Damp Proof Course: The Cheapest Thing That Causes the Most Expensive Problems
PremiumWhy the DPC matters, how it connects to your existing house, and the common mistakes that cause damp problems later.
A roll of DPC costs less than a takeaway. The remedial work when it fails runs to several thousand pounds, plus replastering. That's the entire argument for paying attention to this section of your build.
The damp proof course is a thin strip of plastic sitting in the mortar bed of your walls. It stops moisture rising through the masonry by capillary action. Get the height right, lap the joints properly, connect it to the floor membrane, and it works invisibly for the lifetime of the building. Get any of those things wrong and you'll be dealing with damp patches, peeling plaster, and mould for decades. There is no cheap fix after the fact.
For your extension, the DPC itself is trivially simple. Your bricklayer lays it as part of the normal bricklaying process. It adds maybe fifteen minutes to the job. But two things make it worth understanding: the junction where your new extension meets your existing house, and the connection between the wall DPC and the floor DPM. Both are invisible once the build progresses past this stage. Both are impossible to fix without ripping things apart.
DPC work happens after your foundations are poured and inspected. Your blockwork needs to reach DPC level (minimum 150mm above external ground level) before the DPC strip goes in. The DPC-to-DPM connection must be complete and inspected by building control before the concrete floor slab is poured, because once that concrete goes down, there's no inspecting anything.
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