I agree with the others wholeheartedly. Do not leave the existing roof. Reasons:
1. The old roof will inhibit heat flow from the new roof, shortening its life.
2. You wont get a chance to find and fix any bad decking, and re-nail, or screw down decking where nails have backed out.
3. You cannot effectively apply ice dams material in your valleys, rakes and hips.
4. Weight: Why leave dead load that serves no purpose in place?
5. Once you install No. 2, that it. Next time all MUST come off. (Resale)
6. Looks: Recovers often look crappy, especially 3 tabs.
If you need more than all the above, do whatever you want, cause you already have your mind made up, or the roofer you are talking to is a used car salesman.
There are many,many,many,many factors when deciding on a recover.
1.Existing layer count.
2.Manufacture warranties and restrictions.
3.Local codes and requirements.
4.Condition of the roof being recovered.
a.Is the roof leaking ?
b.Are the shingles curled,blistered,deteriorated etc.
c.Is the decking,delaminated,sunken,rotted etc. ?
d.Are there structural damage such as busted rafters,warped rafters,fallen bracing and is the structure capable of holding another layer by region ?
e.Snow load and ice factors.
f.Customers financial status.
g.Your financial status giving you the ability to walk away from a paying job.
h.Possibly one of the more important factors in a recover,,,,Was the workmanship of the previous roofer according to spec ?
i.Replacing any and ALL roof components such as;
1.Step flashing at walls,chimneys,skylights etc.
4.Counter flashing at chimneys and walls.
5.Pans and aprons and side skirts for skylights.
6.The length of fasteners being used to recover.
So see,,,there is more to consider than just saying,,,I am not doing it.In my region I have done them and will continue doing them.,.,But only after a few factors are addressed. :thumbsup:
And I can honestly say that my recovers look very clean and nice and I have never had a single issue from the several thousand I have done.I will do them all day long and sleep in peace at night.
I will put the looks of my recovers against the visual of most peoples single layer installations.
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