Window Board - and lead

[EdTurtle]

Hi, I have a block wall (thermalite) upstairs, the bottom half is plasterboard and the top is float and set, then the whole wall is skimmed. Not sure why they did it this way, its in a dormer - the bottom half backs on to the downstairs loft, and the top backs onto the outside.

I am in the process of plastering the whole room, and this particular wall has always cracked on the join between board and float/set. This is because the plasterboard isn't particularly well attached to the wall behind - i can count about four nails just banged through into the blockwork. I think to attach it better (don't want to remove and start again - I'm not 100% that its block all the way down - it could be part stud wall below) i could drill a hole - bang a plug through the wall into the blockwork and then fix with a 50mm screw. It looks like the gap behind the board is about 15mm. I would then couple this with expanding foam in the gap as well. Does this sound ok?

Second part, is how to attach a window board?! There is a gap of around 15mm, then plasterboard - doesn't give me anything to attach the window board to? I had considered foaming it to the plasterboard when I'm filling the gap as above. There is lead there under the window sill, which was folded down when the old window board came out, if i fold it down again how would i get the foam down the back of the board? And if fold it down again after I've foamed, then the window board would sit on the top of the lead - no problem, except i couldn't then foam the board down could i?

Any ideas please? I've included three pictures, showing the window, then lead (which i folded up to see behind), and a closeup of behind the plasterboard. Hopefully I've explained the problem - it is quite hard to explain though.

Thanks

 

[DAZB]

Drilling a few holes big enough to apply some Insta Stik or Mega Stik type foam will help bond the board to the wall and also secure with plugs and screws so it doesn't push it away from the blocks. If you have a 15mm gap then use a 25mm X 15mm timber to slip in to the gap which can either be stuck with adhesive or screwed through the board with 32mm drywall screws and then it also gives you a bearing for a small stub cill.

 

[EdTurtle]

Drilling a few holes big enough to apply some Insta Stik or Mega Stik type foam will help bond the board to the wall and also secure with plugs and screws so it doesn't push it away from the blocks. If you have a 15mm gap then use a 25mm X 15mm timber to slip in to the gap which can either be stuck with adhesive or screwed through the board with 32mm drywall screws and then it also gives you a bearing for a small stub cill.

Thanks for that, makes sense about the foam pushing the board out, i hadn't considered that. So, what about the lead, should it be folded up, or down?

Just been up to check, its proably more like 10mm at the most, and it does reduce to probably 5mm in places - it varies all over the place too- not sure i can get a slip of wood in there.... would some wooden tapers work, cut to match the actual size of the gap at that point? Would foam sprayed around the wedges/tapers hold it in place?

Thanks

 

[DAZB]

I would fold the lead up then it would sit upright behind the cill and any wind driven rain cannot get over and into the room should your sealant / trowel mastic fail. The foam mentioned will work as it is designed to stick full sheets of plasterboard to walls so the tapers will be fine with it.

 

[EdTurtle]

Great thanks - although it will make it a bit more tricky to set the board in, as the lead will push the board out towards the front of the wall.

Thanks

 

[EdTurtle]

Another quick question, once i've screwed the board to the wall, so its firm. Would it be a problem to squeeze some mortar down the back of the board? Reason being is the top of the wall will be sand and cement rendered, so it would be nice to have the render go down the back a bit as well, so the join between plasterboard and render is behind the board. It will adhere to the vlockwork, but will it adhere to the back of the board? I will still drill some holes and get some foam in as well, which will stop the cement render just falling behind the board?

Thanks

 

[DAZB]

It will stick to the blocks but not so well to the plasterboard and would be more likely to crack under any movement. I would fill with foam carefully and trim off any excess flush with the top so your cill will sit flat on top.