4. Foundations
Time to get outside and do some building work – yay!
That’s the good news, the less-so-good news is that the most critical part of the entire build is the first thing you need to do, there’s no easing yourself in i’m afraid.
Setting out
There’s not a whole lot more to say that’s not covered in the video but one thing you can do to make your life easier is to use batter boards. As the string lines intersect each other, it’s slightly more accurate and easier just to move the string along the batter boards rather than trying to move the stakes to get your outline square. Here’s a great video on how to make batter boards.
I’d also recommend picking up some line marking spray to mark the ground under the string lines. Now you can remove the string lines but your building outline remains!
Concrete slab
So there are two main options for foundations: a type of pier system that holds up a timber floor or a concrete slab. The piers can be pre-made plinths, timber/metal posts sunk into the ground or of course concrete blocks like mine. Ground screws are becoming more frequently used too.
A concrete slab is a very popular choice however and potentially speeds up the build as you can go straight on to the walls without having to do a timber floor. It’s worth considering contracting this out as getting the mix right is important. A concrete company will usually be able to pump in the mix from the road through special pipes (this can even be done over the roof!) or it can be wheelbarrowed in. Of course you can go it alone and buy/hire a concrete mixer.
Thermal efficiency: A concrete slab has great thermal mass. One of my favourite bloggers has a great article on the topic but essentially, in comparison to an insulated timber floor, it will take longer to heat up in winter but will retain the temperature of the room for longer once the heaters are turned off. Conversely in summer, it will help keep the room cooler.
You can of course add insulation to the concrete bases. House extensions usually have the insulation below the concrete slab or at least below the top layer of screed. However in Nick Somerville’s build, he added the insulation above the concrete base and added rigidity with chipboard.
So how do we go about making this magical stone?
- Take the outline of your building and dig down around 5cm wider than where the outside of your walls will be. At this point you can do the shuttering – basically timber boards held in place by stakes to contain the concrete.
- Add your hardcore and compact it- this stabalizes the ground.
- Add 50mm of sand blinding – this prevents the hardcore from piercing the DPM.
- DPM – a thick sheet of polythene which prevents moisture rising into the concrete.
- Rebar/wire mesh – this should sit in the middle of slab to add rigidity. It’s not always a necessity but worth looking into.
- Concrete – poured to the top of the shuttering and levelled off with timber or a float if you want it extra smooth. It needs to be kept wet for a few days until it cures. Full hardness will be achieved over a few months.
Once the rest of the build is complete you can add 25mm PIR insulation, vapour barrier, chipboard, underlay and flooring!
So why didn't I go for a concrete base?
- I’m on good ol’ London clay which shrinks and expands with its moisture content. The garage slab cracked in numerous places probably because of the clay beneath.
- There are quite a few trees in the vicinity and their roots can break concrete over time – think of the pavement on a tree-lined street. Additionally the planning department were concerned about an oak with a Tree Protection Order so may not have granted planning permission with a concrete slab.
- The slope of the site would have meant the slab would have been extra thick on my mum’s side.
- The cost – using block piers is about as cheap as you can get, although the timber floor does offset that somewhat.
What would I have done differently?
Well in some ways, nothing! My foundations have stayed really solid and there’s been no movement at all 5+ years after I set them in place.
However I could have dug deeper, laid down 4 inches of hardcore and then poured a good amount of concrete before setting the blocks on top. At the least, I probably should have cemented the bottom block to the hardcore under it. I’d also recommend pouring concrete or postcrete around the blocks rather than just filling in with hardcore.
That said, sheds are usually built on paving slabs on a bit of sand and a garden room is basically a glorified shed so I’ve not been sweating it too much.
The 15cm building regs rule
At 7:03 into the video I talk about having the floors 15cm above ground level. If your garden room is under 30m2 then building regs won’t apply and you can disregard this as it eats into the 2.5m permitted development height – those centimetres will feel precious later on in the build!
For me I had an overall height of 3m and the land at the back of my garden room is significantly higher so if my foundations look high, that’s why.
Whether you are using piers or a concrete slab go for a few centimetres above ground level (maybe 2-5cm) and if you’re concerned about water getting in, dig out the soil from around the building and put gravel in to allow rainwater to soak away. Have a look at Graeme’s build to see what I mean.
Task: Divide this part into two sections.
- Set out the outline of your building, it’s exciting to see the footprint. You may find it’s too big and you have to go back and revise your plans or perhaps you have more space than you thought.
- Make a decision on the type of foundations you are going for and go for it. I’d recommend getting the foundations done way ahead of your intended build schedule.