When you are working on a roof you need to determine how many squares or 100 square foot sections of shingle that you will need to purchase.
There are two methods to estimate the amount that you will need.
The first is to actually get up on the roof and measure the actual square foot of the roof surface.
This is easy enough for a home owner because it will give you the chance to inspect your roof closer.
If you are specking jobs you will need another method that will give you your square footage.
First you measure the width and length of the building then you measure the pitch or slope of the roof.
Most roofs are are between a 6 and 10 to 12 foot run pitch.
This means for every 12 feet of run distance the angle of the roof increases 6 to 10 feet in height.
Some roofs are much higher and some can be close to flat on mobile homes.
The higher the pitch the more shingles that will be needed.
Use this chart and diagram to find your Slope / Pitch factor
4:12 = 1.05
5:12 = 1.08
6:12 = 1.12
8:12 = 1.20
10:12 = 1.30
12:12 = 1.41
14:12 = 1.54
To calculate your approximate need use this formula
Length x Width = SQ
SQ x Slope factor = answer
30 x 60 feet = 900
For a 8:12 roof 900 x 1.20 = 1080 square feet
add 10% for a gable roof
add your starter strips
and you have your Total
Below a 4:12 pitch your fudge factor of 10% extra material will be fine to cover the difference between a flat and slightly pitched roof.
Over a 10:12 roof really requires someone with skill to install the shingles especially over single story heights. If you feel brave enough to do it then fine but you really should just pay a professional that understands the dangers.
You can see from the chart above that from a 4:12 roof to a 14:12 roof there is a 50% increase in materials. So the calculation is important unless you can get up on the roof and measure actual square footage.
Additionally you will need a starter strip along the bottom and side edges and you will need extra shingles for the ridge cap and valley areas where they are cut to fit.
If you need to special order your shingles make sure they will accept all unused packages. This way you can over order by a small amount and still make sure that all the shingles you are using are from the same color batch.
We should take a moment to speak about color batches. Some roofing materials have a slightly random color by design to give a texture to the roof but you must make sure that all of your shingles for your one job have the same color batch number.
Variations in materials will change from day to day at the manufacture. If you have color batches that are slightly different you may not notice from the ground but if you have a batch that just doesn’t match you will have a roof that looks like repair work has been done on it.
Other materials that you will need include
Plumbing Vent Flashing
Water Membrane for the Valley and roof edge
You should figure about 400 nails per 100 square feet for three tab shingles.