Injected DPC always required?

[mcmurray]

I understand that damp proofing is a contentious subject but I must ask.

The cottage I've been looking at doesn't have an original DPC due to its age and has never been injected, as stated before it had new floors poured 30 years ago with a DP membrane and all the external walls stripped back to bricks, batons fitted and plaster boarded. I've two questions.

1) Due to the plasterboard sitting proud of the bricks a quick check with the damp meter would reveal nothing... right?
2) As it stands the house was without a musty smell and had no obvious signs of damp from the floor / foundation, all decoration was 30 years old but without signs of damp penetration. Would I be forced to have an injected DPC as a matter of course and should I object as it all seems fine without one?

Alastair

 

[FamilyWiggs]

Alastair, I'm not an expert but my understanding is broadly:

a) no, DPC is not needed, and in many cases, not necessarily helpful;
b) it depends upon where the membrane in your new floors extends - if into and under the walls, then I suppose you may be ok.
c) the fact that your house has been (tanked? and) dry-lined suggests that the membrane is in the floor only and all that can happen in that case is that any moisture rising from the ground meets the membrane and is pushed outwards towards the walls, where it can then cause you a problem.

A damp meter is unlikely to detect anything on the internal face of dry-lined walls. However, in a similar situation to you, when we ripped out the dry-lining we found sodden walls behind.

I think you need to know the state of the walls behind the dry-lining, before deciding what to do.

 

[AMc]

Note - I'm not a damp expert just a home owner

1) Correct
2) Depends. If you're home valuation for a mortgage includes it as a necessity then you may end up with a retention which you can only get removed by showing a bill for an injected DPC. The best way to avoid that would be to get the surveyor to check for damp problems as opposed to simply documenting the absence of a DPC and inserting the boiler plate text that one should be injected. This will depend on the abilities and experience of the person involved and that's down to who you employ to do the valuation/survey.

My 2p worth is if there is no visible sign of a problem inside and there is no sign of a visible problem outside (mould, moss, salt staining etc.) then doing a lot of invasive investigation may be a mistake unless you plan on major work anyway. Checking that ground levels outside are well below internal floor level would be a given though. If moisture has a path from under the slab and out through the walls it may not be a problem at all.

My recent experience is that we have had to follow a similar path of concrete slab over DPC with dry lined (and insulated) walls. We removed concrete pointing and used lime in the walls to allow a path out. The subsoil was bone dry 1.5m down. While it does give me pause I also unearthed a well which shows water 3m from finished floor (and presumably the water table) so I have a degree of confidence in what we've done only time will truly tell.

 

[MdB]

The subsoil was bone dry 1.5m down. While it does give me pause I also unearthed a well which shows water 3m from finished floor

Oh, I so wish... Our water table is around 0.3m down....

 

[christianb]

I had a mortgage retention removed by writing a stinking letter to the lender, pointing them in the direction of overwhelming evidence of the futility - and dangers - of retrofitted dpc's. That, though, was in a cob building. I also engaged the services of a sensible damp expert who wrote a letter saying that he would never support the use of chemical DPC's in earth structures.

Have a shufty through SPAB guidance and see if there's anything that might help your cause.

 

[Flyfisher]

Additionally, if the house is listed, the relevant CO may have a suitable letter to send to the mortgage company about the issues of injected DPCs.