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Blocked Cavity Wall

Night Owl

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HNY everyone.


My damp woes are continuing in my late 1950s bungalow, however I seem to be finally making a little progress and need some advice please.


Main bedroom on front of house has a damp issue.  I got a damp survey done but all they seem to want to do is a chemical injection and membrane plus replaster etc.  Not happy with that so declined the offer (8K quote!!).  As I'm refurbishing the property I decided to remove the plaster and have a closer look.  Here's what I noticed in one of the particularly damp areas.




This is on the front outside wall which is of brick cavity construction.  The floor is solid concrete with a bitumin layer then parquet tiles on top.  Originally the plaster was put on below the parquet tile level which would have been the cause for a lot of the damp rising up the wall.  I've removed all that now.




The wall to the right in the picture above is internal however I think the damp may have been tracking from the front of the house along the channel I've now cleaned out.

Looking outside, there isn't any leaking drains, standing water, poor mortar joints etc so not sure where it is getting in.  Decided to check the cavity so removed a brick right on the damp patch.  As suspected, the cavity was blocked with old mortar.  Dug it out but found it extended up another brick course, so removed a second brick above which finally revealed some cavity insulation. 




Dug down a little and found a nice cavity below the slate DPC.  Temporarily stuffed it with a plastic bag whilst I cleaned up.  Still got mortar stuck between the brick course but will hack that out later. 


1. What I would like to know is what you would advise to get it all out along the wall. Would it be best to do a little at a time or support the wall somehow and remove longer runs of bricks at a time? 

2. As the problem seems to be across two brick courses, how easy will this be?

3. Would it be better to tackle this job from outside?

4. Would you advise a wall to floor seal or leave it open to breathe?


Thanks for any advice in advance.


Edited by Night Owl
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Thanks ProDave, will definetly do that, it took long enough getting to this stage.


I couldn't believe how much mortar was lying behind the bricks.  It wasn't loose and took a lot of chipping at awkward angles to get out.  Once out though it revealed a nice clear cavity down to foundations and showed up insulation above.

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Id agree with Dave. You need to wait and see what happens. 


On the basis of what you have shown, id expect to dry up pretty quick. 


But always a possibility its something else.

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Thanks for the reply.  I'm going to leave it for a bit to see if that bit dries however, if the problem is all along the walls then it could be tracking from anywhere.  I might have to remove a few more bricks around the room where it's damp at the floor/wall join to see how widespread the problem is.

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Had another check today to see if things were drying out in the area of the cavity that I cleared.  I'm not sure if it has made any difference - possibly the higher damp area has dried post brick removal.




Interestingly, the brick dust that was produced when drilling out the lower brick is now very damp where it has been lying infront of the opening - it was dry during drilling.




I'm wondering if the damp I'm looking at is being caused by condensation?  There is no heating in the property and RH is always above 90% - today it was around 7 degrees and 93%, pretty much the same as outside.




Also had a look inside the cavity with a camera and will post a couple of pics shortly.  The problem is not just isolated to this area and is in other rooms on both internal and external walls - although only appearing damp in the channel where wall meets floor.  How easy can water track along a slate dpc if it manages to get in? 

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Inside the cavity looking down from DPC (i.e below the slate) it seems to be pretty clear.




Although there was a great lump of brick in there




Above the slate line a different story.  Its only a 50mm cavity and the slate sticks out into the cavity from inner and outer leaves leaving just enough room to fit my fingers through - this is why the fallen mortar hasn't dropped right to the foundations and has built up on the slate




Completely clogged above the DPC.




I can't put a dehumidifier in the room to try and eliminate the condensation theory as there are so many gaps around windows and only a partial ceiling at the moment.


Any input most welcome.

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It's been pretty dry for the last three or four days.  Not much difference noted inside but suspect it is too cold to properly start drying out.  Looked outside today and it is interesting to note that the ground has dried up apart from an inch or so around the base of the wall.  I presume ground water must still be finding a way up the wall from the foundations below ground.




Been looking at ACO drains and French drains as a possible solution to help with the situation possibly connecting up to a new soakaway.  The whole front of the house is concrete though so will be a pain to excavate. 

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At what level is the damp proof course in the inner leaf relative to the bitumen on the floor slab (this would have been used to stick the parquet down as well as give some damp proofing to the slab). If these are at different levels and not linked could the damp be tracking via the floor into the wall? 

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Hi, thanks for the replies.


The property has been in this state for about 8 months now.  Had windows open throughout the house every day over the warmer months, but only got the plaster removed over the last three.  Lots of draughts throughout due to old windows so there is airflow.  May set up a fan in this room to see if it helps.


kandgmitchell, the dpc (slate) on the inner leaf is practically level with the bitumen on the floor slab, directly below the brick I've removed.  Water could well be tracking under the slab to the edges.  Plaster was installed below the dpc/bitumen on the walls which I'm in the process of removing and cleaning out the wall/floor channel.




Started stripping out the living room today which has exactly the same problem.  I know there is water under the property as I have an old boiler room in a very small 'half' cellar that has filled up by about 3 inches of water over recent months during all the rain.  I plan to dry this out later in the year, tank it and fill it in up to floor level.  Where the water is coming from is unclear, I know the water table is quite high but to my knowledge, the property has never flooded.

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  • 1 month later...

Left this wall for a few weeks however it doesn't appear to be drying out as I'd hoped - the cold weather not helping.  Decided to continue cleaning out the cavity, it was pretty blocked up!




All cleaned up as best I can now for this section.


image.png.c139d42b046a86d6f83e88091a51bb9d.png  image.png.25986083241ba9e5b8aed701684910ef.png


Will brick up the opening now and leave to see if it makes any difference. 


Should I replace the wall insulation removed behind the bricks I took out?  It was the white wooly type stuff that had been installed.  Will not replacing it cause a cold spot at the lower section of the wall or will no insulation mean it will be better ventilated? 


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Moved onto the bay window section today.  Floor sounded rather hollow in the front part of the bay and started to crumble when hit with a hammer.  Totally gone, so dug it all out.




The 'cavity' between the main floor slab and inner wall of the bay had been deliberately filled with rubble. A thin slab had been laid over the top.  No damp proofing on this part of the floor.  The main slab has bitumen however there was some damp between this and the wooden floor - possibly due to a water leak last year.


Any suggestions for damp proofing this area and building up the new part of the floor?  

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