Log Cabin Base Ideas, Concrete, Slab or Timber?

A garden building such as an office or a log cabin is no different from any other structure. Without a strong foundation, it will be unsustainable and eventually unusable.

Creating extra space in the back garden has become extremely popular in recent years as an affordable alternative to extensions, loft conversions or even the expensive upheaval of moving home. Subject to certain conditions, it won't even require planning permission.

However, although it's a fast and cost-effective option, the principles of construction apply. Fortunately, the days when your only choice was a concrete base are over and there are now several types of foundation, including concrete, slabs and timber. Let's look at each one in more detail.

Now you neeIn order to support an average-sized log cabin, a concrete base should be at least 100mm thick and laying it involves a lot of preparation. You need to clear away any vegetation or other debris and dig out the ground to at least 150mm and fill the hole with aggregate which needs to be tightly compacted. Place a polythene damp proof membrane over this. Before you can add the concrete, you need to create a wooden frame to contain it while it sets, using planks that are at least 25mm thick to withstand the pressure. After adding the concrete you must ensure that it is perfectly level, using a piece of timber or other smoothing implement.

Now you need patience while you wait for the concrete to dry, which can take several days. You can remove the frame after about 24 hours, but we'd recommend leaving it for at least three days to avoid any damage to the edges. Ideally, you should allow 28 days for the concrete to reach its full strength before you return to it for the construction of the log cabin.

The process of laying a slab foundation is very similar in its preparation. Again you have to dig out the area to about 150mm and fill it with hardcore or some form of aggregate, which you need to compact. Cover this with a layer of sand and cement in a ratio of about 4 to 1, about 50mm deep. Smooth off this layer before you lay the paving slabs. This double layer should be sufficient to raise the base of the cabin above the level of the surrounding ground so that rainwater can drain away properly.

Choose flat paving slabs rather than the textured kind. They should be a minimum of 40mm thick. As you come to lay them, make sure each one is level and that no gaps are left between them. If necessary, use a rubber mallet to make a perfectly even fit. Use a spirit level as you go and if there are any inconsistencies add more of the sand and cement mixture underneath.

There's no need to bind the slabs together, provided that they are tightly and evenly laid. Your foundation is now ready to take the weight of the log cabin.

This is an excellent solution if you've earmarked an uneven site for your cabin, perhaps across a bed bordered by paving. It's wise to use tantalised (pressure-treated) timber because this will resist damp and rot far more effectively than untreated wood.

Clear the ground of any rocks or vegetation and make it level. Place a membrane over the entire area otherwise, you'll find weeds growing up between the timbers. Create the outer frame using butt joints and then fix them together with 6" nails or screws designed for outdoor use. Add the timber bearers at 18" intervals, check that everything is level and then screw the timber boards in place, leaving gaps of 4-6mm between each one. Finally, even if you're using tantalised timber, give the finished decking a preservative treatment. Bear in mind that a timber foundation will add to the height of the finished structure, which must be a maximum of 2.5m high to fall within with planning permission exemption.