3. Foundations, Posts & Floor Joists
The Cabin has an unusual construction which has actually worked out really well.
The whole building is raised above the ground on a series of posts. This means that the foundations consist of a series of individual post holes filled with concrete. This is a one man job, and can be accomplished using single bags of ready-mix saving the hassle of working with a concrete mixer and all the mess that goes with it.
Benefits compared to a concrete slab foundation:
- Posts can easily accommodate uneven ground.
- There is no need to level the site first which can save many hours of work.
- Tree roots are far less likely to cause damage as they can find there way between each foundation.
- Drainage is improved with the soil beneath contributing to the total soak-away potential of this area of the garden.
- Maintaining an air flow under the building reduces risk of rot, which always starts with timbers closest to ground level.
The whole building is built around a set of 3×3 in (75×75 mm) posts, laid out in a regular grid roughly 5ft (1.5m) apart. Each post is held in standard 3in square metal fence post spikes, which are set in concrete. Unlike typical fence installations, these posts will take almost no sideways forces which is the weak point when they are used for fences as they catch the wind and leverage all of this force onto the spikes causing them to fail prematurely. As the posts in my building are all tied together they will be thoroughly braced, so the spikes are simply carrying the weight of the building vertically.
3.1 Setting out the posts
I used string lines to ensure the holes were dug in the correct place, then concreted in the met-posts checking alignment as I went. I left the concrete slightly raised and trowelled it to create a slope so water shed away from the metal spike.
3×3 treated posts, 2.4 m long, were then clamped in the Met-Post shoes, checked for vertical with a spirit level and braced temporarily with tile battens to keep them in place. Their height will not matter as I will cut them all off to the same level
The middle row of posts as shown on the plan above, were a little different, being only 2×3. This is because the one on the far left is also going to serve as one of the mullions in the panoramic windows to the rear, so needs a slimmer profile. The two posts in the centre of the building are only 20cm long as their purpose is only to brace the floor. They don’t need to come up higher. (BTW Having these central supports for the floor is why I could get away with using 2x4s for the floor joists as that made the span was less than 5ft)
The next two steps help tie the structure together allowing the temporary bracing can be removed as they are completed.
3.2 Wall Plates
Using a spirit level the posts were marked at the same level and all cut off in situ.
Two 2 x 3in (50x75mm) timbers were then screwed down into the top of the posts to make an effectively 3 in wide by 4 in deep (75 x 100mm) wall plate that continued all the way around the top of the outside posts. (See Elevation above)
Using two 2x3s together like this helps with joints. Say it is necessary to place a joint in the lower piece: cut it to ensure the joint falls in the middle of a post top. Either side of the joint can then be screwed into the post. Make sure the top plate does not also need to join here, so that it bridges across the joint, maintaining the integrity. Staggering the joints at the corner posts can achieve a similar effect.
3.3 Floor Joists
I used 2 x 4 in timber for the floor joists. The main joists were run lengthways, and screwed to the inside of the posts leaving the bottoms some 6 to 8 inches clear of the ground. Down the centre two joists were used, one on either side of the central posts. Joist hangers were used to run the cross joists which were spaced at ~16 inch (400mm) centres.
In this picture (from a later stage in the build) you can also see the 3x2in posts used along the centre line. The ones in the middle were cut off flush once the joists were in, but the one at the back goes up to the wall plate doubling up as the central mullion for large 4 pane window at the back.