Supporting window frames with check reveals?

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Is it possible to fit UPVC window frames with UPVC exterior sills (No concrete prestressed sill) to a partial fill cavity wall with check reveals?

If so, What supports the frame with it sitting above the cavity void? I can understand with a stone sill, that it can be sat back over the cavity and the window bears down onto it.

Screenshot 2022-05-10 at 22.03.41.jpg
 
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My guess would be to stick in a cavity closer, then some steel straps across the inner and outer skin.

The problem with your proposal is you would need large upvc cill projections which would look rather ugly.

I’m not sure about how the cill end caps would fit as the cill would be cut around the window - you have to be careful as upvc cills are hollow and the water drains into them and discharges through holes behind the front external lip.


Upvc windows with face drainage, sat on creasing tiles or stone cill would look nicer
 
Having further looked, I don't think the check reveal will work out well for the sizes that UPVC sills are available in. The available sill sizes are, 125/150/180/225. 60mm of that sits under the frame & a 40mm over hang is required leaving too little or too much.

Because I'm considering using a fibre cement wall cladding on parts of the upper elevation, this attached to battens adds an extra 50mm, I'd have to have standard reveals with 225mm & 180mm sills for the cladded and none cladded areas. This would mean the window was set back by 75/80mm from the front wall. The cladding company recommends not using any stone window sills for some reason. A photo on their website doesn't look like theirs much of an overhang.

you have to be careful as upvc cills are hollow and the water drains into them and discharges through holes behind the front external lip.

Upvc windows with face drainage, sat on creasing tiles or stone cill would look nicer
I didn't realise they drained from the sills, thought it was always from the bottom of the frame onto the top of the sill.
With creasing tiles do you use a stubby sill?
Example of Cedral cladding with red brick house.jpg
 

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