How to replace window sill above crumbly mortar?

[brokecamel]

Hi all,

I'm redoing my kitchen, which included removing an old tile windowsill. Beneath the tiles was about in inch worth of crumbling mortar. I used a villaboard made level with spacers to give me a flat surface to re-tile the backsplash, however I'm not sure what the best approach for replacing the window sill is given the condition of the surface.

In hindsight removing all of the mortar and lowering the windowsill down to the brick was likely the best option but I've already tiled and don't want to risk doing "demo work" near the new tiles.

I've bought some timber that perfectly fits in the width and depth of the window, and 19mm is also the perfect height from the mortar to be level to the existing window sill. Is just a generous amount of liquid nails the appropriate way to fix it to the mortar?

Thanks!

[JacobZ]

Hi @brokecamel,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Fixing anything to crumbling mortar is not going to be a good option long-term. Looking at the mortar, it looks to be fairly weak, so removing it really is your best option. If you are careful and mask off the glass and tiles using painter's tape and a decently thick drop cloth, then you should be able to remove it without causing any damage.

I'd suggest using a cold chisel and hammer to break up the mortar. Work outwards from the cracks, as they are already weakened, and you will likely find that it comes away relatively easily. Once you can get down to the brick level, angle your chisel at the bottom of the mortar to peel it away from the brick.

Once the mortar has been removed, I'd suggest using window packers to pack the sill to the required level and use this STAHL Masonry Kit 7mm Drill With 50mm Green Wall Plugs to fix it in place. Predrill the sill, then use the pilot holes to mark the locations for the plugs. Use the masonry bit from this kit to predrill holes for the plugs and tap them in place. Once the plugs are in place, set the window packers over the location of the plugs, as this will ensure the sill is supported underneath the screw, so it doesn't bow downwards when it is screwed in.

You can then use a countersink bit to create a countersink for the screw heads to sit in and install the sill.

Once in place, use some Earl's White MulTfill to cover the screws, allow it to dry, give it a sand then prime and paint the sill.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[brokecamel]

Wow thanks for the detailed reply Jacob. I figured that approach would be the way to go, though only in hindsight.

I might get a start this arvo and finish it off next weekend. I'll post an update how I go.

Cheers.