Replacement window cill

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Hi,

I want to replace a window cill in a curved bay window as part of remedial work I'm doing to deal with blown plaster and improving the u-value of the bay wall which is solid brick.

See thread in Building forum

My plan is to sand & cement render the wall and then affix insulated plasterboard which is then skimmed.

There will be a lip with the original cill as the finish wall will come out further than previous.

The original cill looks to have been bedded directly onto mortar (with no dpc) which likely explains the cracks in the top surface, possibly from moisture contacting the wood.

My plan is to replace the cill and place a roll of dpc beneath the replacement and have the new cill be deep enough to overhang the new wall finish.

Things I want to know:
1 - Suitable material for the replacement cill. Softwood or MDF? Are there other materials I should consider?
2 - What sort of thickness should I be using, 18mm?
3 - Best way to secure the new cill: A bed of mortar with dpc? Mechanical fixings? A combination? Gripfill?
4 - I'd like a bullnose finish but don't currently own a router. Can anyone recommend a router to purchase for such a task?

Many thanks
 
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MR-MDF in 22 or 25mm can be purchased ready machined with a bull nose edge in a variety of widths from timber merchants (NOT DIY sheds, please note). This assumes that you are OK with a segmented window board (which are fairly common) - do you have a circular saw and guide rail (home made or otherwise) to make thecangled end cuts?

If you want a true curve that requires a very long trammel arm with a 1/2in router fixed on the end and is quite a complex operation - especially for a router novice. You still need to start with segments which are end jointed (ideally with biscuits and glue)

If you want to rout your own edge profiles it is a little bit more complicated than getting a router with a follow bearing 1/4 round cutter because that won't generate a true bullnose (you need to leave about an 8 to 10mm flat in the middle of the edge withbthat type of cutter so the bearing can run against a flat surface). The topic has been discussed on this forum several times.

Personally, I think a biscuit jointer (to joint the segments) might be more useful tool than a router for this task

You can bed the sill on mortar or packers then drill and counterbore (and if needs be SDS and plug) then screw down. Fill the screw holes withb2-pack wood filler and machine sand (2-pack doesn't shrink but can't be hand sanded). A DPM will be needed beneath the packers or mortar
 
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I put "window board" on measure sheets when installing them, however they are commonly referred to as a window sill by the general population. Same sort of thing as the use of the term "staircase" or "noggin" IMHO
 
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MR-MDF in 22 or 25mm can be purchased ready machined with a bull nose edge in a variety of widths from timber merchants (NOT DIY sheds, please note). This assumes that you are OK with a segmented window board (which are fairly common) - do you have a circular saw and guide rail (home made or otherwise) to make thecangled end cuts?

If you want a true curve that requires a very long trammel arm with a 1/2in router fixed on the end and is quite a complex operation - especially for a router novice. You still need to start with segments which are end jointed (ideally with biscuits and glue)

If you want to rout your own edge profiles it is a little bit more complicated than getting a router with a follow bearing 1/4 round cutter because that won't generate a true bullnose (you need to leave about an 8 to 10mm flat in the middle of the edge withbthat type of cutter so the bearing can run against a flat surface). The topic has been discussed on this forum several times.

Personally, I think a biscuit jointer (to joint the segments) might be more useful tool than a router for this task

You can bed the sill on mortar or packers then drill and counterbore (and if needs be SDS and plug) then screw down. Fill the screw holes withb2-pack wood filler and machine sand (2-pack doesn't shrink but can't be hand sanded). A DPM will be needed beneath the packers or mortar
Definitely not up to a true curved window board!

I’m still not fully decided on proceeding with the insulated plasterboard which is what will drive the need to replace the window board.

If I get a waterproof render plus a skim I could refit the existing board with a dpc added.
 

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