The famous line from Forrest Gump, “My mom always said life is like a box of chocolates. You never know what you’re gonna get.”, can be applied to roofing, particularly on older homes.
In my opinion, to properly replace a roof, it should always be stripped to its roof decking when delving into things.
Though most local building code these days does allow for 2 total layers of roof, I personally do not prefer layovers whenever possible.
That being said, similar to Forrest’s box of chocolates, when you strip off all the existing roof shingles from someone’s roof, you often obviously have no idea what the roof decking underneath it all will look like until you get it off.
Although you may be able to gather somewhat of a sense from looking at the underside of the existing roof decking, by venturing into the attic, and seeing how things appear directly underneath the decking, you will not have a true sense until the old roof is off.
Once it is off, either one of three things will happen:
- The roof decking will be perfectly conducive to fastening the new roof system on top of it.
- The roof decking will have areas that need shoring up, but overall will be in good shape
- The roof decking will be a total mess and it will need to be overhauled in some capacity to properly put the new roof on.
Roofing code states that a new roof system must be applied on top of a solidly sheathed roof deck (a roof deck with a minimum of 3/8” sheathing, installed with no more than ¼” spacing between sheathing panels).
When an existing roof is taken off, and it is found that the roof decking beneath it is compromised to some degree (either some type of rot has set in over a wide span or the spacing between the existing sheathing is too wide, etc.), new roof decking needs to be laid.
As long as when you are fastening the new sheathing over the existing sheathing, you are doing so in a manner in which it is appropriately fastened to the roof rafters or trusses and can properly carry the new roof system through the most common severe weather conditions of your particular region, it is perfectly fine to do so.
It is SUPER important to make sure that your roof decking is solidly sheathed.
The whole goal of a solidly sheathed roof is to make sure that your new roof system can properly “grab” on to the area it is being fastened to.
If ever thinking about getting a new roof done, if whomever is doing the roofing work has any doubt that your existing roof decking is conducive to receiving the new roof system once the existing roof is stripped, a conversation should be had as to what the possible cost would be to install a new sheathing system over the existing roof decking if need be.
Again, this is something that is often found needing to be done for older roofs (in homes 50+ years old) and is less likely to be found needing to be done on more modern homes.
-Tom
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