Although many home owners, and sadly a fair number of roofing contractors in Vancouver do not know how to measure a roof, it is a fairly simple process. All you need is a tape measure, pad and pencil. Your goal is to determine the area your roof in order to come to a close estimate of the material that will be required to re-roof your home. Generally when you re-roof your home you will replace the tar paper, shingles, plumbing stacks, flashing and if required your fascia boards and skylights.

If you don’t have it already, I would install ridge cap ventilation at the same time. I personally recommend ridge cap ventilation to all sloped roof homes. It is easy to install and unlike soffit ventilation it can’t become clogged with insulation.

Estimating the area of your roof comes down to calculating the square footage of each portion of your roof and adding them together. If your roof is a very simple shape this is a pretty quick process. A basic rectangular roof on a rectangular building is as easy as it gets. In that case measure the length and width of each side, multiply for the square footage of each side and add the square footage of each side together. Don’t bother with inches, round all measurements up to the nearest foot.

As a general rule add 10 percent for waste, for more complex roofs you will need to add 15% and for very simple you can get away with 5%. You will likely need most of this excess material but if you have any unopened bundles you can return them to the store, although there is usually a restocking fee. As a contractor I am occasionally short material and it is particularly annoying to leave a jobsite to buy 30 dollars’ worth of material especially considering greater Vancouver traffic.

Plumbing stacks are simple, count them and measure the diameter of each to make sure you get the right size replacement. If your stacks are in good condition it is perfectly fine to reuse them. Flashing is simple to measure as well. You can buy it pre-bent or if you need unusual pieces and have a pair of sheet metal pliers you can bend the sheet metal yourself.

The process for every roof is basically the same. Break up your roof into triangles and rectangles and use the area formulas for a triangle and rectangle.

Rectangle or square area – Length x Width.

Triangle area – ½ Base x Height.

I get asked all the time whether it’s possible to measure from the ground? Strictly speaking yes it can be done, if you remember your trig and your roof is relatively simple. You will need to know the pitch of your roof. Of course if your roof is flat you won’t even have to remember your trig.

Personally I would never measure from the ground myself and I wouldn’t recommend it because you will be more accurate doing it from the roof, you can determine the number and type of penetrations you have on your roof, you can determine how many layers of shingles are on your roof, you can check the pitch and you can get a feel for the shape of the plywood or shiplap that is under your shingles.

If you are ever given an estimate from a roofing company and the estimator did not go onto you roof, promptly throw that estimate in the recycle bin. It isn’t worth the paper it is printed on, and it is highly likely he has overestimated the size of your roof so that he is not short material.

A roof such as the one in the diagram to the right merely needs to be broken down into basic shapes in order to estimate the square footage. Roofs with this complexity (I would call it average) would require you to allow for 10% waste. Roofs with more valley’s, dormers or odd shaped roofs require an additional 15% material for waste.

To estimate the roof on the right I would again round all measurements up to the nearest foot.

  • Area A – rectangle
  • Area B – rectangle
  • Areas C , D and E can be broken into two triangles and a rectangle each.
  • Area F (parallelogram) same as a rectangle
  • Area H is a rectangle and there are 4 of them in the diagram, each side of each dormer.

If you were replacing the flat roof portion that would be rectangular as well of course.

Determining the Pitch of Your Roof

Roof pitch is given as a fraction over 12. The numerator is the rise and the denominator is run. Since the denominator is commonly quoted as 12, the easiest way to determine your roof pitch is to use a 12 foot measurement for the run. If the run is 12 feet, the rise in feet will give you the answer to the pitch. If the rise is 4… then it is a 4/12, if it is 6 then it is a 6/12… etc.

If you don’t have enough room… such as when measuring dormers you can half the numbers. Make the run 6 and if the rise is say 3, you have a 3/6 pitch which (multiply both by 2 to give you it out of 12 = 6/12….

Or you could always use inches if you wanted to stay with 12. Just remember the smaller the run the more likely you are to introduce errors.

Keep in mind higher pitch roofs are contracted at higher rates. Any roof over a 7/12 pitch means material left on the roof will likely slide off, which greatly slows work. The additional safety setup and slow place of work can add considerable cost to steep sloped roofs.

Rise of 6 feet over a Run of 12 feet. This roof would be a 6/12 roof, referred to as a “six twelve” in conversation.

Even if you are hiring someone to do the job for you it is a good idea to measure your roof yourself. There are a lot of roofers out there that simply can’t measure and much more often than not they will overestimate the size of your roof. Since roofers charge and work out rates by a “square” – 100 square feet, you could end up paying more than you should.

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