Hi Stewart
When I come across these sorts of situation I look at the problem from several aspects.
1) can water/damp/moisture rise up from whatever I'm putting my 'item' on?
2) can water/damp/moisture run down the outside of my 'item' and creep underneath it to cause problems?
3) can very strong wind blow/force water underneath the 'item'
4) how does any excess water run away ...ie can it get flooded
So in the example of the summerhouse that we bought
it stands on a concrete slab. The slab has been built to building regs although not strictly necessary. However, what this means is that (1) is not a problem
The design of the building is that it has a wooden frame (made from sawn treated timber) that sits ontop of the slab. On top of the wooden frame sit 'sole plates'. These are made from Meranti, the cross section has a lip on the inner face, then a flat top and then a slope. So the prefabricated walls sit on top of the flat top. The inner lip prevents any water being blown in as in (2) and (3) above. The slope allows water to run off (4). The diemnsions of the 'sole plates' are such that they overhang the concrete slab and so prevent water getting onto the slab and creeping underneath (2).
On the underneath of the sole plate there is a drip channel and this encourages water from the slope to fall off and not run back underneath and onto the slab.
I further improved their design by putting lead flashing over the sawn frame and lapping it over the edge of the concrete slab. This is inboard of the drip channel. So, in theory, that prevents any water ingress andf so hopefully you can apply these design principles to your own situation.
That's the theory. In practice there is a wee flaw in their design (and if you'll let me hijack your thread :wink: ) is as follows.
The design of the summerhouse is hexagonal and so the sole plates meet at 45 degree angles. Over the last few months (it's only been laid down about 4 months), the Meranti has shrunk slightly and so these joints have opened up. Now the installers realised that this might happen and put silicon rubber in the joint faces...but possible either not enough or the Meranti has shrunk more than expected. Bottom line is that with the recent heavy rains, the summerhouse now leaks !!! :evil: The worst gap is about 4mm.
Now I can't decide whether to put brown silicon into the cracks (and become part of the annual maintenance regime) or try and cap the joint with a thin piece of Meranti (siliconed underneath). Cutting out a rebate on both sides and infilling with Meranti will merely compound the problem, i think.
Roger
