The next thing to do would be to have the windows replaced altogether. Or actually, to have a lot of the roof replaced. Leaky windows lead to rotten beams! But my priorities for now are to sort the house out, not the garage, so I decided on a temporary fix. I bought a piece of new perspex and intended to fix it to the inside or the window pane. Not ideal I know; that way the water will gather in the window pane, and rot the wood. But I'm not ready to go onto an old slate roof to repair it from the outside! And not to pay someone else to do it either. Yet. So I thought I'd screw the perspex to the inside, and file a gully in the wood, and insert a piece of hosepipe into the gully. The hosepipe would hopefully drain the window pane!
I found a dry Saturday to do it and it worked out as I had planned. Now I have to wait for the rain to come back (when I'm at home) and then I can see if my construction sort of works!
Fixed! Although the perspex is so clear it's hard to see on a picture
From the side
With drainage system