You must remove any rotted pieces of wood on the roof and replace them with your plywood.
How to fix a flat roof with felt.
You can cover the surface of the felt with gravel for added strength.
Apply some bitumen adhesive over the repair and beyond to suit the patch and apply it.
Step by step guide for felting a flat roof step 1.
If the old layer of felt roofing is in good enough condition then a liquid system repair can be applied.
Apply another layer of adhesive to the top for maximum waterproofing.
Fill the hole with a layer of roofing cement.
Cut a piece of new felt to use as a patch big enough to give above a 50mm 2 inch overlap beyond the repaired blister.
For older felt roofing a patch of torched on roofing with add an additional layer and fix the damage.
Cover the edges of the roof with strips of felt before laying out the final layer.
Press the patch firmly into place and make sure that the edges are well sealed.
Fibreglass flat roof repair.
Most cans of cement work well for any roof but you may want to check the label to make sure it is appropriate for the kind of roof you have.
You will follow the same process as in step 2 for the last layer.