DPM or not?

[Dave Reynolds]

Hi folks, having a nightmare deciding how to approach a damp subfloor void and looking for some experienced advice. Got a Victorian terrace with a musty smell downstairs. Lifted floorboards and found damp soil beneath, lots of buliders debris, bitumonous DPC visible in some areas. Insufficent ventilation, 2 airbricks at the front, none at the back. High mositure readings in most downstairs lower walls and a visible (and growing) damp patch in the alcove next to the rear fire place. Some damp joists. No leaking pipes, but there is high ground level on one side that I cannot change. Had a surveyor and builder look at it and I want to give it a belt and braces fix. So current plan is clear oversite, expose existing DPC (if possible), increase ventilation (2 more air bricks to front), 1 to rear, maybe also use rear chimney. Chop out old rear fireplace (suspect dampness is being drawn into it -> patch on wall). Replace any damp joists, chemical DPC, re-plaster with sand & cement mix. Builder and surveyor both recommend putting down a DPM held flat with a layer of gravel on the damp soil. I am unsure about putting a DPM down as it will not be joined to the DPC. Worried it may make the lower walls damp. Are there any opinions on this out there please?? Also any thoughts on using the rear chimney to vent the subfloor?? Dave

 

[JohnD]

dpm will cut water vapour evaporating off the damp soil.

Assuming the water table does not rise above your dpm, it will not reach the walls so you will not need to turn up much

the walls will be exposed to the airflow so water will continue to evaporate off them and be blown out by the airbricks. It will probably only rise about two courses in clean bare brickwork. Put additional airbricks round your damp patches.

You must have airbricks on at least two sides to get throughflow.

dig out the fireplace because it will have no dpc and be full of rubble. You can put a hearth slab on honeycomb sleeper walls.

I quite like using the chimney to ventilate the subfloor, but you never know what a future occupant may do.

You mention a chemical dpc. Do you mean silicone injection? Waste of time, money and effort. Completely useless.