Understanding our 1930s DPC

[cjhox]

Hi Everyone,

TLDR; How should a DPM in a new floor slab connect to the DPC in the walls? Does it seem like I should add a DPM to the walls based on the plastering we found?

I'm trying to get my head around how the (rising damp) moisture management in my house should work. It's a 1930s semi with cavity walls and two courses of engineering bricks as the damp proof course. As far as I can see, there is not a damp proof membrane within the wall (i.e. not also a layer of bitumen etc bedded on top of the engineering bricks).

We're doing some renovations at the moment and have all the old plaster off the walls, this was mostly the original crumbly lime-based stuff, however there have been isolated areas in the kitchen where a much tougher substance had been used on the bottom 0.5 meters. This seems to be original/near original to the house. To my untrained eyes, it looked like more of a cement than a plaster mix – some sort of tanking? Furthermore, in a few places on the ground floor, such as the dining room, the bottom 1 meter of the original plaster had been replaced by something much more modern, a pink-ish plaster mix, this is likely 10-20 years old based on what we know of the previous owners.

During the last couple of years in the property, before starting the works, we’ve not seen particular evidence of damp ourselves. The attached neighbors house has holes from chemical damp proof injection, though.

From here, we intend to dig out the old concrete slab (half the ground floor) to insulate and install under floor heating, and also re-plaster.

Question 1) For the slab, we’re obviously going to need a DPM, but I’m a little unsure of how it would get tied into the DPC of the walls. Is there an established method for this? (It’s doesn't look like there was a DPM for the existing slab, i guess the original floor tiling covered this function).

Question 2) For the re-plaster, we were assuming we’d batten-out, insulate and plasterboard, but I’m concerned that the odd plaster sections we found indicate some ad hoc remedial moisture management. We don't want to create a problem, so should I be considering adding a DPM above the engineering bricks (or chemical DPC) while everything is exposed, or some other plastering strategy instead?

Question 3) Out of curiosity, how do engineering brick courses not get compromised by being bridged by the mortar they’re laid with? I assume it’s a special mix?

Thanks in advance.

 

[noseall]

Question 1) For the slab, we’re obviously going to need a DPM, but I’m a little unsure of how it would get tied into the DPC of the walls. Is there an established method for this? (It’s doesn't look like there was a DPM for the existing slab, i guess the original floor tiling covered this function).

You cant lap it onto (already built) walls. Best you can do, is make sure that the polythene extends above the DPC.

Question 2) For the re-plaster, we were assuming we’d batten-out, insulate and plasterboard, but I’m concerned that the odd plaster sections we found indicate some ad hoc remedial moisture management. We don't want to create a problem, so should I be considering adding a DPM above the engineering bricks (or chemical DPC) while everything is exposed, or some other plastering strategy instead?

Cavity wall construction suggests that you should not need additional internal membrane.
Rather than fix battens to the wall, why don't you say foam fix 50mm PIR foil sandwich to the walls, then trap them back with 25mm x 50mm battens, then add another 25mm PIR between. Foil tape all joints on both layers.

Question 3) Out of curiosity, how do engineering brick courses not get compromised by being bridged by the mortar they’re laid with? I assume it’s a special mix?

Thanks in advance.

Are you sure there is no physical DPC present?

 

[tony1851]

@Nose, being in Staffiordshire you would know these bricks as Staffordshire Blues, some of the finest and hardest bricks made.
They are often used in two courses as a dpc (in hard 1:3 mortar) so there probably wouldn't need to be a rolled-out dpc as well?

 

[^woody^]

Yes lots of houses back in the day had solid blues as the DPC.

As years go by, fine cracks in the joints and changes in the ground water profile can mean the DPC gets overwhelmed.

If a floor DPM is being fitted, then that definitely will divert more water to the walls, and more precautions are needed to reduce the risk of rising dampness. Get advice on a system for the site specifics.