Basement windows in masonry walls

dave11 said:

I'm about to replace the original basement windows in a house built in 1951. They are below grade with window wells, which will also need to be replaced. The replacement windows I bought fit perfectly for width, but need 1.25 inches added in height to fit the opening, which is concrete block on both sides and below, with a 2x10 sole plate above.

The current windows are wood-frame hoppers, that sit on a mortar bed placed atop the blocks, and sloping away on both sides. The mortar outside the window has cracked and broken away, exposing the open tops of the blocks to water and air outside.

Obviously the new base for the windows will need to seal the openings on the blocks, which was once done by the mortar. Is P.T. lumber the only way to go? And would plywood be preferable to solid stock?

What about a piece of composite decking used here?

Tapcons with flush heads to secure it? Then seal with silicone?

Thanks for any advice.

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