fixing an internal window ledge (cill)

[jim]

Rejuvenating a window & the final puzzle is how to mount the new window ledge.

The ledge is timber with a tenon which fits into a mortice slot in the timber window frame.

But when slotted in there is a 6 to 12mm gap underneath the ledge to the underlying block & brick work (which spans across the cavity & butts up to a dpc against the outer leaf.)

Q: How is this void best filled?

TIA

[R]

Layer of plaster or light "render" and fit board onto, dress off excess.

[Roger Mills]

Build up the level with bonding plaster. When that has dried, fix the window board to it using Gripfill.