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Damp walls due to DPC Bridging in 1950's semi

Postby satkin859 » Fri Apr 29, 2022 1:20 pm

Hi folks.
Some damp issues in a 1950's semi, cavity wall, suspended timber floor property I purchased in December.

The front, back, party and kitchen wall all have damp issues. See reference floor plan below.:

Floor plan

I have had multiple companies round, all of which have confirmed there are multiple bridges of the DPC due to the following:

- Cavity walls have wet sand/droppings above DPC level
- Fire place hearth in living room is backfilled with coal/sand above DPC level
- Fire place removed from dining room by previous owners left hearth in place which is also backfilled with coal/sand above DPC level
- Concrete floor in kitchen (1 inch thick) is backfilled with coal and
sand above DPC level

Since moving in, I have cleaned the cavity walls out to around 150mm below the internal cavity leaf DPC (one brick lower than the external skin)

I have also removed everything from underneath the hearths in the dining room and living room, with a plan to replace the hearth in the living room with suspended concrete.

living room hearth before removal.jpg
Living room hearth before removal

living room hearth after removal.jpg
Living room hearth after removal below DPC

Dining room Hearth before removal.jpeg
Dining room hearth before removal. Now removed all the way down to oversite.

I am taking out the concrete and backfill from the kitchen tomorrow and moving back in with parents until all work is complete.

We have had new airbricks installed (double the amount) with telescopic sleeves

We are reducing external ground levels slightly and putting in some drainage due to being on clay soil. The oversite below joists is lower than external ground level.

I have a joiner on standby ready to replace all ground floors with new pre treated timbers, wrap in DPM at wall bearings and sleeper walls.

I am installing a new chemical DPC at the lowest mortar joint above DPC level just to be on the safe side.

My question however, is now that we have just about removed all the bridging. How do I make good the actual walls?

We have been told that we need to protect against next door neighbour. As its more than likely their chimneys are also backfilled with rubbish and making the party wall at those locations wet. And it has been suggested to install a meshed membrane to the chimneys and walls for this, with plasterboard dot and dabbed on top.

I'm just wondering if there are any other alternatives or advice?

Kind regards
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Simply Build It

Postby ajbb77 » Sat Apr 30, 2022 6:56 pm

Chemical DPCs are not a failsafe system and are generally only guaranteed to prevent damp rising up the walls when used with proprietary cementitious plasters up to at least 1 to 1.5m because it's impossible to know that the chemicals have penetrated the mortar bed completely. If I wanted to be safe I would want to check that the existing DPC is still in-tact and functioning and if in most places it appears to be broken then I would want to install a new physical DPC in the course above the existing, or perform a patch repair to the existing if it's mostly in-tact. Plasterboard is appropriate because it avoids the issue of water-borne hygroscopic salts coming through from the damp brickwork, though they can migrate through dot and dab so battens are preferable. Consider also other sources of damp beyond the bridged DPC like a leaking subfloor pipe, or a high water table, which are often overlooked.
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