Thickness of Adhesive

[CHarr]

If I'm bonding 12.5mm plasterboard to brickwork using plasterboard adhesive, can someone tell me what thickness of adhesive I should allow for once the boards have been fixed (ie the typical finished depth of adhesive + plasterboard, measured from the brickwork)?

Thanks, in advance.

 

[PrenticeBoyofDerry]

It would depend on the flatness of the wall. But once the board was been fixed and pressed to wall, the dabs generally end up at about 20mm-30mm. but will vary depending flatness and level of wall fixing to.

 

[noseall]

The overall plasterboard + adhesive thickness is around 25mm when using 12.5mm boards. this would indicate an adhesive thickness of 12.5mm also.

This means that 25mm electric back boxes can be screwed to the wall without chasing.

If we are dabbing a wall with heating pipes, then it is wise to try and cover the pipes wholly with the plasterboard even if it means thickening the dabs or chasing the pipes in a bit.

 

[CHarr]

There's a big difference between 12.5mm & 30mm! Is this because of making an assumption about how flat the wall might (not) be, or based on 'more is better', or does that 20-30mm include the thickness of the board?

 

[PrenticeBoyofDerry]

There's a big difference between 12.5mm & 30mm! Is this because of making an assumption about how flat the wall might (not) be, or based on 'more is better', or does that 20-30mm include the thickness of the board?

It all depends on the surface you are fixing to, if the wall strays out 10mm, 15mm, 20mm, 30mm.......... and you want the board to be level once fitted, then obviously that variation should be taken in to account.
IF you had a good flat, even wall, with very little variation, then it is possible to get small dabs and the the boards closer to the wall. As noseall has pointed out you often come across services and accessories for plumbing and electrics, which can also effect the size of the dab, you could of course chase/sink pipework, cable and backboxes within the wall to reduce this if needed(make yourself aware of chasing depths in relationship to part A of building regs).
A standard backbox for sockets is 25mm, but some are 35mm and some are 47mm. The depth of the backbox would often depend on the type of face plate being fitted and accommodation of cables.
Copper pipe will also require protection against the contact of lime.

 

[CHarr]

This is in a bathroom, and there is no wiring that needs to go behind the PB, nor plumbing, as I am constructing a floor-level void 100mm x 63mm on one of the walls using studwork, and will screw the PB to that on that wall. Apart from the toilet soil pipe (which goes straight out, through the wall), all the pipes will run there. I don't even need a backbox for a light switch, as it is located outside the bathroom.

I've finished hacking off the existing plaster today, and I find that the walls are very level and flat: I'll aim to get quite thin dabs of adhesive, I think.

 

[PrenticeBoyofDerry]

If you need an installation guide it maybe worth a browse of this:
http://www.british-gypsum.com/~/media/Files/British-Gypsum/SITE-BOOK/SITE-BOOK-Full-Publication.pdf
Section Six, Starting at page 240, will give you some basic instruction and advice on this type of system.

 

[noseall]

If I were to measure the overall thickness of dab + 12.5mm board, on a thousand extensions and on walls with no pipe/cable interference, then 25mm would come up the most.

Trust me.

 

[PrenticeBoyofDerry]

I think the measurement of 25mm given by noseall, in general is a fair one, give or take a couple of mm.
If room space is the issue here, also take in to consideration the product being used to finish the walls off with.
*As plaster finish will add approx. 3mm to depth
*Tiling dependant of tile depth and adhesive screed, could add on 7mm-20mm
*If the boards are tape jointed to taper edged boards then there will be no additional space taking up.

It may not be a big issue, but I have come across situations where it has been important, to accommodate appliances and furniture.