Concrete Windowsill Sizes in 9inch hollow blocks

[RDWRER]

Does anyone have experience in fitting precast concrete sills in to old walls built from single block. In this case it is 9 inch hollow block (cavity block), but similar in principle to solid block I guess. This is a retrofit window and cills will need to match existing which overhang perhaps 60-75mm outside.

All local merchants seem to sell 250mm deep concrete window cills. They seem too deep and i'm wondering if they're only suitable for modern cavity wall construction? I've seen some 210/215mm cills supplied by specialist precast concrete guys - is that more like what i need?

Example of what is it should probably look like in my head, but can't find anything authoritative on cill sizes and what would have been used on original building.

Cheers

 

[noseall]

Does anyone have experience in fitting precast concrete sills in to old walls built from single block. In this case it is 9 inch hollow block (cavity block), but similar in principle to solid block I guess. This is a retrofit window and cills will need to match existing which overhang perhaps 60-75mm outside.

All local merchants seem to sell 250mm deep concrete window cills. They seem too deep and i'm wondering if they're only suitable for modern cavity wall construction? I've seen some 210/215mm cills supplied by specialist precast concrete guys - is that more like what i need?

Example of what is it should probably look like in my head, but can't find anything authoritative on cill sizes and what would have been used on original building.
View attachment 368929
Cheers

Your maths doesn't seem to add up.

You say you have 9" (228mm) hollow blocks, which actually 215mm in real life. Then you say the existing sills over hang the masonry by say 60mm+

So, 215m (wall thickness) + 60mm (sill overhang) = 275mm overall. Then you say a 250mm sill will be too deep? Eh?

You also confusingly ask if a 215mm sill will be ok, affording you no overhang on a 215mm thick wall at all.

Confused.com

 

[RDWRER]

I'm assuming the window sits right on the back of the concrete sill on the upstand. I'd need say 50-75mm internal reveals, so in my head the cill is sat in the middle of the block with some infil behind it internally.

Above image just for reference for a windowsill in 215 block

 

[noseall]

I'm assuming the window sits right on the back of the concrete sill on the upstand. I'd need say 50-75mm internal reveals, so in my head the cill is sat in the middle of the block with some infil behind it internally.

View attachment 368983

If you are detailing the windows and sill, so that you can accommodate insulation and thermal breaks etc, then you need to illustrate your design strategy prior to deciding which width sill works bets for you.

The window jamb itself will be best positioned so that it has cold one side and warm the other, with the jamb itself located over that cold/warm join - or as near as damn it.