Cutting an internal Wall of a cavity for UPVC door.

[Allex]

I have been given a measurement of 800mm to cut an opening in the outside leaf of the wall. What should the measurement be for cutting the inside leaf of the wall?

 

PS I think it should be 820mm (10mmeach side over). Thanks.

[DevilDamo]

That will be quite a narrow door. On the assumption it’s brick, can you not achieve 910mm on the outside? You’d do the same on the inside and then finished reveals with plaster or plasterboard.