DPM or not?

Discussion in 'Builders' Talk' started by Dave Reynolds, Sep 19, 2021.

  1. Dave Reynolds

    Dave Reynolds New Member

    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
  2. ElecCEng

    ElecCEng Screwfix Select

    I would match the number of air bricks, i.e. 4 front, 4 back. Put the DPM on the bottom of the joists, insulate and seal. Leave the inner surface of the bricks as is.

    Don’t bother with the chimney for ventilation. I think you are correct that the damp patch on your wall is related to it. Fix it and leave as is, otherwise you’re introducing another possible issue.
    rogerk101 likes this.
  3. CGN

    CGN Screwfix Select

    More air bricks, clear out any crud. High ground level is probable cause. You won’t cure it with injectable dpc.
    ElecCEng and rogerk101 like this.
  4. Adama119

    Adama119 New Member

    Hi Dave,
    I've recently had the exact same issue in my house. It's an 1870 victorian terrace with cavity walls and floor.

    Damp is in the hallway. We had a new front door fitted about 8 years ago and no ventilation holes were added below. Our old front door had loads of them but we didn't think anything of it until the patches appeared.

    We took up the floorboards to find damp and uninsulated pipes, debris which had pied up under certain walls and breached the dpm (right under where the damp is showing) and zero airflow. The smell was bad and there was white mould on the ground.

    I created a new vent below the door which has instantly created a cool airflow, removed debris back below dpm level and insulated pipes which had been sweating through the summer months and showed signs of the green discolouration copper makes.

    I'm certain this has helped but will create more ventilation below the door if it doesn't fix it.

    Since I've been researching it I've also read that old houses like ours weren't built to endure climate change and 2021hot weather but were built to maintain heat during the much colder victorian winters. It said that to fix this we should have air blocks every 1.5 metres outside to tackle the rising humidity created in our cavity spaces below. We now have one vent on each of the 3 sides of our bay and under front door. Also two out back infront of largest room and one under the kitchen. All are now about 1.5 to 2m apart.

    I hope some of this helps.

    Last edited: Sep 23, 2021
  5. GRH

    GRH Member

    May I respectfully suggest, as this is an old building, that you re-plaster with NHL 3.5 lime and sand mix, this will enable the wall to breath, cement won't and the moisture will find another route that may lead to damp
    ElecCEng likes this.

Share This Page