Internal window boards/sills

[Draff]

Couple of questions here:

When is the recommended time to do this - before or after plastering?


(Hopefully this next question makes sense ) We have PVC windows fitted and I need to build up the inner wall to the window level by first putting in a wooden shutter and then seal in the DPC course with a cement mix:

- what ratio of sand cement mix is needed?
- how far should the mix come up to on window frame?

Chippy has gone on holidays for 3 weeks and I have some time on my hands this week...

 

[Carpenter]

Window boards, door frames and the like should always go in before plastering starts as plasterers use these as reference points in establishing coat depth and trueness in the finish. A 3: 1 sand/ cement mix will be more than adequate although you should be using some aggregate in the mix, which then makes this concrete. Bring the conc. fill up the the level of the underside of the frame (sill level) only- any more and you won't be able to remove the window in the future should a replacement ever be required. Don't forget to include some rigid insulation behind the DPC (on the room side) to minimise any cold bridging.

 

[Draff]

Thanks Carpenter...

-the windows are sitting on granite sills and the granite sill suface is farely rugged. There are few small gaps when the window is sitting on the sill. The people fitting the windows filled any gaps between window and sill with silicon. I'd still like to cover the base of the window by a couple of mm with sand/cement mix to ensure all gaps are sealed. Aside from problems if we replace the window, will this cause any other problems?

-all outter facing walls will be slabbed with Kingspan. This will cover the rigid insulation point you made. Correct?

 

[Carpenter]

Thanks Carpenter...

-the windows are sitting on granite sills and the granite sill suface is farely rugged. There are few small gaps when the window is sitting on the sill. The people fitting the windows filled any gaps between window and sill with silicon. I'd still like to cover the base of the window by a couple of mm with sand/cement mix to ensure all gaps are sealed. Aside from problems if we replace the window, will this cause any other problems?

-all outter facing walls will be slabbed with Kingspan. This will cover the rigid insulation point you made. Correct?

By covering the frame you are going to lose the depth of the frame when you come to adding your window board- it doesn't look great and can actually look mean (like the frame was the wrong size/ someone made a mistake). If you are still unsure about it you can run more sealant around the frame and sill before fitting the boards- even if you were to concrete up over the frame this will shrink on drying and you will still have a hairline gap. When fitting window boards I would always caulk the joint of the board with the frame anyway, before decoration. It serves a dual function: eliminates a dirty gap which inevitable opens up as the timber boards dries and gives a further defence against draughts and air leakage.
I would still put some rigid insulation immediately behind the DPC of the sill, 20mm of polystyrene or Kingspan would be better again.