I'm in an older house with fibrolite cladding and alu window frames fitted into the fribolite, with wooden surrounds to seal the edges, for which prolonged water exposure in one has partially rotted the bottom strip of wood, see the photos below. The alu window is installed over the top of the fibrolite, so the whole lot seems reasonably waterproof (the flashing at the top and the wood surround down the sides looks fine), it's only that lower piece of wood that's rotted, presumably from water running off the alu frame and then sitting on the wood for awhile. It's nailed onto the fibrolite, not onto wood, so it looks like only this one strip of wood has rotted, not any framing timber behind the fibrolite:

 

 

 

 

 

 

The obvious solution here is to replace it with another strip of wood (the existing one is nearly forty years old so it's held up pretty well), however I'm wondering if I should do anything extra to deal with water ingress at the bottom of the window frame. I was going to spray some penetrating fungicide up underneath the alu frame to deal with anything that may have got in there and then replace the wood strip, but since I've got the thing opened up I could also do other things. I'm not certain whether (say) sealing it up tight will mean moisture is kept out or kept in.

 

 

At the moment I'm considering using a thinner piece of wood so I can run it up under the alu frame rather than having it flush, which is where it looks like the water is getting in now, or running a bead of sealant along the top edge of the wood before I fit it to form a watertight seal between the window and the top of the wood.