from Practical Sailor/ Waypoints
Most people are not sufficiently expert to be able to carry out any but the most minor repairs to their sails and it is better not to attempt to mend sails yourself. They should be sent back to the manufacturer or to a professional sail repairer. However, it is essential to learn how to spot signs of wear so that you can deal with it before it gets worse. The easiest way to inspect your sails is to lay them out flat on an even clean surface and work around the edges and over the belly of the sail. Pay special attention to the points where the sail is attached to the spars and rigging.
The head - examine the headboard (the reinforced part at the head of the sail) and make sure that the rivets are in place and that the stitching is not weak anywhere. The latter may well wear around the head and you will need to check the reinforced patches in this area.
The tack is subjected to a great deal of strain and can distort. If the stitching breaks the whole sail will become misshapen. On a main sail, look for signs of wear on the bolt line (rope edging) where it rubs on the boom and for damage to the stitching on it.
The clew is subjected to the same type of wear as the tack, but on headsails is particularly vulnerable to chafe against the standing rigging.
The Luff - on a mainsail, check that all the slugs are properly attached and not broken and that the reefing cringle is not distorted. Look out for chafe along the bolt line on a mainsail and for strands of broken wire on a headsail luff. The hanks on a headsail must be firmly attached and free from any corrosion.
The foot - on a mainsail with a roller reefing system, check for signs of chafe where it has been rolled around the boom. If there are slab reefing points or eyelets check that these are not pulled or torn.
The leech - examine the batten pockets for damaged stitching and if there are any leech line attachments make sure they are in position. Genoa leeches on cruising boats are particularly vulnerable to chafe against the lifelines or shrouds.
General points - always be alert to the possibility of deterioration of the fabric due to sunlight. If the cloth is thin or brittle over a small area it can be patched, but with larger areas the sail should be replaced.