How to Build a Concrete Base for Your Log Cabin

In most cases a 100mm thick concrete slab is an ideal base for your log cabin providing the ground is reasonably level and firm. The slab described here is the “above ground” method and includes a damp proof membrane to stop damp rising up through it and to prevent the newly laid concrete drying too quickly by draining rather than by evaporation.

Firstly, establish if your ground is level enough over the area where you wish to position your cabin and that the ground is firm. If in doubt, excavate the soft ground and replace with well compacted hardcore, usually about 150 mm thick. If the site is significantly sloping, undulating or a particularly damp area, we can recommend other base solutions.

The base area should be 100mm bigger all round than your log cabin (measured to the outside faces of the cabin walls). Use 100 x 25mm timber to form the frame or shuttering for your base. Measure and mark out the correct size on the ground. Place the timber on the ground in position and knock into the ground some posts along the outside edge of the timber. These will hold the timber on its edge and stop the sides bowing out when the concrete is poured. Ensure the top of the framework is absolutely level then screw or nail the timber to the posts. If any of the posts stick up above the timber frame, cut them off with a saw. This will enable you to tamp down the concrete to make it level.

Make sure your frame is absolutely square by measuring the diagonals of the frame from corner to corner. These measurements should be exactly the same. You can now lay down a sheet of 1000 gauge polythene to form a damp proof membrane. Pour in the concrete then with the aid of another person level the concrete using a length of timber. With the timber on its straight edge, tamp down the length of the frame. These vibrations settle the concrete and it will find its own surface level. Tap gently on the sides of the boards with a hammer, moving all the way around the frame to release any air bubbles from the frame and the concrete. A tamped finish is not suitable for the cabin, so smooth the surface using the straight edge in a sawing motion whilst moving across the area. The straight edge must stay in contact with the pre-levelled shuttering at all times to ensure the surface remains level.

When the concrete is hard, remove the timber shuttering and lift out the posts. Normally, at above 10 degrees, the concrete will cure overnight to a sufficient hardness to build on the next day. It will continue to harden for the next 30 days.