Can I put new shingles over my wood shingle roof?
I have been asked a few questions lately, so I thought that it might be good to reveal the answers here. I had a real estate professional who is not a home inspector come to ask me if there is any fire code against placing new shingles over an existing wood shingle roof. In this case, the new roof would be composite (asphalt) shingles. This problem is more prevalent than what you might think.
First, what is a new roof? I wanted to take a moment to go over this term, because it can confuse homeowners. Some roofing companies tell their clients that they are putting on a new roof, when they are only placing shingles over existing ones. A “new roof” means that they went all the way down to the framing to place everything new from that point up. Wood shingle or shake roofs typically do not have sheathing underneath the surface. They have wood strips called battens. It is fine to place new shingles over an existing roof system. The concern about putting shingles over the old ones is weight. A single shingle does not weigh much, but pick up one package of shingles. Then consider how many packages you may require for your roof. Most roof framing can support two layers of shingles. This weight will contribute to movement in the home (those cracks in the walls). One home inspector discovered that his home had seven layers of shingles. It was a company house in Sugar Land. The firm’s crew never took off a layer; they just added over. The house did not collapse, but there was some damage to the framing.
Is it a code violation to put new shingles over wood shingles or shakes? You have to check with your local authority, but I imagine that it is not going to violate any code. If the wood shingles are in good condition, and precautions are taken to limit them from becoming a fire hazard, then new shingles can be placed over these existing shingles. Now, if you are a home inspector, the first thing which may pop into your head is that the reason new shingles were placed onto the roof is that there was a problem. If you are a real estate investor, you may want to consider this too. The fact that there was damage which prompted the new covering may mean that new shingles should not have been placed over them. Next, we have to be concerned about the fire protection measures. Again, check what is acceptable with your local code, but this protection may be some type of underlayment, sealing the shingles. Go to the edge of the roof to examine the layers. You will see the wood shingles, an underlayment, then two layers of shingles (two layers of composite shingles along the edge is standard in most installs). There are other steps, such as cleaning, that cannot be readily checked once the roof is on, so this will be all we have to go on.
Carefully study the attic space when trying to make a determination about the condition of the roof. You want to find areas of dark discoloration, which could indicate moisture penetration. Also look for any signs of damage to the shingles from the underside. Most problems occur at the ridges, valleys, and penetration points, like for vents. If you suspect a problem, call a professional to examine it. On a recent roof inspection job, I discovered that the attic ventilation had been cut off during the roofing job. You really have to look carefully.