Party wall DPC and DPM on a hill

[mattchan]

I have had an extension built that has turned a piece of party wall that used to be outside into the inside wall of my kitchen.

 

There seems to be a slightly strange construction to the wall which results in the DPC on my side being bridged by some flinty infill on the neighbours side.

 

I was wondering if i should inject a DPC on the green line in the diagram as shown below and then run the DPM up the wall past this to avoid the bridge?

 

 

Here is a cross section from an exploratory hole cut in the wall. Green is neighbours inner skin and 70mm cavity. Red is the slab butted to the outer wall with the thin DPM directly onto the flint.

[tonyshouse]

No point in injecting, need to tank over it all then I would physically cut in a dpc a tiny bit below the top of the tanking. I would use asphalt, poor alternative is bituthene.

[mattchan]

As in tank the entire wall right up to roof level? Would the DPC then just be there to stop the damp getting to the roof structure?

[mattchan]

@tonyshouse sorry to nudge you on this but I wonder if you would mind clarifying exactly how far up the tanking should go?

[Russell griffiths]

What are you considering regarding insulation, you have a massive cold area against there floor and ground level. 

I would look at your new construction like a semi basement. 

I would want a waterproof system up to 600 above their floor level, then I would add insulation to the exposed area of wall up to the ceiling and joining in with roof insulation. 

Then build a metal stud wall in front of that. 

You will loose 100-125 mm from the room size, but everything else will just be a bit of a poor IMHO. 

[tonyshouse]

I would take tanking up to floor level in the house next door 

[mattchan]

21 minutes ago, tonyshouse said:

I would take tanking up to floor level in the house next door 

 

Ah, so in my diagram above move the green proposed injected DPC up by 6 courses and the pink DPM extension a course above that?

 

 

2 hours ago, Russell griffiths said:

What are you considering regarding insulation, you have a massive cold area against there floor and ground level. 

I would look at your new construction like a semi basement. 

I would want a waterproof system up to 600 above their floor level, then I would add insulation to the exposed area of wall up to the ceiling and joining in with roof insulation. 

Then build a metal stud wall in front of that. 

You will loose 100-125 mm from the room size, but everything else will just be a bit of a poor IMHO. 

 

Thats my next concern really.... the extension sits inline with the existing house. The construction in the other rooms that are on to the party wall is just sand and cement float and set over the brick.

 

The BCO just wants 25mm horizontal batten applied to the wall with 25mm celotex between the battens then boarded over. I just wanted to sort out the DPM etc first.

[Ferdinand]

49 minutes ago, mattchan said:

 

Ah, so in my diagram above move the green proposed injected DPC up by 6 courses and the pink DPM extension a course above that?

 

 

 

Thats my next concern really.... the extension sits inline with the existing house. The construction in the other rooms that are on to the party wall is just sand and cement float and set over the brick.

 

The BCO just wants 25mm horizontal batten applied to the wall with 25mm celotex between the battens then boarded over. I just wanted to sort out the DPM etc first.

 

Listen to yourself not the BCO on the insulation, needless to say 🙂 .

[Gus Potter]

On 06/04/2022 at 00:48, mattchan said:

I have had an extension built that has turned a piece of party wall that used to be outside into the inside wall of my kitchen.

Just wondering.. did your neighbours extend first? Is the outside of the brick on the boundary? Do you know anything about the founds, do the the founds as existing encroach onto your land? Who owns the existing wall?

 

That open cavity.. is it ventilated or just as a result of bad workmanship? Are you adding any load to this "party wall" ?

 

To design this I would first look at how you can do something that does not interfere with the existing wall in any way. Thus you can't be blamed for anything later on. Avoid cutting in DPC's, chemical DPC's are a waste of time and also you need to drill holes in the wall.. you could be blamed for any cracking / the smell of the stuff permeating next door.. and so on. Sounds like you get on ok with your neighbours but a quick dilapidations survey may avoid any potential issues in the future.

 

Quickly.. I would explore this.

 

Clean the face of the existing brick. Apply SBR bond then when tacky a cement screed 10mm thick with waterproofer, then another the day after, then a cement based water proof polymer slurry.. timing is the key here as you want the renders and slurry to set but not cure fully. Take it up say 150 - 300 mm above the neighbours floor level as you show.

 

Now strap and line the walls but don't fix though the renders at the bottom.  Keep a gap between the timber and the masonry / screed of say 50mm. Make sure you isolate the floor slab from the wall using 50mm of vertical insulation, rather than what you often see as 25mm perimeter slab insulation. Keep your DPM on the inside of the perimeter insulation and once the tanking has dried out tape it to the slurry.. that is kidology as that is supposed to stay glued for 50 years but it won't.. looks good on the drawing though. The main thing though is that provided you keep a bit of heat in the wall all will work... the damp won't rise particularly if the neighbours extension is a fairly modern brick and the dew point / thus evaporation won't occur thus drawing further moisture up from below.

 

Maybe go for 30 - 40 mm of insulated plaster board / vapour barrier as a backup. Here you want to actually loose some heat to keep the party wall that little bit warmer. Doing so will mitigate condensation (the dew point) = rising damp. Compensate for this heat loss by improving insulation else where as this is easier to do and often more cost effective.

 

Does that sound like it may fit with your set up?

 

 

 

 

 

 

 

[Temp]

Don't neglect that "cavity tray above roof level". They are some work to fit but they stop water running down inside what was an exterior wall and coming through the brick into your new room.

 

Otherwise +1 to tanking the wall to above ground level then battens, insulation and plasterboard. 

 

[mattchan]

On 18/04/2022 at 23:49, Temp said:

Don't neglect that "cavity tray above roof level". They are some work to fit but they stop water running down inside what was an exterior wall and coming through the brick into your new room.

 

Otherwise +1 to tanking the wall to above ground level then battens, insulation and plasterboard. 

 

 That cavity tray has already been installed. The builders who did that part and laid the floor etc are long gone, sadly they didn't identify the issue above whilst doing the work.

[mattchan]

On 18/04/2022 at 21:32, Gus Potter said:

Just wondering.. did your neighbours extend first? Is the outside of the brick on the boundary? Do you know anything about the founds, do the the founds as existing encroach onto your land? Who owns the existing wall?

 

That open cavity.. is it ventilated or just as a result of bad workmanship? Are you adding any load to this "party wall" ?

 

To design this I would first look at how you can do something that does not interfere with the existing wall in any way. Thus you can't be blamed for anything later on. Avoid cutting in DPC's, chemical DPC's are a waste of time and also you need to drill holes in the wall.. you could be blamed for any cracking / the smell of the stuff permeating next door.. and so on. Sounds like you get on ok with your neighbours but a quick dilapidations survey may avoid any potential issues in the future.

 

Quickly.. I would explore this.

 

Clean the face of the existing brick. Apply SBR bond then when tacky a cement screed 10mm thick with waterproofer, then another the day after, then a cement based water proof polymer slurry.. timing is the key here as you want the renders and slurry to set but not cure fully. Take it up say 150 - 300 mm above the neighbours floor level as you show.

 

Now strap and line the walls but don't fix though the renders at the bottom.  Keep a gap between the timber and the masonry / screed of say 50mm. Make sure you isolate the floor slab from the wall using 50mm of vertical insulation, rather than what you often see as 25mm perimeter slab insulation. Keep your DPM on the inside of the perimeter insulation and once the tanking has dried out tape it to the slurry.. that is kidology as that is supposed to stay glued for 50 years but it won't.. looks good on the drawing though. The main thing though is that provided you keep a bit of heat in the wall all will work... the damp won't rise particularly if the neighbours extension is a fairly modern brick and the dew point / thus evaporation won't occur thus drawing further moisture up from below.

 

Maybe go for 30 - 40 mm of insulated plaster board / vapour barrier as a backup. Here you want to actually loose some heat to keep the party wall that little bit warmer. Doing so will mitigate condensation (the dew point) = rising damp. Compensate for this heat loss by improving insulation else where as this is easier to do and often more cost effective.

 

Does that sound like it may fit with your set up?

 

 

 

 

 

 

 

 

The neighbours haven't extended, the room the other side of the wall used to be their integral garage. From what I can find out from BCO records and the architect who did my plans and theirs, their leaf of the wall has been their since the houses were built and the only addition when the garage conversion was done was to build their floor level up.

 

I don't know the answer to if that cavity is a result of bad workmanship. I assume it is and they just werent too bothered in the 80s when it was built.

 

The wall is exactly in line with the party wall between us for the rest of the house. Foundations were exposed and inspected by BCO when the wall butting up to theirs was built by the builder. We have a party wall agreement in place.

 

As mentioned in my post just above the slab etc is already in and was completed by some long gone builders.

 

So is that effectively a 20mm waterproof render with a polymer waterproofing slurry over then tape the floor DPM to this?

 

Then go over this with an insulated plasterboard?

 

As a point of interest why don't the injectable DPC creams work?

Edited April 24, 2022 by mattchan

[mattchan]

Just for reference the cross section detail above was a quick drawing i did to try and explain the situation I'm in rather than something the builders / architect came up with before / whilst working.

 

Below is a zoomed out view of the inspection hole i showed in the first post. You can see the screed and DPM which is lapped up past the DPC shown in blue in my diagram above.

 

 

Other pics for reference so you dont need to scroll...

 

 

Edited April 24, 2022 by mattchan
Added more pics