T e a r O f f Vs. O v e r l a y
|The question often comes up about whether an existing
composition shingle roof should be torn off or if an overlay (or recover) is possible.
Overlays save on time, labor, and disposal costs. If the existing roof has only one layer,
lays flat, and there arent any problems with the roof deck, removal is not
Regardless of whether you overlay or tear off, you should properly ventilate
your attic. In almost all cases, continuous soffit and ridge vents will provide maximum
cross ventilation. For more information on ventilation, visit http://www.roofhelp.com/ventilation_main.htm.
If any of the following are true, a complete tear-off of the existing roof system(s)
should be considered.
- Too many layers If a roof has more than one layer of roofing, the roof should be
torn off. In most cases this is a code requirement. Codes rarely permit more than two
layers of roofing. To determine this, all you need to do is contact your local building
- Bad decking - if spongy areas are noticed when walking on a roof, or if you see sags
between the rafters or trusses, there's a chance that some of the roof deck may be
deteriorating or be damaged. Deterioration can be attributed to dry rot or delamination of
the plies in the plywood due to glue failure. Dry rot is wood rot caused by certain types
of fungi and if it isn't taken care of, it can spread. If there is suspicion of bad
decking, then a full tear off should seriously be considered.
- Ice dams - ice dams can be a big problem. In areas where there the average January
temperature can be below 30° F, and no ice and water protection membrane is present on a
roof, then a full tear off should be considered. Twenty year old buildings with no ice and
water protection and no prior problems, can suddenly experience thousands of dollars in
damage when a freak cold front hits. For more information on ice dams, visit http://www.roofhelp.com/library.htm
- Incompatible shingles - if a heavy weight architectural style shingle is used to cover a
light weight strip shingle such as 3-tabs, then the roof will look good. However, if a
light weight shingle is used to cover a heavy weight shingle, the light weight shingles
have a tendency to show all the bumps and ridges (called telegraphing) and won't look
- Existing roof is in poor condition - if the existing roof is in really poor shape, such
as tabs being severely curled or if the rows are crooked, then complete tear off and
replacment should be considered.
- Shorter Life Span - There is no known documented research, but most roofing
professionals agree that with an overlay, the average lifespan of the shingles will be
shortened by about 10%-20%.