Crumbling sill - what to do?

[DoNotUseFullName]

I'm gutting the understairs WC and I removed the internal wood sill to find a build up of concrete underneath that's crumbling and will pull away with zero effort.

Not sure how to deal with it, should I just remove it and build it up with fresh concrete? It's roughly 720mm x 120mm x 100mm, would that need rebar?

Should I lay a couple of blocks instead? (I don't know why that wasn't done in the first place...)

I plan to dot & dab the wall, could I do the build up with that instead?

Thanks in advance!

 

[Wayners]

I'd knock off that loose masonry.
Cut thermalite blocks with hand saw to fit.
Cement on

 

[Ivor Windybottom]

Is there some sort of sill or lintel lower down? If so hack out the broken stuff until you get to it then build back up.

 

[DoNotUseFullName]

Thanks guys. As far as I can tell there's no lintel or anything below.

 

[noseall]

Thanks guys. As far as I can tell there's no lintel or anything below.

Remove the built-up mortar and replace the masonry beneath the jambs with a small piece of block mortared in. The remainder can be built up cosmetically.

 

[ree]

OP,
Your using blocks idea is the way to go but first clear out all perished material, & then consider what the finished surface level will be using eg. tile, window board etc. & work up to that as a benchmark.

 

[Ivor Windybottom]

As you're building up from nothing, it's a good opportunity to get some insulation in, e.g. a few layers of tile backer board or PIR sheet. Then the room will stay warmer and you won't get condensation on it, as often happens with bathroom window sills.

 

[DoNotUseFullName]

Thanks again for all the responses. Noted about the PIR.

I've removed the material that is there. I see that the concrete sill the window is sitting on in not actually flat. The void is about 100mm deep at the top but only about 65mm at the bottom.

I guess that makes things a little awkward? Probably why they filled it with concrete in the first place.