I was thinking maybe a bracket mid span on top. The bottom is not possible as shelf will be running along top of window opening. Might need to look at some planed 50mm timber. My Corbels are 300mm high x 200mm projection x 94mm wide. The Sagulator says 25mm MDF on that span and depth ACCEPTABLEAs the corbels will sit either side, there obviously isn't an chance of getting one (at least) in the middle, so MDF is completely out, and even 38mm might deflect on that width. It's possible that you could fit an angle bracket on the top side of the shelf, rather than the bottom,
The Sagulator says 25mm MDF on that span and depth ACCEPTABLE
Oh yeah! OK back to the drawing board lol Although shelf load will be negligible just a few decorative items (glass)But you've typed in 25 centimetres for the thickness, change it to 2.5 and try again.
like some decorative end piece, kitchen cornice or similar good idea as i have a length of cornice left overIf you put a 2x1 on the front edge ( above or below) then you can use thinner shelf.
Probably to put it along the back edge like a sort of upstand. That would add rigidity as well as giving the OP something to fix through into the wall behindIf you put a 2x1 on the front edge ( above or below) then you can use thinner shelf.
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