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!

Chet T16

Sorry for pulling up an old thread but it fits my exact question. Our house is hollow 9" blocks and I'm taking out a sliding door and putting a window in its place (same width)

At the same time i'm removing the uninsulated battened plasterboard and fitting insulated plasterboard, probably 62.5mm

Theres 2 things i'm unsure of:

1. Where the window and sill should be placed. I know the window should be centered but where should the sill be in relation to this? The window has a 70mm frame and the sill upstand is about 25mm. The sill is 9" front to back. The skillfully drawn image is relatively to scale with the window centered on the wall and the back of the sill at the centre line. Should i move the sill back to clear any drain holes from the window? Or centre the upstand to the window?
2. Assuming window and sill in place, how best to finish the void behind the sill/under the frame? Do i need to cap the hollow block openings?

10-10-20

Hi, the window should normally have a drip bar just under the bottom edge. They are commonly from 15mm to 25mm deep. That bar typically laps over the sill's upstand and the sill and bar touch and the window is sealed along that with a bead of silicon. Is that clear enough to work out?

As regards the cavity blocks, you don't need to cap them off just now, just make sure that you run a DPC under and around the ends of the sill and up the back of the sill in one piece. Make sure that the drip void on the underside of the sill (if installed) is far enough out from the projected final wall finish. Then cap off the internal sill by using soap bars or similar to make-up the height required AND maintaining the DPM in position around the back of the sill.

Chet T16

Thank you.

I don't have the window yet, or drip bar dimensions but assuming its there then i'm moving the sill out slightly further than shown in my pic? And if no drip bar its not far off?

Noted on the DPC and soap bars. I'm also blocking up a window in the same room and this void was left behind the plasterboard and didn't seem right!

10-10-20

https://www.boards.ie/discussion/comment/123683292#Comment_123683292

This is roughly it here (taken from an example at Passive Sills. The upstand meets the drip. The weep-holes aren't affected as these should be higher up in the bottom section of the window frame.

https://passivesills.com/solid-block-construction/

RDWRER_

@Chet T16 your diagram is roughly how I did it after advice on this thread. Window placement pretty much how 101020 has it in the detail above too.

On a couple of small windows I just filled in behind the sill with some spare pir board I had lying around. Plenty strong enough to support a little windowboard. On others I used stock bricks on edge.

I just made sure to leave the gap at the back roughly the depth of a course of bricks on their edge (70mm) (or a soapbar if you're using that). This worked nicely with sitting the cill roughly halfway in the opening for the size of cill my merchants stocked.

Insulate board over it, window board, then skim. Looks great, best of luck with it

Chet T16

Great, I think i'm good to go so, I'll have the window early next week so I'll know exactly where I'm going when I see it.

Thank you both for your input, much appreciated!