Penetrating or rising damp problem

10 Aug 2007
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United Kingdom
I have damp (according to the surveyor & damp contractor) along the base of the two walls (front bay & side) of 1930’s semi. I am in the process of buying the house.

The damp contractor has suggested stripping off plaster to 1m, injecting a chemical barrier & replastering.

I have just found out that cavity walls were filled with insulation about 15 years ago. The damp proof course is only 1 course above a level (ish) concrete drive along the side of the house. On closer inspection the there has been repointing (some ok some crumbling) which for the most part actually covers the slate in the dpc.

Is covering of the slate with mortar likely to be the cause of the damp, or will it be a combination of factors?

What are best options for remedying the damp without spending thousands on the contractor’s suggestion.

Would it help to repoint the dpc so the slate is exposed, and/or is it possible to inject or apply a treatment to the course?

Is it possible to inject a chemical barrier into just the outer skin, or will this cause a problem with trapped damp etc elsewhere.
Or is it possible to inject both brick skins without removing the plaster?

Once the purchase completes I’ll be able to lift the flooring to check the timbers for rot.

Any suggestion or pointers gratefully received,


Gordon B
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Take no notice of either of these sharks. Ventilation? was it shut up for some time? Have you noticed the damp? how did they determine it was damp?

If you think rising damp is real, go and have a look at one of the canal bridges, made of brick, with some of the bricks standing in the water.

How high does the water rise up the bricks?
The contractor used a resistance meter to identify the damp.
It's possible it's condensation, the house has been empty for sometime, but I thought this unlikely as the walls have cavity wall insulation.
Maybe it's penetrating damp - I don't know.

I certainly don't want to spend out thousands on unnecessary chemical injections and replacement of plaster.
Anyone any ideas?

Penetrating damp can be caused by the outside ground being higher than the internal floor level. Something as simple as digging soil away from the wall may help in the long run.

If you do need a new DPC and decide to go ahead with the recommendations you may be able to save about 1/3 off the bill by removing the original plaster yourself. It's not a fun job though! Damp treatment companies normally insist on replastering with their own plasterers to ensure the guarantee.

Many old houses must have some degree of damp. After living in the house and heating/venting it for a while you may decide it's not so bad. On the other hand it's a real pain to do the recommended treatment while you're trying to live there.
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Bloody hell, a surveyor AND a damp contractor finding rising damp during a house purchase?! Sounds like standard practice so far! Of course a damp contractor will "find" damp - he's got his summer holidays to pay for!!

What do the internal walls look like to you? Is there any evidence of damp, mildew, blown plaster, salt residues, or any other visual characteristic to suggest that the "findings" of the surveyor and contractor are legitimate?

If I were you I'd wait until you move in there, get some ventilation into the place, check the floorspace and timbers (may be the cavity wall insulation is blocking some airbricks) and have a good look at the condition of the walls. You can post pictures on this forum.

Please don't waste £'s having chemicals injected into the walls, it really is a waste of time and money, and even if they fob you off with stories about it solving rising damp it certainly won't fix penetrating damp (I know, I've had it done!).
Thanks for the replies,
There is some crumbling plaster in and around the bay, but most of the side wall seems to be so sound. From memory there is only a small patch of mildew in the bay area. and no other signs (salt residues etc)
I'll have to wait until I exchange before I can get access to check out in more detail & maybe check out the cavity insulation is not blocking the air bricks.

You were right -it's not rising damp......

Have now been able to look under one small area of floorboards and found out:

Whilst the outer skin slate dpm is 1 course above the outside ground level (concrete), the floor board level is one course below the outside ground level.
See diagram

The inner skin has a slate dpm 1 level below the floor boards. That is, there are separate slate dpm’s for the outer & inner skin – with the inner slate course 3 brick courses below the outer slate course, and 2 below the outside ground level.

Both slates jut out a bit into the cavity.

There is, in the 6inch stretch I had access, to a fair amount of rubble & finer material bridging the cavity, below and above the inner slate and to a lesser extent above the outer skin slate course.

Does anyone have any tips for clearing out cavities in these circumstances:

• The length of this wall to be cleared is over 8m long. The outside is all concreted, and is 2 courses above the inside slate level – so no easy access to clearing out at the lower levels from the outside.
• I can only get easy access to the floorboards for the 1st metre of the 8m run of wall. The rest are under the stairs, or below built in cupboards, and floor tiling and fittings in a downstairs shower room.

The floorboard and joists seem to be in ok condition as they are not tied into the wall.

Thanks to everyone who’s helped.


Not sure how you saw into the cavity by looking under the floorboards...

To answer the question if you are serious about removing bridging substances, you have to remove bricks (from the inside for the sake of vanity) and drag the stuff along.

ie. start at one side of the bay and remove one brick for every four and use wire or bits of metal etc. to drag the rubbish along and out. Then replace the bricks.
Got access to cavity through a hole just under the the slate on the inner skin - see photos (if it works this time) that show the slate dpc below the floorboards and some of the rubble (light brown clour) I dragged out from the hole. The slate can be seen running along at 1 course below the floorboard (and above the hole) in the photos.

My question is has any one got any tips for scraping out of ortherwise removing the bridging substances from along an 8m stretch; given that:

-The outside is all concreted, and is 2 courses above the inside slate level – so no easy access to clearing out at the lower levels from the outside.
-I can only get easy access to the floorboards for the 1st metre of the 8m run of wall. The rest are under the stairs, or below built in cupboards, and floor tiling and fittings in a downstairs shower room.

Thanks for the help to date



The dpc should be a minimum 150mm above ground level. You may also have motar bridging in the cavity by the careless bricklayer.


If you can't then you may have to do this

It is possible, but not easy, to get a lot out with a powerful aquavac or similar. In an older house I managed it by poking the flexible ?35mm? hole through the hole, letting it bend round and along the cavity, and stabbing at the rubbish with it.

If the rubbish had been dry it would have been a lot easier, but it was damp and had solidified so I had to loosen it by jabbing. Sometimes a large bit broke off and blocked the hose.

If your cavity is already wet, and you don't mind making it even wetter for a short time, perhaps you could blast it out with a strong water jet. you would need bricks removed at low level for the stuff to gush out like mud and I imagine need to leave floorboards up for good ventilation so it dried out fast.

The more bricks you can remove, the easier it will all be.

If you can't get at much of the internal wall you might consider taking out some bricks from the outer leaf and recognising that you will have to make good neatly afterwards.

I agree with Masona that you should really take up the concrete outside and lower the level. That will also make it easier for you to remove bricks and rake out the cavity. I wonder where your airbricks are, with that outside concrete so high? Lack of ventilation will make the dampness MUCH worse and it looks like you have a newish joist, so maybe the floor has already suffered from rot.

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