You can either measure the roof from the ground or by standing on a ladder and actually going.
How to fix a flat roof with felt.
Step by step guide for felting a flat roof step 1.
Press the patch firmly into place and make sure that the edges are well sealed.
For older felt roofing a patch of torched on roofing with add an additional layer and fix the damage.
Apply some bitumen adhesive over the repair and beyond to suit the patch and apply it.
You must remove any rotted pieces of wood on the roof and replace them with your plywood.
Cover the edges of the roof with strips of felt before laying out the final layer.
If the old layer of felt roofing is in good enough condition then a liquid system repair can be applied.
Fill the hole with a layer of roofing cement.
Fibreglass flat roof repair.
Apply another layer of adhesive to the top for maximum waterproofing.
You will follow the same process as in step 2 for the last layer.
Get a can of a waterproof roofing cement from the store.
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.