Filler/packing material under replacement windowsills

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I’m in the process of fitting new, deeper windowsills in a bay so that shutters can be fitted on top. The old sills had been bedded on a very deep bed of silicone, approx 15mm deep. I need the replacement sill to be at the same height as the old sills as the shutters have been made to fit the height of the old sill. Unfortunately, I didn’t know how much silicone was under the sill, and i now need to pack up/fill under the new sills by around 15-20mm and fill the gap behind the plaster to give the sill something solid to sit on.

What should I use as the filler material? I’m thinking mortar with gripfill and screws to secure the sills and easifil to blend it into the plaster. Any better suggestions? I’m assuming window fitters come across this problem regularly.
 

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I would use timber, 18mm ply or softwood, packed with plastic shims to level up.

You haven’t got anything too clever to fix onto - ideally you need to screw the timber packer down where you can, along with grip fill to get a solid fixing.

if you can use timber, then the new windowboard can be screwed down to the packer.

whatever you do, the new windowboard needs secure fixing as it will have a larger overhang.

you could plaster with easifill, or fit a timber mould below the windowboard -that can look good actually, it makes the windowboard look thicker (note, ends need mitred returns)
 
Not much to attach the sill to, hence the previous silicone to glue it into place. I’ll just about get a screw into brick if I drill on the angle at the back edge of the sill.

The problem with using timber as a packer is nothing is level or flat. It’s a right mess underneath, hence my thought of fitting the sill on a mortar bed.
 
The problem with using timber as a packer is nothing is level or flat
There's no reason why you couldn't cut timber wedges, though, bedded onto Gripfil or the like. It doesn't need to be perfect, it won't be seen
 
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I would go with either plywood or build up the sill base with mortar.
Then attach the new sill using screws if the overhang is too long.
 
2 pack wood filler. Sets in 5 mins.

Then you can inject foam to seal off or use polymer mastic
 
I ended up putting wooden battens in to give me something decent to attach the sills to. The battens are only 30mm wide and the sills are 120mm so I was a bit concerned there would be enough supports. A combination of screws, gripfill, expanding foam and the fact that it’s a 5 section bay so I biscuit jointed each section of sill to the next and it’s actually ended up rock solid.

Thanks to all that offered advice.
 
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