Older asphalt shingled roof, no leaks, but has some moss build up. Can a metal roof be installed on top of it or must it first be removed for proper installation of a metal roof?
While it’s a commonly adopted practice to install over your existing asphalt shingled roof, we do not recommend this. Check with your local city or county building inspections office to find out if removal is required.
It is best practice to tear off shingles prior to installing a metal roof. One of the biggest reasons for this is your putting on a roof that should last 50+ years, and it is good to know exactly what the surface under those shingles looks like so any issues can be addressed prior to your metal roof being installed.
In my opinion it is always best to remove the old roofing before a new roof is installed. It allows for an inspection of the roof deck. At that point we usually pull a couple pieces of decking off so we can easily get into the attic and look for other problems like mold and eave intake vents that have been plugged up with blown in insulation. Another common problem is bathroom and kitchen vents that are not connected to external stem vents. These are all easier to look at with the old roof removed.
We also like to make sure that the decking is nailed to code. Believe it or not most roofs in our area need to be renailed. Either the builder didn’t nail it to code or they used the wrong type of nail and the decking is loose.
Then there is the fact that an old roof might have insects in it. Cedar shakes and shingles commonly do. By removing it you get rid of most of the bugs and can treat for ants.
The weight of the old roofing combined with the weight of the new roof can cause sagging between the rafters. Aesthetically this is not good. And in time can become a structural issue.
So, tear off the old roof everytime.
If installed correctly, they should be removed to deck the surface. Decking should be replaced if compromised or incorrect type, A licensed and insured roofing contractor should perform an inspection/consulting to preemptively eliminate issues likely to arise. Furthermore, to ensure OSHA, industry standards and manufacturer’s guidelines are being implemented. PRO TIP - Employ a third-party, impartial and seasoned roofer for an accurately unbiased assessment.