The standard way to construct the footings for a modern house, is to dig down to suitable soil and to lay a concrete bed. A few years ago I had a small 4m single story extension added to my house, and the builders had to dig down 2m to before the building inspector was satisfied – it took three people the best part of a week, and then they had to lay a 2 ft layer of concrete after that.
This would be a pretty challenging approach from a shed building perspective. Not the digging part – I like digging – but the cost implications are horrible: cement and aggregate are expensive, and deep holes, means spoil, means skip hire. So the belts and braces approach was not going to work.
It was beginning to look like all my foundation research had been done in vain.
Naturally, I had already been looking around at what the commercial garden office builders do. The majority of vendors build of a system of ‘mini-plinths’ which consist of a base stone or rubber pad about 500mm square, a top stone/pad and a heavy gauge steel bracket to which you attach a wooden subframe. The plinths sit just below the surface, so no digging or concrete is needed, which is cool.
Or is it? The very small number of vendors who don’t use this type of system build on a concrete slab instead and insist that this is the only reliable way to hold up a substantial garden room. They also go on to point out that assessing the suitability of a site for any other system requires specialist knowledge, without which you are gambling on the stability of your building.
Now, if you have just spent a week obsessing about soil bearing capacity, the ‘slab’ view is going to come as an unwelcome surprise. After all, no one in their right mind would lay a concrete slab if they could possibly help it. Never mind the skill, mess, expense and delays involved, concrete is a disaster from an environmental perspective and so more polar bears will die.
And of course, this gets you thinking: if you ran a garden office company, you’d be pretty keen on this plinth system for the very same reasons. And they you get to thinking, hang-on, a lot of these companies have only been around a few years – what if the slab-merchants are right, and it really is reckless to go around building on plinths without doing a full soil engineering survey? What if it is only a matter of time before the plinth supported buildings start to sink into oblivion? YOU NEVER KNOW FOR SURE.
We are going to have to do those sums after all.