Bitumen roof felt alternative to bitumen DPC for deck?

[Adam]

Hi All,

On the last part of "preparing the ground" from this page:

says "Place squares of bitumen DPC membrane over the top of the concrete...".

I forgot to actually look up what this was (I now know "DPC" is damp- proof-course) so was then wondering round B&Q looking for something which looked plausible containing bitumen, and ended up with a roll of bitumen shed felt. Is this a reasonable alternative? I notice on the B&Q website the roof felt comes under the "damp proof" category.

Thanks a lot, Adam

[The Medway Handyman]

I use this when building decks

the felt back to B&Q & swap it for

[Dave Liquorice]

The important bit is "damp proof" it's to stop moisture wicking up from the ground via the support structure into the timber causing it to rot. Traditionally slate would have been used, latterly bitumen based felts or plastc DPM as used in walls.

Shed felt has stones on the surface and is a bit thin. The stones will tend to push through the soft bitumen possibly making holes, so it's probably wise to remove as many as you can (stiff brush should do it). Then make a pad from a folded strip say 4 or 5 layers thick that is big enough to fully support the timber over the concrete/brick/slab support. Make all your pads the same number of layers thick or the levels will go out as the structure settles. They still might, thats the advantage of slate it doesn't compress.

[Adam]

decks

> Take the felt back to B&Q & swap it for that's only 30cm wide, but my concrete slabs are 40cm across. Presumably it's a problem if there's a gap in the DPC?

Adam

[Dave Liquorice]

You only need the DPC between the timbers and the slabs not over the entire surface of the slab.

[Phil L]

Personally, I wouldn't use anything underneath timbers in a situation like this - it merely holds any water in place, right where you don't want it - on the end grain...far better to make a gap somehow between end grain and concrete, so that air can pass underneath - how this is acheivable is another matter, probably involving metal bolts or suchlike

[Adam]

Sure, but there'll still be a gap, unless I'm miss> Personally, I wouldn't use anything underneath timbers in a situation like

This seems like a good point. There will be rainwater above the DPC and damp below the DPC, so what's the point in it? Why not just lay the joists directly on the concrete?

Thanks a lot for the helpful replies so far :) Adam

[Dave Liquorice]

length that they are (or would be...) in contact with the concrete support.

Any rain water will soon evaporate and dry in the well ventilated void. The damp from the ground will rise up due to capillary action and be there all the time...