't properly fixed down or at all watertight, but I was determined not to suffer any more water damage. It's also the day I got my worst build-related injury (so far, at least).
The way you get the front and back pieces on when you're working by yourself is to attach a piece of wood to the bottom of the end joist to act as a ledge. You can then rest the 4.8M piece on that at one side, while screwing in the other, then go to the other side of the roof, screw that in, and remove the ledge. I'd done all that with no issues on the south, but was struggling to fit the ledge on the north - I was using a short piece of 2x4 as the ledge, screwing it into the underside of the joist with the drill. Rather than the screw spinning, the 2x4 was, and to stop it from doing that I braced it against the side of my head. Stood on a ladder, obviously. The screw wasn't driving in cleanly, so I decided to put the drill into reverse to bring it out so I could try again... but didn't adjust my bracing first, so the 2x4 spun in the opposite direction and cracked me one on the right temple.
No concussion, and I didn't fall off the ladder, but I did turn the air blue, get tinnitus in the left ear for a week, and grow an impressive bump on the template. After a few hours of rest I was able to carry on, but it was a very strong reminder that building sites are dangerous places and you need to step back and think if you're having a problem with something.
A couple of days later the EPDM membrane arrived, and I was able to get it onto the roof to make it fully watertight. A day of struggling with the T&G to get it sited correctly and screwed down, then a day to glue the EPDM to the OSB and attach the roof trims, led to the picture we saw at the start of this post:
I've since attached a gutter along the south edge (the other three edges have upstands) and the water is being directed to the apple tree in the northwest. Eventually I'll have a rainwater collection system, but I want to be done-done first.
Honestly, it's close now.
I've got some more wall straightening to do, and in particular, there's one bale on the west wall that I need to shorten in-situ. I'm pretty confident that I can just cut the strings, pull off the overhanging bit, and re-tie the strings, but I've not done it yet so it could be much harder than that. I'll get a clamp on before I cut the strings, just in case...
The walls are currently protected by the roof overhang, with tarps hung externally beneath that. Before the tarps can go, I need to render the walls with lime render. This will be a bunch of work by itself, and needs me to install the windows, re-seat the doors, trim the walls, go around and stuff all the gaps between bales, fix render stops to the appropriate places, and probably a thousand other things I've forgotten. I've also decided to use lime render internally, as well as externally, to improve the shear resistance of the walls (much of the resistance to lateral forces comes from the render, not the bales + compression, and I'm still a bit nervous about wind). This can't happen until the weather has improved though - April or May - and I need a ceiling and floor internally to render *to*, if I want a tidy job of it inside.
The ceiling and floor are part of "first fix", along with a bunch more stuff that will turn it into a habitable space, bringing in services, etc. I've ordered almost everything I need, and it's coming this week - principally insulation for roof and floor (rockwool, which cost as much as the straw bales making up all four walls!), but also a great deal of wood (matchboard for the ceiling and underside of the roof outside; more joists for the suspended timber floor; battens for miscellaneous purposes). All that implies a bunch of awkward work, but at least I'll be dry when doing it. My current plan is to do the ceiling - 150mm depth of rockwool, with 50mm beneath the joists if I can, to maxmise acoustic insulation. I can then put the floor in, with the joists in hangers (my life would be so much easier if I'd installed these back in September, but I'll manage). I've still got old decking boards to use as floorboards; it'll be interesting to see if they're usable, or if I'll need to order *yet more wood*.
I've got to fit the windows - I'm planning for a 440mm HDPE "bubble window" on the west side, looking out onto the apple tree, and I've got 1.2M of casement windows to fit on the east side, from a local second-hand shop. Both require precision, so I've been putting them off. The casement window is getting a window box attached to the uprights, while the bubble window - being lightweight and very well-sheltered - is going to be attached to a thin piece of ply, which will itself be attached to some battens running between floor and wallplate, nestled into the straw.
The doors used to open and close cleanly, but I managed to knock the uprights for them a little off true when I fixed them to the wallplate. It's not much, but there's a bunch of futzing I need to do to sort that out - and I'd quite like it to be done before the lime render goes on, just in case I have to change plans there.
I've also got to sort out the voids between ground level and foundation, and generally rodent- and pest-proof the whole thing. If a wasp colony decides to take up residence, I will be very disgruntled. No evidence of mice so far, but the cat did take out a pigeon that was sheltering there yesterday.
Do I want a green roof? I'm still undecided about that. The EPDM isn't offensive to look at, but it's also not lovely, and it's quite a large expanse to stare at when I'm working from the home office. I think I'm leaning in the "no" direction, but only because I'd prefer it to be done. I can always come back to this another day/month/year, after all.
I'd pretty much trashed the garden while working on this. It's mostly tidied up again now - and is no longer a no-go area for the rest of the family - but the lawn is a mess. I'm trying to re-seed it, but if that fails, I'll have a turfing project to do.
Finally, there's decoration and furniture. Once it's dried, the lime render will receive either limewash or silicate paint and there's a bunch of neatening work to do around the edges (literally). The eventual plan is to use this as an educational space for our kid - we're planning to homeschool, assuming he's amenable once that age comes around, and it feels like having a completely separate building for that would be helpful. Hopefully it won't take me another six months to get the rest done.
Ideally, I'd get the chance to do this all over again with the lessons I've learned, but I'd need to find a paying client to make that happen - I've literally no space to put another one. Do get in touch if you're vaguely in the area and might be interested!
Questions? Comments? Criticisms? Contact the author by email: gemini@ur.gs
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