Roof Expert Needed (Math Related)

[Jim Smolka]

As an engineer, numbers must make sense, so roofing should be no exception.

A roofing contractor came over to give an estimate. To simply things, lets assume that the front of the house is 46 feet wide and the side is 30 feet wide. The pitch of the roof is 7/12 (7 feet up for every 12 feet of Horizontal run) and is a simple 'A' type roof. So far so good. Now for fun part. The estimator used the following equation:

((Side of House * Front of House) * (1 + Pitch))*(Waste)/100 = # of Squares.

Filling in the values

((30' * 46') * (1 + 7/12))*(1.1)/100 = # of Squares. = ~24.03 Squares

where a square is 100 Ft^2 and a waste factor of 10% or 1.1 multiplier.

Now for the Engineering Math that is based of the principles that the side of the house is 2 right triangles and the right triangle theorem of A^2+B^2=C^2

=(((Side of House/2*(Pitch))^2+(Side of House/2)^2)^0.5*Front of House)*2*Waste/100

Filling in the values:

=(((30/2*(7/12))^2+(30/2)^2)^0.5*46)*2*1.1/100 = 17.57 Squares.

To double check the above equation, half of the side of the house is 15', the rise over 15 feet with a 7/12 pitch would be 8.75' Therefore the 'C' part of A^2+B^2=C^2 is 17.365 feet.
Then to determine the area of half of the roof is 17.365' * 46 = 798.8' So double it and then multiply by the waste factor of 1.1 results in 1757 ft^2 or 17.57 squares.

All this to show that 'engineering math' results in 17.57 squares and 'contractor math' is ~24 squares.

OK now for the question. Who's math is correct and what is the standard for determining the # of squares when the roof top is not measured and only horizontal numbers are used???

Thanks,

[oldE]

I'm assuming you'd like him to shingle the parts that extend out over your walls, around the waste stacks, the power mast and chimney? If your overhangs front and rear are about 2 feet and your eves are extending another foot and a half, you can eat up another three squares in a hurry. Is he using a ridge vent strip to cap it all off or using shingles to cover the ridge?
When I've done my buildings, I measure the actual dimesions of the roof. I still end up with a part bundle. Man, my knees hate roofing!
Les

[Jim Smolka]

Opps, I forgot to metion that the dimensions of 30' and 46' include the overhangs. Anotherwords, the dimensions are the shadow of the if a light was directly above the roof.

Chimneys are on each end of the roof and shingles do not go all the way around. I will go up on the roof and and get the actual measurements. Yes a ridge vent will be used.

[oldE]

Another factor I forgot to mention. Depending upon the length of the roof, your waste will vary. For example: if your roof was exactly some multiple of the length of a shingle, there would be almost no waste. If, on the other hand, you threw away the centre half (or two thirds) of the end shingles, your waste will be up.
Also, when you start, you invert a row, to cover the gaps (assuming 3 in 1 shingles) in the first course.
Les

[island911]

math refnc check

[competentone]

Originally posted by Jim Smolka
...what is the standard for determining the # of squares when the roof top is not measured and only horizontal numbers are used?

I'd say the "standard" is to measure the roof top.

[williecoyote]

I get 15.97 squares, (same as island911). Since you are shingling the roof, you need to measure the area of the roof. You can figure this if you know the dimensions of the building, with overhangs, and the pitch. You will need slightly more than 16 squares because you will need starter course, and ridge caps. I would think in the 18-19 square range.
I don't know if the estimator is trying to scam you, or if he's just stupid.
fwiw
My dad had his house in Raleigh re-roofed. Home Depot quoted him $21,000.00 for material, and labor. He found someone else who did an excellent job for less than $7000.00. It took the HD estimator 2 hours of walking around the house to come up with his bid. The contractor who did the job wasn't there 20 min. to do his estimate.

[juanbenae]

island rules !

[Jim Smolka]

Yes, the 15.97 number is correct according to engineering equation above. (The 1.1 multiplier for scrap results in the 17.57 squares).

The house is in a high wind area since we are located at the base of a small moutain. Therefore, a high wind shingle is required. Another reason for considering other materials too (metal, slate, rubber [fake slate], etc).

[M.D. Holloway]

Roof Math in Texas:

1 Roof = 4 Mexicans + 1 hail damage insurance claim

[TSNAPCRACKLEPOP]

"SOME" roofers over estimate, then return the extras for cash or credit.