How to Measure Roof: Step #1 – Making Your Drawing
A good place to start is to make a birds-eye sketch of your roof. Visualize the roof as if you were a hundred feet above it looking straight down. What would you see? Draw the outline of the entire roof. Then show every eave, rake, valley, hip and ridge. Show every detail, including pipes, vents, chimneys, skylights and solar panels. These things all affect your estimate of roofing costs. Try to keep your drawing somewhat proportional, but it doesn’t have to be drawn to perfect scale.
How to Measure Roof: Step #2 – Measuring the Slope
I like to measure the slope first, because it is both important and easy to forget. This method requires a carpenters level (any length will do) and a measuring tape. Set one end of your level on the roof with the other end pointed directly “downhill”. Raise the downhill end until the bubble is centered. Then keeping your measuring tape vertical, measure the rise, which is the distance from the bottom of the level to the roof.
If your level is one foot long, you’re all set. What you see is what you get. If your level is two feet long, divide the rise by two. It your level is three feet long, divide the rise by three.
On roofs with high profile roofing components such as tile or wood shakes you may need to lay a board across a few courses to get a true reading.
Sometimes it’s easier to measure the slope of the fascia board or drip edge instead of the roof surface. As long as it’s parallel to the roof, it doesn’t matter what you measure.
Record the slope right on your drawing.
How to Measure Roof: Step #3 – Taking the Measurements
Next, record the measurement of every single vertical, horizontal and diagonal line on your drawing. They will all be needed to figure the right amount of roofing material that you will need. Also be sure to measure each roof section from the eave, straight up the roof to the very peak.
The easiest tool for measuring most roofs is a measuring wheel. It is fast and doesn’t require anyone else to hold the other end of a tape. But it is a mechanical device and you need to make sure it is working properly.
The most accurate tool for measuring is a tape. It is also handy for some roofs such as barrel tile, where a wheel doesn’t roll very well. I use both a 25 and 100 footer.
It’s a lot easier if you round each measurement up to the next larger foot. I’ve also found that if the roofers are careful, that’s just about the right amount to allow for waste on most roofs. If you have roofers on piecework, rounding up also eliminates virtually all squabbles over roof size.
How to Measure Roof: Step #4 – Calculating the Area
When you have all the measurements you need, grab a calculator and figure the square footage of each roof section.
No matter how complicated the roof is, you can break it down into a series of rectangles and triangles. From there, it’s just simple math. For the rectangular sections, multiply the length times the width. For the triangular sections, multiply the length times the width, divided by two.
It’s a good idea to number each section, so you don’t forget any. Then just add all the sections together for the total square footage. Dividing the total by 100 gives you the number of “squares” of roofing material to order.
To figure how many shingles you will need for your starter course, just add up the lineal feet at the eave and divide by the length of one shingle. Don’t cut all your starters at the beginning of the job though. As you shingle each section, you may be able use what you cut off the top course for the starters on the next section.
For your hips and ridges, you may have the choice between bundles of pre-cut shingles or cutting your own from three-tab shingles. Either way is OK but they don’t all have the same coverage. The pre-cut type will have the coverage printed on the bundle. A bundle of standard three-tab shingles will cover about 33 lineal feet of hip or ridge.
How to Measure Roof: Step #5 – Ordering Your Materials
Be aware that some shingles are packaged three bundles per square, while others are packaged four (or more) bundles per square.
You will seldom estimate the EXACT amount of roofing you need for the job. Having a little left over is usually preferable to not having enough to finish the job. But try to keep a few bundles out of the weather in case you need to return them. Suppliers might charge you a restocking fee, but they don’t mind taking them back as long as the bundles are in good shape and you still have your receipt.