I'm trying to decide whether to use a damp proof membrane (DPM) at the base of my hut's timber post foundations. The larch posts are resting on 600mm x 600mm x 50mm concrete slabs and will be mostly protected from rain by the overhanging structure. Should I put a DPM under the base of the posts or not? Pros would be that DPM will prevent dampness from the ground travelling through the slab into the timber. Cons may be that the DPM will simply trap a layer of moisture at the base of the post encouraging rot.
Also wondering about a layer of DPM between the top of the larch posts and the C24 2'x6' treated horizontal ground beams?
Any thoughts or experiences?
A year and a half after installing the posts and beams and all but one (of nine) slabs remain bone dry. Debris and dirt will probably need cleaned out of the holes once/year. The ground is decades of pine needles and absorbant, so very little surface water. However one post received a bit of run-off in the extremely wet downpours that are becomeing more frequent. Some mud was beginning to accumulate around the post. I simply dug a channel and lined it with slate and gravel to tempt the water in a less harmful direction. I also placed a couple of bricks under the beams, so that if the Larch post did rot, it would not matter. All the posts and slabs are accessible except this one, which is why I adopted a belt & braces approach. I'm not that worried because replacing a post will be a fairly simple job with a car jack, hand saw and new post. I was very keen to avoid introducing too much concrete and man-made materials into a woodland. Trying to adhere to the "Removeable at end of life" ethos of huts. However piling concrete blocks on the slabs would, I accept, be a more permenant, worry-free solution. Having said that I am happy that this method is working well.