Screw Pile Foundations & Base for Shed/Garden Room

In the next while I'll need to replace my Argos flatpack as it is (a) too small and (b) too flimsy for my liking. I intend to build a timber frame shed with 4x2 (possibly 6x2) frame, OSB inside & out, probably cladded with cement board + render. The plan is for the roof to be apex and tiled, and it'll have windows & doors: it'll be a shed to begin with, but I'm going to leave plenty of scope to easily convert it to a garden room/office by adding insulation etc. It will be there a good long while, but if I (or someone else) wants it gone I want that to be a relatively easy job.

Obviously this will need a base, which again obviously will need to get the wooden base of the shed far enough off the ground to prevent wicking & rot etc. I don't want to use concrete blocks because they're ugly and inconveniently porous.

In a perfect world, the base would be concrete with DPM & DPC. But there'd be several difficulties with that, the most decisive one being that I just don't want to have to break my back digging trenches.

So I'm looking at alternatives, one of which is ground screws/screw pile foundations. Which look like they might be perfect in theory, but the only information I can get on them is from people trying to sell them plus about 3 reviews on Amazon. And it's not very informative as to spacing, weight bearing, longevity etc.

Has anyone any experience with these? Or an anyone recommend an alternative to concrete?

Thanks.

Lumen

I used ground screws. They worked out fine but I had very specific reasons for using them.

https://www.boards.ie/discussion/2057890777/shed-build-help/

If I was doing it again I'd probably pour concrete piers/piles as it's easier to get them accurately located and level.

I don't think your reasons for avoiding blocks are good. Porosity is easily dealt with and you can paint them black if you're bothered by the ugliness.

You don't need to dig trenches for a slab base.

chooseusername

How do you plan on insulating the floor if you wish to turn it into a garden room/office later?

https://www.boards.ie/discussion/comment/120256639#Comment_120256639

Rigid PIR on top of the OSB floor of the shed, with more OSB on top for the new floor. Aluminium tape at all joints in the PIR, which will address air tightness & vapour permeability. I intend making sure the shed ceiling is high enough to accommodate the PIR (which will also be on the walls).

That's one reason I need the frame raised for ventilation underneath and also a ventilation gap for the wall cladding: when the PIR is eventually on drying to the inside won't be an option.

https://www.boards.ie/discussion/comment/120256311#Comment_120256311

Thanks for the link. It's very helpful how someone has already successfully done a similar job. I will shortly be resurrecting that thread with questions for you!

Yeah, I'm beginning to lean back towards blocks + DPC. I've several reasons for avoiding a concrete base, which can be summarised as too much effort, too much money, too much embedded carbon, would kill nearby plants, less natural drainage in the garden, etc etc.

Lumen

Stating the obvious but the foundations should match the weight of the structure. I think my shed had a design weight of about four tonnes, all wood (this was when construction timber cost about €500/m3).

Roof weight is particularly important as heavier finish requires heavier structure which requires heavier structure and so on.

If you keep everything light and do the calculations properly (i.e. proper engineering for efficiency) then you get into a virtuous circle.

My rough calculations say my shed should be around 4.5 to 5 tonnes

You're right about having the foundations match the structure, but my options stay the same: slab base, ring blocks/piers on compacted hardcore, or screw foundations.

Blocks on hardcore are the best match for me (althoughIcouldchangemymindaboutthat again!). Not ideal, but I don't ideal is on my list of options.

Calahonda52

While Lumen is the master craftsman par excellence in this thread, what I did in a previous house was 4 decent square concrete pad footings at the 4 corners, well down for frost heave, built up with blocks and then put four galvanised rsjs as the base, just four points to level, bolted down to the pads.

a layer of drainage membrane to stop weeds and then some 404 crushed stone

The bolts were long enough to allow shims under the rsj for final levelling

I will be dong it again in a new house and this time will use some 12" land drainage pipe as permanent shuttering to the concrete up-stand off the square pad rather than blocks, with the bolts set in place..I see loads of farm gates hanging off land drainage piers.

I did a poly tunnel during covid and used 6" wavin in the same way, got tops of the pipes level to each other, used post setting cement as a rapid setting to hold them in place and then poured concrete down the pipe with the bars for the tunnel already set at the right level.

The holes I dug with a 4" chisel on a rented kango hammer and a post digger to remove the soil.

All done nice and easy

https://www.boards.ie/discussion/comment/120260562#Comment_120260562

How did you secure the structure to the RSJ? Assuming the structure was timber frame.

Also, what did you use as shims? I was thinking slate, as I can at least be confident it'll take the weight.