Build Cavity Wall below DPC

Hi everyone

I am about to have a go building my house extension, but have not have any building experience in building a wall before. I would appreciate for any responses to the following questions:

1. When building a cavity wall with engineering bricks below the DPC level, should both skins be constructed at the same time? I wonder, if it okay to build the inner skin first and concrete the floor slab. Then build the outer skin.


2. When building the inner skin of the cavity wall, below the DPC level, do you plumb the wall on the interior face or the exterior face (this is the face in the cavity)?


3. How long mortar joints normally take to dry, before they can stand up to rain water - after a heavy rain, we get quite a bit of water in the trench.

Many thanks!

DIYFun said:

1. When building a cavity wall with engineering bricks below the DPC level, should both skins be constructed at the same time? I wonder, if it okay to build the inner skin first and concrete the floor slab. Then build the outer skin.

Click to expand...

That's fine as long as you appreciate that it is the outer skin that determines the buildings' overall size which means you will be working to a rigid cavity size. Also bear in mind that the wall will need to withstand, compression from both the concrete and the compaction of the in-fill material. This may mean having to wait a while until the mortar has hardened considerably. If there is a tall body of masonry I would advise you build both skins, fill the cavity and then do your in-fill etc.

DIYFun said:

2. When building the inner skin of the cavity wall, below the DPC level, do you plumb the wall on the interior face or the exterior face (this is the face in the cavity)?

Click to expand...

Always on the face side and not the cavity side.

DIYFun said:

3. How long mortar joints normally take to dry, before they can stand up to rain water - after a heavy rain, we get quite a bit of water in the trench.

Click to expand...

In the fairer months the mortar will stand a downpour after a few hours. In the winter it can take all day to get any kind of resistance. Just drop a board over the top of the masonry to shelter it from the rain. Standing water will not harm the mortar it will only help strengthen it.