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.
Me? No DPM. You want the air to get in here and dry the posts.
Watch for water pooling. If it pools cut a channel away from your slab so it runs away and allow the slab and post to dry. Reach under and clear around the edge of the slab to encourage drainage, taking this smeared route to your channel going away from the post hole / slab.
Mark
Nice woodworking I like the joint detail.
On wood I usually work from a basic premise, this is that the grain in wood is tubular and will wick up whatever it gets stood in, trees work like that. The trick with fence posts then used to be to stand them in a bucket of creosote or something else nasty thin and oily so that the wood grain had wicked up something that repelled water and sealed the wood itself. The nastiness killed anything that likes to eat wood (fungus, rot etc). I have been told that thinned engine oil recovered from an oil change may work as well (thin with white spirit) but never tried it. I have seen fences painted with this gunk survive quite well though.
Mechanically speaking getting the wood up off the floor is good, water does not like going up hill, concrete piers or blocks then are useful and IMHO better than a flat slab especially one that is sat below ground level and offers a place for water to puddle should the worst happen. A flat slab above the surrounding ground level is better than one below etc etc etc.
A piece of DPM is better than nothing so not to be sniffed at. On its own on a flat slab below the surrounding ground level though it could make things worse as it will slow the posts drying after a wetting.
Altogether then do them all and you posts should last as long as the rest of your hut.
Other than that your mileage will vary with how wet the ground is, whether it ever puddles on the slabs and a whole other bunch of variables.
I think only the hut builder can answer many of these questions for themselves. You are ahead of me in this respect. When I get to build time I will be doing all of the above. I am of the view point that wood structures need a good hat (roof) and a good pair of wellies (floor post treatment and placing).