Building up kitchen shelf

Hi,

I’m redoing the tiles in my kitchen. After taking them off from the kitchen window it’s apparent that they were simply stuck to some plasterboard which was resting on the window surround. I need to build it up about an inch before I lay the new tiles.

What is the proper way to do this?

Thanks

The skim bead is already rusting, and there are gaps in the p/b fitting to the frame.
It would be best for you to remove the bead & p/b.
It would also be a better job to build up the sill board with two layers of backer board not p/b.
The top board should be sloped forward just a touch so that water will be shed onto the sink.
If you can set the bb sill back a little you could use a ripping of bb as a front dropped edge and bring the tiles over the corner and down.

Great, thanks