When setting backing boxes into a wall that is going to be plastered at what depth should they be set - I am assuming that as long as they are not proud of the expected finish level then they are ok ? Thanks Justin
They should be flush, ideally. Some brands of sockets and switches have a bit of a "lip" on them, so the sockets will still be flush even if the boxes are 2-3mm proud. More importantly, don't sink them too deep. You will leave a gap between the socket and the box and debris (bits of plaster etc.) may fall into the backbox. When chasing out your hole for the backbox, have a box handy for checking the depth every so often. Boxes for sockets are 25mm deep, and typically 16mm deep for lightswitches.
I would suggest using 25mm deep boxes for light switches and 35mm deep for sockets 16mm is usually ok for light switches unless you decide to use a dimmer and 25mm is usually ok for sockets unless you decide to change the socket to a flush plate type socket
Agreed, I was just trying to "keep it simple" for the purposes of the thread, those I quoted being the most common sizes, but it's a fair point.
i would think you probablly wan't to aim for about a mm or so below the finish level. a 16mm box is considered plaster depth so i would screw those direct to the surface to be plastered and recess other sizes of box to leave about 16mm sticking out. if you are using normal sockets/switches you can get away with them slightly proud but with flat plate units you won't. on the other hand you don't really want to leaev a big gap as that kind of defeats the perpose of a backbox. the normal way seems to be to plaster over the boxes (which are padded out with cardboard and/or tape) with the finishing coat and then to break away the plaster from over the boxes after its set.
telllondon Does this mean you cannot retro fit flat plate sockets in a house with 25mm back boxes unless you change all the boxes for 35mm ones.