Roofing Needs Require Some Math

By Dwight Barnett
Scripps Howard News Service

Q: I hope you can help me figure out the amount of materials I need to reshingle my hurricane-damaged roof. I measured the roof on one side, and it is 60 by 24 feet. If I double these measurements I come up with 2,880 square feet.

I went to two home improvement stores. The first told me I need about 30 squares and 30 rolls of No. 30 felt paper underlayment. The second told me I needed 60 squares and 20 rolls of No. 15 felt paper. Which is correct, or how do I correctly calculate my roofing needs?

I have to allow for two dormer windows, and a portion of the roof over the garage is leaking. I have a ranch-style house with about 2,000 square feet of living space. A roofing company told me it would cost about $185 per square for this repair job. Is that normal, or are we in Orlando, Fla., being taken advantage of because of the massive damage to our city? Roofers are in short demand, with a three-month waiting list. I want to order the materials as soon as possible because I am having major leaks, even under the tarps that I installed over the missing shingles.

A: I can understand why you might double the square footage to find the total amount of shingles needed, but that's the wrong way to go. The home has a pitched roof, thus the rafter is actually longer than half the width of the home. The length of the rafter depends on the pitch or angle of the roof and the amount of overhang at the eaves. The pitch is the rate of vertical elevation of the rafter in inches for every 12 inches of horizontal run.

For a home 24 feet from front to rear, the total run to the center of the home will be 12 feet. If the rafters rise 4 inches for every foot of run, that would be a 4-in-12 pitch, commonly written as 4/12. Add a two-foot overhang and the total rafter length would be 14.66 feet.

Magic? No, just plain old mathematics. The roof is a triangle that can be broken down into ratios. Here is a list of common rafter lengths for common pitches. For instance, a 4/12 pitch roof with a 12-foot run would be 12 x 1.055, which equals 12.66 feet. Add 2 feet of overhang and the total would be a rafter that is 14.66 feet long. So:

Roof: Ratio of rafter pitch x length

3/12 x 1.035
4/12 x 1.055
5/12 x 1.085
6/12 x 1.120
7/12 x1.160
8/12 x 1.205
9/12 x 1.250
10/12 x 1.305
11/12 x 1.360
12/12 x 1.415

To determine the pitch of the roof, measure over on a level line 12 inches from the edge of the roof on any part of the roof and measure vertically to see what the rise is.

You also mentioned dormer windows, which leads me to think that the pitch of your roof is at least a 6/12. A 6/12 pitch gives you more head room inside the attic. Ready? Ratio from the chart above for a 6/12 roof is 1.120. Take 12 (half the depth of the home) x 1.120 and that equals 13.44 feet. Add 2 feet for overhang and you have 15.44 feet rounded off to be 16 feet for the rafters length.

Take 16 x 64 (60 feet long plus 2 feet overhang each end), which equals 1,024 square feet for the front roof area. Multiply that by 2 to include the back roof area and you have 2,048 square feet.

Roofing is sold in 100-square-foot lots, so you need 21 squares of shingles and 2,100 square feet of 15-pound (15) felt paper and about 50 pounds of galvanized roofing nails.

I think $185 per square is a decent price to remove and replace shingles. Don't forget to allow for some decking replacement at the eaves where the water runs back up and under the shingles. Also use a metal drip edge to protect the decking.

(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors.)