Preparing Your Old Roof
So, after inspecting your existing roof and checked maximum number of roofs allowed by code, you might have come to the decision that you can lay your new roof, on top of your old one. What you will need to do now, is to prepare the old roof. One of the first things you will need to do is to make sure that all old roof shingles are flat and replace any missing shingles. If you come across any curled or buckled asphalt shingles, nail them down. This goes for split or warped wood shingles as well. Once this is done, remove the hip and ridge shingles, including nails – and then sweep the roof. This is done so that the new roof will lie straight and for it to look nice and even. If you are looking to hide the old roof, I would advise removing about 6 inches of old roofing materials from both eaves and rakes and this is best done using a utility knife for asphalt and a hatchet for wood. After this has been done, add drip edges and nail down a board measuring 1 by 6 inches. The board will not only help to hide the old roof but it will also give you a good base for the weakest part of the roof.
Checking the Rest of the Roof
Once you have prepared the roof itself, move on to the chimney/s and vent flashings. Check that they are in good working order and remember to check the mortar on the chimney/s as well. Always install new valleys, using roll roofing or galvanised metal if you are roofing over asphalt shingles and galvanised metal valleys for roofing over wood shingles. Once you are doing any type of work to your roof, it is always a great idea to check over your gutters. If they need repairing or replacing, this is a great time to have that done. It is best to do it at the same time as you are prepping the deck. The reason for this is that some gutters are best nailed to the roof deck and this is obviously best done before the shingles goes down.
You could be surprised to hear that many roofs are not perfectly rectangular and I would always measure the roof surface before starting any major work. It is best to measure it from rake to rake and from eave to ridge. If it is a bit uneven, take this into account and make sure to compensate for this. You can make these compensations by shortening the horizontal courses near the ridge. If you have to make vertical compensations, make sure to hide uneven shingles by the rake that is the least obvious. Best of luck!