All re roofing jobs require a decision about whether or not to rip up the old finish roofing down to the deck below. Doing this increases the cost of the job, whether you are doing roof repair of roof installation.
However, roof repair needs to be done in the following circumstances:
1) When 2 or more layers are already there.
Most codes require you to take up roofing if two or more layers already cover the decking. The reasons are that the combined weight of the three layers may be too great for decking and that you may not be able to fasten the new roof securely through the previous layers. However, there are serviceable roofs of three layers, particularly if the third is metal roofing layer. If you are in doubt about adding a third layer, consult a home inspector.
2) When any part of the roof deck is bad.
A sagging or springy feeling when you walk on the roof indicates a decaying deck. When deterioration is detected, professional roofing contractors usually tear off the finish roofing in at least that area of the roof so that they can properly do roof inspection, inspect and repair the deck.
3) When a building inspector requires a whole new roof.
Sometimes, but not usually, local code officials will require you to remove the old layers if you are planning to put down a new one. In fact, some building inspection departments are very fussy about old decks and even rafters being brought up to modern code standards. Sometimes they have good reason, and sometimes they are too cautious.
There are 200-year old rafters in New England, undersized by modern code standards, that have withstood exceptional snow loads and other tests of time. If your local department requires an upgrade, but you think your roof is solid and no roof replacement is needed, try to persuade roof officials of that. Mainly the codes do not apply retroactively to old construction, but they do rightly call for roofs that can stand up to the expected snow and wind loads.
Those are the three conditions for removing old finish roofing. Generally if you have one single layer, you can cover it.
If you are removing old roof shingles, there is a technique that can make the work go faster: use a long-handled spade-type shovel. Its slight angle at the end makes it a good lever-action nail-puller, and it keeps your face high above the dust. CAUTION: wear a respirator. Many old roofing shingles contain asbestos. As an added precaution, wash your work clothes – separately from other clothing – immediately after removing the roofing.
Another trick is to rent a small, open-top auto trailer that can be manually pulled around the house. Set it against the house wall below where you are working and throw shingles into it, moving it around the house as you go. Doing this saves hours of nasty cleanup and can also be used during other roof repair projects.