We are about to pop a couple of new buildings up in our garden – a workshop / mancave and a summer house / garden office thingy. Some extensive googling about construction methods has led me away from a traditional foundation and stick build towards using galvanised screw piles, a C24 support frame and SIPs for the floor, walls and roof. I like the ease of construction, speed of build and lower carbon footprint when compared to traditional construction. Both buildings will be a decent size – workshop will be a smudge under the 30 m2 limit for building regs and the summerhouse probably about 20 m2.

Has anyone ever built anything using screw piles and SIPS? Clearly SIPs are a bit more expensive than other build methods, but the lack of Groundworks should offset this. Plus, no diggers and no mess etc. Clearly subject to ground survey etc.

Pointers, tips, links, references, recommendations all gratefully received.

Like the approach this guy takes for foundations for garden rooms:
https://www.youtube.com/channel/UChqWFbSX8STP_c8W0RVW1Xw/videos

I had a sloping site, so used DIY concrete micropiles. 6″ cardboard packing tubes as formers, post hole diggers to dig the hole. frame on top. still there 5yrs later.

I have seen some horror stories about SIPs, including a £1million + new build which had to be completely redesigned as a result of SIPs which failed, and two residential blocks of flats which need to be completely re-roofed as a result of SIPs failures.

The problem with SIPs is that they are extremely susceptible to moisture ingress – if everything isn’t exactly perfect, and any defects do come to light, as a result of the ‘structural’ part of them it can be a really big job to put right.

watching with interest as planning a workshop build this year which was delayed form last summer (due to the world falling apart – you may of heard about it), and planning in using sips. however was planning on a concrete raft foundation as quicker, cheaper and easier thank screw plies or pier and post

Our house is a SIPS construction. On the moisture thing, I ended up specifying something a bit beyond regs for the roof. Essentially I added an extra air gap outside the SIPS, to create a way for any moisture to get away. It was in recognition of some issues that have been found with SIPS designs where the ridge has, if I remember correctly, a non breathable finish directly on the SIPS panels and ridge timber, holding moisture in in the event it gets in. Of course it shouldn’t but if it does you want it to be able to get out.

Beyond that, I can’t comment on longevity – the SIPS only went up 2 years ago.

I guess SIPS would be expensive for an outbuilding though. Another option to consider might be prefab panels but the kind that look like insulated stud.

Footings for brick towers we about 18″ deep and filled with concrete

four along the 4.8m side and one row in the middle of the 3.6m side. The back was bolted to the existing garage wall.

The cabin is 3.6m x 4.8m and the water in the hot tub is over 1 tonne. The curved wall with the grass is next to the cabin and does not extend under it. Still standing after many years and all designed and built by me 🙂

We work in timber frame/offsite construction & have used SIPS several times. They are a great product, but in my experience they do have some issues.
1) Sometimes, even though they are ‘structural’ they require additional timbers or steel to ensure they span the required distances.
2) They need a tolerance designing in, as its quite common to get creep on the joints.
3) They’re expensive, if as a business like us, you are buying them in from a supplier & then selling them on as an installed product. It works better if the manufacturer is installing or a self builder.
4) I have yet to have it proven to me that for any given U-Value, they are cheaper than studwork & insulation.
5) It is easier for shoddy installer to achieve the necessary air tightness when compared to timber frame & manually installed air barrier.

I (structural engineer) did drawings/calculations for pretty much exactly what you want just before Xmas.
We didnt use SIPs though, just regular timber construction that any half competent ameteur builder/carpenter can do. As above, I’d avoid SIPs if you cant guarantee doing it right, and the cost is likely to be higher at your scale anyway.

We used screw piles as clay soil, sloping site, and nearby trees would have demanded a huge amount of concrete. If not needed, Jambo and WCA have easier and cheaper solutions.

I’ve also done a timber and screw pile rear extension on a mid terrace with no rear access, so a no dig and no concrete solution was needed as everything had to be carried by hand through the front door/living room window.

Mine is on a slope as you can see and there is a huge oak tree just behind the fence on the right of the photo. Ground is not clay though thankfully.

Maybe worth joining the ‘Self Build Garden Rooms, Offices and Home Bars’ group on Facebook. Ran by Liam of Oakwood Garden Rooms who does the YouTube video linked in the first response to this thread. Really active and helpful group and someone will have used SIPS.

Partial depth self poured ‘pads’ would be my last choice. Sonotubes are designed to get below the frost line and provide a smooth wall surface so that they resist frost heave. UK frost line might only be a few inches though.
With footings, You also want to get down to undisturbed soil regardless of frost depth. At the side of the house where your foundation was excavated, you’re most likely dealing with poor load bearing uncompacted backfill. same consideration with backfilled landscaped gardens.
I wouldn’t be following WCA’s approach tbh. Hopefully there’s no structural fasteners connecting the joists onto the leger board and its just load bearing..

Looks like screw piles are just the ticket but presumably you still need geotech to spec the number and sizing?

ayjaydoubleyou
Free Member

I (structural engineer) did drawings/calculations for pretty much exactly what you want just before Xmas.

Structural Engineer you say, could I possibly pick your brains regarding the validity of some timber frame constructions details?

To the original OP, I wouldn’t discount a normal timber frame construction. Still rapid to construct, provide an excellent structure and cost less than SIPS.

Also planning a self build this year. them screw piles look like a good idea but where do you get the massive pneumatic screwdriver thing? not sure theres much of a saving in getting a contractor in to install them.

I used these screws for a building I put at the bottom of the garden. I was in amongst mature trees so traditional founds weren’t an option. I think I used about 20 for a building 17m2 based on the loading info on their website. They’re hand driven and easy to install, though whether they’d stand up to a structural engineer’s calcs is debatable, but I decided for the nature of the building they’d do the job!

If anyone has foundations ready, plus a lorry arranged for delivery plus a crane, then these prefabs will be worth a look. A fairly solid construction prefab, with no bids yet, at a good price.

https://www.sweeneykincaid.com/Auctions/SaleDetail/3902