We are often asked about the foundations needed for log cabins. We always provide guidance and specifications for larger log cabins, with the plans, but here are some notes explaining about the different considerations when laying foundations for log cabins.
The video below shows use of concrete piers on a granny annexe building foundation:
Types of foundations for log cabins
Several different types of log cabin foundations; the type you can use is determined by the topology of land, slope, and by soil conditions. For instance, this can be demonstrated with the difference between sloping and flat land.
Sloping sites for buildings
If you have a sloping piece of land, a concrete slab, or suspended concrete floor is less practical than doing a pier foundation. This is mainly because, if you consider a sloping ground, you have to do a lot of landscaping to level the ground and once you do that, you will often have a cut into the ground on one side. You then end up with a wall of earth and build a retaining wall to stop moisture coming running down slope onto slab that the building rests upon. On sloping ground, this option is therefore a lot more expensive.
If you build piers, this reduces the landscaping necessary. At the highest point of land, you have the lowest piers and lowest you have highest piers. It is a more cost effective solution, as you don’t have to do a lot of landscaping and you don’t use as much concrete, so it is more environmentally friendly too.
Where you have heavy clay under top soil, you have to be careful with pier foundation, as you have to have much wider holes as clay is not a very stable substrate. If you build in limestone it is very stable, although you then have problems of digging holes in rock, but you don’t have to go down too far.
So, for sloping ground the pier is often the best option. Second best is to level out and build a retaining wall and then for smaller buildings, a decking frame on top of that for building to sit on.
For very small building, then there is less work to level the ground (such as with a 3 x 3 metre cabin), but as you get to larger structures, such as 8 x 15 metres, you have to move huge amounts of soil.
Level sites make the better option
On a level site, you can use a concrete slab, but you must take account of the soil substrate under-ground. Especially with larger buildings you have to be careful that foundation depths are correct. With clay soil, with significant clay content, you may need to dig out to the clay and put hard core down before you put down the slab and rebar into the slab to give it reinforcement. If it is rock or limestone, its’ a pretty solid base and you can dig out top soil and use a lighter cover of hard core and place the slab on that.
This is not intended as a definitive guide to log cabin foundations, and you will need to judge your foundations depending upon what the building inspector requires.
However, the building inspector will usually have a very good idea of local conditions. They will help to advise you as to what you will need to do. Otherwise, with significant log cabin sizes, you may need to take cores and samples to assess the foundation for your new log cabin.
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