how to fix ?

Patch it with aluminum strip suitable sheeting is in B & Q Its about same thickness as origianl roofing.
Don't bother with fibreglass it will open up due to constant movement.
From B & Q .
Materials: Aluminum sheet, Builders external gap sealant used for windows and door frames[ White or clear]. Self tap screws approx 3/4 to 1 inch long guage 5 or 6 & pop rivet gun and long rivets about 1/2 inch. for riveting into voids where screws won't grip. a twist bit for tight fit. Don't use drill bit bigger for ease of placement or the rivet will pull through. Water and rag.
Press down around split to find out where the wood frame is and mark positions as at those points you will have to screw the plate down and pop rivet in the voids.
1. Measure length of split add at least 4 inches for a
2 inch overlap each end of split.

2: Measure width add 4inches to allow 2 inch overlap on each side of split.

3: Don't cut the application nozzel on the mastic tube leave it so thin bead comes out and run a bead on flat area all around the split.
4. Coat the plates/strips with mastic smooth out with wet finger, then run a bead around plate approx 1/8th in a second bead right on the edge turn it over and lay it in position.press down from middle to outside edges to bed the plate down into the mastic.
5. Drill out pop rivet and or screw holes start at front work to back fixings every 3 inches along edge approx 1/4 inch in. Wipe off the surplus mastic oozing out smooth off edges with wet finger. Once edges sealed count job as done.
Email robertwhiting76(at)hotmail.com if you are still not sure how it's done and I'll send drawing.
Substitute the (at) and brackets for the normally used email symbol meaning same thing. If you haven't a rivet gun then try screws all round first and any that don't grip replace with rivets if needed.

Bob
June 2009