Roofing Over an Existing Roof
If you are hoping to roof over an existing roof, please check the building codes and manufacturer’s warranties before doing this as you do not want to void them. Luckily most of them often state that an asphalt or wood shingles roof will be adequate to use as a deck for a new roof.
If your existing roof is asphalt you will be glad to learn that you can generally roof over it with new asphalt shingles, wood shingles or shakes. Providing that the house framing and rafters are strong enough, you can even lay a tile roof. What you need to do is to check that the old roof surface (and this includes the sheathing underlay) are in good working order. If the sheathing has water damage you will need to remove it and with that – the old roof as well. If it is only partially damaged you can normally get away with patching it but this is only good for small areas. Make sure that the asphalt shingles are flat as the new roof will look lumpy if it is laid on curled shingles. Any missing shingles needs to be replaced before the new roof is laid. However, if the area with missing shingles are large, you are often better off to remove everything down to the sheathing. With new wood shingles, you will need to install furring strips between the two different roof surfaces in order to provide for ventilation.
Wood Shingle Roof
If your existing roof is made of wood shingles it is often sound to use as a nailing base for either wood or asphalt roofs – sometimes even tile. Remember that wood shakes will last longer if you lay them over spaced sheathing. If you want to install it over a wood shingle roof, put down a layer of felt between the different courses of shake. So, as mentioned, new wood shingles can go down on old wood shingles but I would strongly advise against this. The reason for this is that the new roof will last longer if it is not in contact with the old wood, which could possibly be rotten. If you are removing the old roof, make sure to lay the new one over spaced sheathing so it can breathe. If you are laying asphalt shingles on the wood, make sure that the wood is flat or otherwise add a layer of felt over the wood shingles first.
Wood Shake Roof
If your existing roof is made up of wood shakes, make sure to get an engineering report to confirm it is strong enough to support the new roof if you want a tile roof – with permitted building codes off course. The tiles can either rest or be nailed to the shakes. If not – the shakes will have to be removed.