Concrete Window Sill in Cavity Block wall - uPVC window

RDWRER_

I need to fit a concrete sill in a 9inch cavity block wall, but can't work out the correct sill and where it should be placed in the wall to give me the right overhang and window position.

Local merchants all seem to stock 250mm wide sills, with 50mm wide upstands. Do I sit one of those half way across the wall, and put the window on the upstand then fill in behind the cill?

uPVC window supplier seems to specify 50mm drip on the bottom of the window, so does the window overhang the sill upstand slightly for this drip?

Thanks

10-10-20

Hi, I presume given the 9" blocks that it's a garage and not a livable area?

A 50mm drip on the window sound excessive, you can normally get them to modify it to 20mm. Some can be trimmed, some cannot!

The window position is normally center of the wall, so lay the sill so that the front of the upstand is roughly in line with the back side of the uPVC window's lower drip flange. The sill overhang should be around 7cms with the drip channel exposed - even after rendering (don't forget to include ~12mm for render). Move the sill back and forward to ensure that the drip is sufficient, then fit it securely. Don't forget the correct detailing of the DPM under and around the sill and bring the DPM up the back-side of the sill. The uPVC window is supposed to float in the frame of the window, never touching the reveals or lintel; make sure you have good packers and at least a 5mm gap all around to facilitate that. Concrete screws won't secure the windows in a cavity-block window, ensure that the supplier is providing frame straps (as some were not).

Hope it helps!

RDWRER_

That's a great help, thanks. Old property so garage and house both built of cavity block

Yes, I measured the 'drips' on my recently installed windows and they are more like 20-30mm, not sure why the invoice said 50mm… Come to think of it all of the windows have face drains on so I'm now realising the little piece under the window is probably not an actual drip but just a filler piece? It is flush with the frame and siloconed to the sill so there's no drip happening there!

Good point on fixings, the width should work out block gauge all being well.

My project is a second hand window i picked up, so there is no lower 'drip' or trim fitted, but again it has face drains, so hopefully just some trim will cover the gap down to the cill. I guess the flush trims are factory fitted as i can't find anything to buy to fit.

Cheers

10-10-20

The drip trims are just to stop water from tracking in and over the upstand of the sill, so make sure to seal the sill to window interface with a good clean bead of good quality silicone specifically for windows. I like backfilling the gap under the window and around the sides with Soudafoam SWS, but I'm not sure how high airtightness is on the list of priorities for you. Anyway, good luck with the work!