A.Christopher DeBlois, a
structural engineer with Palmer Engineering Co. in
Tucker, Ga., responds: The simple answer is
that if the roof is fine now, it will almost
certainly be fine when you add the new roofing.
Most residential roofs are sized or designed for
basic loads of 10 pounds per square foot (psf) for
dead load (self weight), and 20 psf for live load (
primarily snow, ice, and rain). The old roofing
— standard asphalt shingles —
probably weighs about 2.5 psf. The 2x8s themselves
weigh about 2.3 psf depending on the species of
wood, and 3/4-inch plywood or 1-by roof decking
weighs about 2.5 psf. Add a pinch for 30# felt, and
you get an existing load of about 8 psf. This
assumes that you don’t have insulation or
a finished ceiling on the rafters, but I am
guessing that a 30-year-old house probably has a
roof over an unfinished attic.
The fancier and heavier architectural shingles
will weigh in at about 4.0 psf. Adding all this up,
I get a total load of 32 psf (20 live load plus 8
existing dead load plus 4 new dead load).
That’s less than a 7% increase over the
standard design capacity of 30 psf, so
you’ll probably be fine.
A more detailed analysis of your question
depends on the answers to at least five
- What is the species and grade of the
- Is the framing in good condition?
- Did you measure the 16-foot rafter span
following the slope, or did you measure
- Is there a finished ceiling under any or
all of the roof?
- What is the ground snow load where you
In order to give a more precise answer,
I’ll make some assumptions. I’ll
assume the rafters are #2 Southern Yellow Pine
(SYP) or of equivalent quality, and I’ll
assume the material is still in good condition.
Since you said "16 feet long" instead of "16 foot
span," I’ll assume you measured the
16-foot rafter length up the slope. At a 4:12
pitch, this gives a design span of 15 feet 2
inches. To give myself a little wiggle room, I will
add another 2.5 psf to the existing dead load for a
drywall ceiling with insulation.
Now I have enough data to actually calculate the
capacity of the roof. For #2 SYP rafters at 16
inches on-center and spanning 15 feet 2 inches, I
calculate an allowable load of 45.2 psf. Note that
I have taken advantage of two upgrades permitted by
the wood design code: a 15% strength increase
available for multiple member use (typical for
rafters and floor joists), and a second 15% load
duration increase that applies for snow loads that
are only present part of the year. Subtracting 10.5
psf for the existing framing and roofing, and 4.0
psf more for the new roof, and rounding down, I am
left with an allowable roof live load of 30
A roof load this high can only be caused by
snow. In this case, I conclude that you can add the
new roofing if you live in a locale with ground
snow loads of 30 psf or less. For the curious, the
dividing line would fall approximately as follows:
south of Pennsylvania, south of Michigan and
Wisconsin, south of the middle of Iowa, south of
South Dakota, and west of the Dakotas at an
elevation below 5,000 feet above sea level.
If the framing details for the house
you’re reroofing match my assumptions,
then you should be able to add the new layer of
shingles. If you’re in doubt, either
consult a local engineer or tear off the original