Measuring A Roof

Do you know how it’s done?

Measuring a roof from the ground can be a much safer way to get the information you need to do a roof. This week I was asked to teach a new solar estimator how to measure a roof from the ground. The basic idea of measuring a roof from the ground may sound ridiculous but it can be done, and done very accurately. I think everyone should measure at least the perimeters of the roof from the ground because the risk of a fall walking along the eaves of the roof can be avoided. If you are a professional and you are up and down ladders, crawling around steep, wet, damp and slippery roofs year in and year out, the odds are against you. These tips can reduce the risk of a fall. This web page is to show how the whole roof can be measured from the ground and a order for materials be placed.

So, What tools do I need to measure a roof?

1, Tape Measure. a 25 foot or longer works good. I use a laser for speed and it is very accurate. There a measuring wheels that work well also.

2, Graph Paper, some basic graph paper will help keep straight lines and scale your roof drawing. I use a two-foot scale drawing and most sizes homes will fit on a 8.5 x 11 sheet of paper.

3, Calculator, is a important tool since it will help you with complex math with less or no errors.

4, Pitch Gage, will be needed to find the pitch of a roof. There are pitch gage Apps for smart phone free to download.
Familiarize your self with the names of the parts of a roof, like the Gable, Hip, Valley, Eave and Ridge. Take a minute before you measure and Google your roof then highlight these parts of the roof like seen here to the left. It will help with your drawing later. Start from the left corner and start taking measurement from the eave to eaves all around the home. Using the graph sheet you should end up right where you started. If you don’t, then you made an error some where.

Once the site drawing is done look at the drawing and in a red marker draw out squares like seen in this drawing. {drawing has been cropped}

A, Is { 48′ x 24′ } B, { 16′ x 24′ } C, { 12′ x 24′ } and D, { 6′ x6′ } Do you see the squares? Add these four totals and you have the foot-print of the roof which is 1860 sq. ft. Now we need to add the pitch factor which is the rise of the roof. Most homes have about a 4:12 pitch or 18.5 degrees. This is as low a pitch as you want when installing asphalt shingles. This roof we are measuring today is a 6:12 pitch or 26.5 degrees. The pitch factor I use for this is 1.12. This factor should be times the house foot print, 1860 sq. feet and your roof size equals 2084 or 20.84 sqs. Lets call it 21 sqs. Determine the waste factor and you have the total roofing sqs. you need to complete this roof.

Now to determine the length of the valleys and hips needs to be calculated using the hip factor of 1.50 for a 6:12 pitch. So here in the front right side is a hip roof and the width being 24 feet. x 1.50= 36 feet, divided by 2 for each side which equals 18 feet for the hip length. The valleys are the same formula as the hips. For this short valley on the right being 6 foot equals 6 x 1.50 = 9 ft.
The eave should be looked at by a ladder to see how many layers of shingles there are on the roof. With a good roof flashing count, you can place a materials order to the supply house. Here are the pitch factors rounded up for calculating the sqs. of a roof.

Degrees converted to roof pitch

9.5° = 2:12,  14° = 3:12,  18.5° = 4:12,  22.5° = 5:12,

26.5° = 6:12,  30° = 7:12,   33.5° = 8:12,   37° = 9:12,

40° = 10:12,    42.5° = 11:12,   45° = 12:12