Roof and Roofing Q&A
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At the risk of a book length reply, I am going to redo my roof this
summer, and am wondering if it is better to remove the old asphalt shingles, or
(as many people have told me) just lay the new shingles on top of the old. There
is only one layer on the roof now.
JG from Exeter, NH
That is actually a great and very straight-forward question, with an equally
great and straight-forward answer. You can generally lay a new roof over the old
without any problems as long as:
1) You only have one layer of asphalt shingles on the roof. Most roof frames
can support two layers of asphalt shingles, rarely three. Depends on the
framing, of course.
2) The shingles lay fairly flat. The new shingles will take on the shape of
the old, so if the old shingles are very curled, twisted, or buckled, the new
roof will eventually look the same AND the life span of the new shingles may be
decreased. This may also void the manufacturer's shingle warranty.
3) There are no existing leaks from undetermined sources. Just putting a new
roof over an old roof will not necessarily fix underlying problems, such as with
flashing or roof vents. These leaks will most likely recur unless they are
properly repaired prior to installation of the new roof.
4) You were satisfied with the performance of the old roof... e.g.. no
occasional leaks during rainstorms, etc. This refers back to item (3)... new
roof over old roof may not guarantee elimination of leaks.
5) An examination of the roof deck or underlayment shows it to be solid.
If there is any widespread warping or softness, repair will necessitate the
stripping of old shingles. If the area is large enough, it makes sense to
restore the entire roof to new condition.
Ideally, your or your contractor should repair, reseat, or (ideally) replace
all flashing around chimneys, valleys, vents, etc. He should also cut off or
replace any curling shingles so the new ones lay flat. If you have had ice dam
leakage, he should also remove the bottom few feet of old shingles and install
an ice and water shield right against the roof sheathing plywood.
You can then be assure that your roof will have a long and healthy life!
The contractor replaced the asphalt shingle roof on my house. I
can see the nails sticking out from the plywood for 1/2"
inside the roof. It wasn't so with my old roof. Will this shorten
the life of the roof or cause any other damage to the structure?
It is perfectly normal for roofing nails to extend through the
roof plywood, or "sheathing". I am surprised that you
didn't notice them from the previous installation. Over time, the
moisture in the attic space can cause the bright ends of the new
galvanized nails to darken, making them less obvious. If your
previous roof was 15 or more years old, that may explain why they
seemed to be invisible.
When installing new shingles on your roof, the contractor
installed them from the bottom up, so that each successive layer
of shingles covered the nails from the lower row. Thus, very few
nails are directly exposed to the weather. Good roofing practice
requires that any exposed nails be sealed by placing a small
amount of roofing cement or silicone caulk under the nail head
before setting it. Clear exterior silicone caulk is the obvious
choice for a lighter colored roof.
If your new roof was installed over the old roof (up to two
layers are permissible provided your roof structure can hold the
weight), roofing contractors must use nails that are adequately
long to extend through the deck. Though there is consensus as to
the shortest nail that is adequate for roofing purposes.... 3d (1
1/4")... it is more typical to find a roofer using a 4d (1
1/2") roofing nail or, in some cases where roofing materials
are thicker, a 6d (2") roofing nail. These easily penetrate
through two layers of shingles and the sheathing. Your roofer may
have opted to use the same nail length throughout. Again, this is
perfectly acceptable roofing practice. If you roofer used a nail
gun, it is almost guaranteed he used the same nail length for the
entire job. It's just easier, more efficient and the difference in
cost is minimal compared with the total material's cost.
Remember... a roofers TIME is more valuable than the cost of a few
In climates that experience extreme wind and rain and/or
significant snow, many roofers install a product known as an
"ice and water shield" underneath the shingles. This is
a self-adhesive, rolled product that is installed directly onto
the roof sheathing. It replaces the once commonly-used tarpaper
providing a second line of defense against leaks. The advantage of
the shield over tarpaper is that it "self-seals" around
the nails providing better leak protection than either shingles or
a shingle-tar paper combination. This product can be installed
over the entire roof if necessary, but is usually installed along
the bottom three or six feet of the roof to protect against
ice-damming leaks. Also, the use of this product is determined by
the slope of the roof and the climate... the more moisture and
snow AND the flatter the roof, the more protection the shield can
In today's newsletter, you stated the following: "If your new roof was installed
over the old roof (up to two layers are permissible provided your roof structure
can hold the weight)."
Most communities that I've been in allow only 2 layers of shingles, however
there are some that allow three. People should check with their local building
supervisor or town hall to find out what the ordinance is for their community.
Based on your statement, someone might rip off the shingles if they already have
two layers because of your blanket statement, even if they are allowed a third.
The cost for laying shingles over an existing layer compared to ripping off the
old ones first could be quite substantial.
With regards to second and third roofs, do they last as long as the first or are
there diminishing returns?
TM from White Meadow Lake, NJ
You are correct... local codes are much more stringent than the national code
when it comes to reroofing. The national code does allow two layers of asphalt
shingles over an existing layer. In areas with snow loads, though, it is less
common to get approval for a third layer... especially heavy-duty 30 year
shingles! But the three layer allowance is dependant on an inspection of the
roof structure by the local building department to be sure it can carry the
weight. So thank you and consider your message passed on!
You would think that ANY scrupulous roofer would let the homeowner know all
their options, including the permissible number of layers allowed. But that is
my naive-side talking, so ignore me on that count!
There are a few reasons to NOT layer an asphalt roof that should be considered
by the homeowner. The first reason is that I have seen many roofs with leaks
that reroofing didn't fix! This is usually due to problems with flashing that
were not apparent to the roofer. Proper reflashing can require the roofer to do
quite a bit of shingle removal and replacement prior to installing the new roof
layer, somewhat diminishing the price advantage of layering.
Also, the second roof will not last as long as a new single layer. The primary
reason is heat... the hotter the roof gets, the shorter the life of asphalt
shingles. Most people would intuitively think this is nutty, especially if they
have ever been on a roof. I mean... how much hotter could it get? The truth
is... lots! Remember that the temperature of the roof is affected by how well
the attic is ventilated. Asphalt roof manufacturers have proven that a cool,
well-ventilated attic leads to longer roof life. When you "layer" the roof, the
lower layer acts as an insulator so heat does not radiate into the attic space
as well as a single layer. Since the upper layer becomes hotter than it would
otherwise, the roof's life is measurably shortened. Unfortunately, I have no
data to give you regarding the actual reduction in roof life. (If any reader has
this information I would be glad to receive it.)
The other factor that may shorten a layered roof's life is the unevenness of the
old roof. The new shingles do not lie on a smooth decking... they lie on top of
old shingles. This unevenness will eventually reshape the new shingles as they
soften with the sun's heat causing them to bend to conform to profile of the old
roof. Even the slightest bend is a potential weak spot that may lead to
premature breakage... especially if the roof is walked on.
As you can probably tell, I am a one-layer sort of guy. So are the growing
numbers of roofers who are tired of callbacks and dissatisfied clients. But I
appreciate the additional costs of stripping an old roof. So all I can say is...
let the buyer beware!
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