I live in the Pacific Northwest and we've had a very wet winter. I have a home-built 42 inch oven with a barrel vault arch and a 12 inch landing. I suspect that the landing is absorbing the rain and it's trickling down under the oven floor. It takes a long time to get the oven hot and when the dome is hot, the floor is still 250-300 degrees and doesn't bake well. I never sealed the exposed fire brick on the landing. What product should I use and also should I grout the spaces between bricks to keep water out?

thanks for any advice!