Damp proof membrane

[akist]

Some of the walls we are building are built on top of an existing concrete slab. The builders have started by placing a plastic sheet "membrane" for dump proofing on the slab and are placing blocks on it.

I am concerned that we are actually depositing blocks on a "rug" with no adhesive connection to the concrete floor below but only their weight to keep them in place.

If you wanted to build blocks off a concrete slab would you also put a membrane to start off with? Are you not meant to put mortar below the membrane so at least the membrane sticks to the floor below?

Sorry for asking basic questions, just curious.

 

[acurachris]

The builders have started by placing a plastic sheet "membrane" for dump proofing on the slab and are placing blocks on it.

What are you building? The membrane is a "damp proof membrane (DPM)". It sounds odd to me to build on top of the membrane unless it's a gas/radon barrier. Normally you would build the blocks off the slab and lap the membrane into the DPC course which would normally be mortared below and above.

I'm not a builder though so perhaps someone else will offer better advice.

 

[Notch7]

You might want to change your thread title.......

usually a layer of mortar is put down, then the damp proof course, then mortar and then blocks.

The damp proof course has a textured surface to lock into the mortar.

it sounds like your builders have laid a complete sheet of dpm then started laying on top of that. Im not sure if thats good practice or not -it allows for a continuous damp proof layer across the floor but introduces a slip layer under the blockwork does it introduce much weakness in a single skin wall.......maybe

Mind you thats pretty normal practice for the inner skin of a cavity wall, a dpc is laid and the dpm is lapped over it to link the damp layer. However the inner walls are also locked to the outer with brick ties.

I think I would have used dpc under the walls, then put down dpm in each separate room, returning up the walls and cutting off after screeding.

 

[^woody^]

DPM = no mortar
DPC = mortar

 

[akist]

1) There is an uninterrupted strip of membrane built from the inner skin to the outer skin (higher on the inner skin). There is mortar above and below this membrane in between courses, but was the membrane placed when both sides of the mortar were wet? Or was it placed over already dried mortar, eg a day's old? Who can tell.

2) There is another strip of membrane right on the concrete slab on top of which we are also building an inner skin with concrete blocks. There is no mortar under this membrane, it sits straight on the floor. The blocks are quite heavy.

3) There are wall ties everywhere.

From all the above I instinctively do not like is placing membrane over a surface without any bonding material, or placing it on top of already dried mortar.

 

[acurachris]

There is an uninterrupted strip of membrane built from the inner skin to the outer skin (higher on the inner skin).

It sounds like the membrane is being used as a cavity tray. I think a picture would help.

There are wall ties everywhere.

That's not a bad thing!

 

[^woody^]

From all the above I instinctively do not like is placing membrane over a surface without any bonding material, or placing it on top of already dried mortar.

Where is the building/floor going to go?

 

[akist]

No, it's a great thing. My worry is the adhesion between layers as we place a piece of plastic in between.

 

[akist]

Building has cavity walls. Outer skin is bricks on underground foundations. Inner skin is 50% concrete blocks on underground foundations. The other 50% of inner skin is concrete blocks on existing concrete slab. There will be 100mm Celotex over slab and then 65mm of concrete screed.

 

[^woody^]

But every single building built from the last 1800's onwards with a DPC of anything but bricks will be sliding on its DPC. Likewise for floors when DPMs became a thing in the 60's.