Backing coat depth

by Keith
(Sussex)

Hello Paul,

I have Thermolite block walls to plaster and will be plastering up to window reveals that have the wooden cases already fitted. If I rule off these for the backing coat I presume that the skim coat is going to be proud.

Will a sand and cement render shrink back enough to allow a skim coat up to the wood or should I dot and screed a couple of mm lower than the window surround to give room for the skim coat?

Excellent website by the way and very good videos, I shelled out £10 for the online viewing and it is money well spent, thanks.

Answer

Hi Keith

It will not shrink back enough, so the render can be applied flush but you will need to either try and almost push it back by about 1mm when using the float or after floating when it has firmed up gently scrape it back around the edges. But don't dig in too deep - just 1mm will do the trick. It is a similar process as for angle beads.

Paul
PS I'm sure you know already but - Thermalite blocks can provide very high suction so control this either by damping the walls down with much water or a weak solution of PVA.