Damp Proofing

Penetrating Damp gable-end wall after loft conversion

Anonymous user 03/03/2024 - 3.07 PM

I live in a 1930s semi-detached house which has recently developed penetrating damp. The ground floor walls are cavity with solid 9-inch walls on the 1st floor covered in original probably lime-based render (1st floor only). About a year ago we had a hip-to-gable loft conversion added. The new gable-end wall is timber construction with DPM and external cement-based render mixed with water-repellent. Since the recent wet weather penetrating damp has appeared in large patches at the top of the 1st floor just below the new loft gable-wall. Our builder cut inspection holes into the plaster in the loft wall to rule out problems - it is completely dry inside. The problem appears to be where the old and new renders meet in the gable-end wall. A line runs along the length of the building here which may have allowed water to be drawn into the solid wall and ultimately right through to the inside. I think this is because the damp first appeared in 2 distinct patches where the new/old renders meet on the outside. It has since spread horizontally along this line. It may also be that altering the roof structure of the house fundamentally changed the way water interacts with the 1st floor subjecting it to much higher levels of damp than the render was designed for i.e. previously it was protected by guttering and eaves collecting rainwater from the roof so it was subjected only to rain that landed on it directly. Now water collects high up at the apex of the gable wall and runs down (water-proof render in the loft) saturating the original render on the 1st floor. My builder says they have never come across this problem before although has admitted that there will always be a weakness where old/new renders join but has offered nothing beyond that. I obviously need to get the problem repaired and don’t know whether this is just “bad luck” and something I need to suck up or whether the builder shares some responsibility and should have designed the loft conversion in a way that anticipated / prevented it from happening. I have suggested that we jointly appoint a specialist building surveyor to diagnose / specify the correct repairs and agree to be bound by his decision on culpability / proportion of repair costs. I’m perfectly willing to accept 100% of costs if that is the finding but don’t want to pay if the loft/render design is at fault in any way. They seem reluctant to agree to this even though the surveyor would also hold fees in abeyance until work completion and proportionately split them according to the judgement – so zero up-front costs. The alternative is that I appoint my own surveyor and then pursue them if he finds them at fault in any way – likely expensive for both sides and something I would rather avoid. Generally I have a very good relationship with the builders and would much rather deal with this amicably. Any advice on possible causes could and recommended courses of action gratefully received.