My builder left a gap under the door sill outside. He's no longer available. So I have to fill it up. I've tried shims but the gap is uneven in many ways. It just doesn't work. I have been considering filling the gap with some mortar patch and then using white silicone caulk over that when it dries.

Winter is approaching so I have to figure this out. Whatever I use must be stiff so the sill won't bend. Hope that some can offer some advice.

Thanks.