Damaged Sill

[318Mike]

I had something like your window on our kitchen window sill.
I removed everything, cleaned it up then left it to dry out, then I covered it all with tile cement and fitted a porcelain floor tile on top, the type of tile that looks like a floor board.
A good layer of cement allowed me to set the tile at a angle that would shed water from the plant pots my wife insists on keeping there, she also chose the tile ) )-;

 

[Gilwern]

The window sill platform is very uneven and bumpy.

What's a way to even this out? The goal would be to create a smooth and even surface to place the sill on.

I have been playing around with plastic shims, but it feels very non-ideal.

 

[Gilwern]

I had something like your window on our kitchen window sill.
I removed everything, cleaned it up then left it to dry out, then I covered it all with tile cement and fitted a porcelain floor tile on top, the type of tile that looks like a floor board.
A good layer of cement allowed me to set the tile at a angle that would shed water from the plant pots my wife insists on keeping there, she also chose the tile ) )-;

If you look at my latest image, do you think your cement method could work?

I think the goal would be to create an even surface over the bumpy brick and plaster section.

 

[318Mike]

If you look at my latest image, do you think your cement method could work?

I think the goal would be to create an even surface over the bumpy brick and plaster section.

Yes, it would, my old sill was very similar to yours underneath.
I used a hammer chisel and chipped away all the plaster and cement bit by bit. Its not difficult, just take your time and nibble away (NOT one big hit) and always aim the chisel laterally, that is along the wall not towards the tiles or the window. Its a no rush job, just take your time.

I also made sure nothing loose was left behind under the frame or between the bricks etc by fiddling it all out with a bit of wire coat hanger and using an old Henry hoover to suck everything out.
I then filled all the gaps with cement, fine sand and cement mixed a a drop of washing up soap to make it smoother to work with. When that layer had dried fully the second layer was first in dobs and clecked to get the angle right, then I filled inbetween the dobs and smoothed it all out before laying the tile down. I have a dislike for window sills that when knocked have a hollow sound underneath!
My last job was to seal all the joints between the window and tiles with mastic.

Tips
Before you lay the cill use a spirt level to check the window is level, if it is not, it is best to lay the new cill at the same angle as the window or it will appear (to the eye) as odd, and whatever you do ensure the new cill does not slope back towards the window.

If you want a photo just ask,

 

[Gilwern]

If you want a photo just ask,

Thank you for the detailed explainer. A photo would be excellent if you have one. Your plan sounds feasible. Two more questions if you have a moment:

1. If I cock up the cement process, how difficult is it to remove the cement?

2. When you say 'in dobs' what do you actually mean?

 

[diy_fun_uk]

You're at risk of overthinking this job.

Get some packers in
Check for level, secure packers with CT1 adhesive or equivalent
Apply CT1 adhesive or equivalent to all touch points
Insert new sill, gently pressing onto adhesive and checking level
Leave to set

Job done.

 

[318Mike]

Unless the whole surface is filled with packers and glue I cannot see how voids can be avoided (sic).
Agree it would be quicker but......

M-in-Law had ants enter a room via a void under a sill. Initial entry was through an air vent.

 

[Gilwern]

You're at risk of overthinking this job.

Get some packers in
Check for level, secure packers with CT1 adhesive or equivalent
Apply CT1 adhesive or equivalent to all touch points
Insert new sill, gently pressing onto adhesive and checking level
Leave to set

Job done.

Appreciate the reality check. You're probably right — I think it's a case of diving in, trying something and seeing if it works. Worst case can always re-do it with lessons learned etc.

 

[diy_fun_uk]

Unless the whole surface is filled with packers and glue I cannot see how voids can be avoided (sic).
Agree it would be quicker but......

M-in-Law had ants enter a room via a void under a sill. Initial entry was through an air vent.

Millions upon millions of sills around the country have voids