A couple of lessons you learned in Geometry class will help you estimate the cost of your new roof. Do you remember “area of a rectangle”? It’s all about measuring and multiplying. After a bit of outdoor work and a couple of phone calls to local suppliers, you can easily figure the costs for re-roofing.

Even if you won’t be doing the work yourself, this bit of homework will help you understand quotes you get from roofers. We’ll start with the pieces, then do some math.

Parts of a roofing job:

  • Tear-off: Removal of existing roofing
  • Disposal: Cost of getting rid of the debris
  • Sheathing: Boards or plywood which form the roof deck
  • Metal trim: Drip-edge and flashing materials
  • Felt underlayment: A moisture and dust barrier beneath the shingles
  • Shingles: The part you see
  • Fasteners: Nails, staples, etc.
  • Vents: Various products used for airflow
  • Ridge-caps: Finishing touch, may be part of “vents”

Measuring your roof:

In the US, roofing products are expressed in square feet or linear feet. You’ll need to know lengths and widths of the different parts of your roof. Since we’re “estimating,” don’t worry about fractions. The tricky part is hovering in mid-air to get the numbers. Fortunately, roofers have developed a sophisticated method for doing this. We call it “pacing.” Here’s how:

You’ll need a 25′ steel tape measure. On your driveway or sidewalk, mark the distance of 7 paces. See how close you can get to 21 feet. Try to adjust your “walk” so that one pace = 3 feet. Next, choose an easy section of your roof to measure – maybe the front eaves. Pace the length of it first, then measure to see how accurate your paces are.

Next, pace off other parts of your roof. Draw simple pictures and take notes. Most people have a bit of trouble measuring slopes, but you can do it by “guess-timating.” Don’t worry; the pro’s do this, too. Here’s an example:

The “gable end” of your house looks like a blunt arrow, pointing up. If that side is about 30′ wide and has 2′ overhangs, then half the roof (one section) is about 17′ on the flat. Suppose your peak appears to rise about 10 feet above the window tops. That gives us a triangle 17′ on the base and about 9′ tall. Use your tape to lay that out on the driveway, then measure the hypotenuse (the length of the roof’s rake edge). If you recall the Pythagorean Theorem, you can do the math on a $10 calculator. Anyway, given the numbers listed, your rake edge is about 19′ long. Close enough.

Add all the pieces:

Each section of your roof should be either a rectangle or triangle. Multiply length x width for rectangles. Area of a right triangle is half the same math. Round everything up. Add all the sections together for total square footage. Divide by 100 to get “squares” – a common measure for shingles, 100 square feet. Now let’s run the numbers:

Roofing costs by parts:

  1. Tear-off: A labor item. Know how many “squares” your roof measures, then ask for quotes. Height, access, and slope matters, too.
  2. Disposal: A good guess is $20 per square plus trucking costs. Varies by region.
  3. Sheathing (if needed): 3 sheets to the Square, plus nails. $10 per sheet for OSB makes a good ball-park number.
  4. Metal trim: Add linear feet of all roof edges. Use 40 cents per foot as an estimate. Pieces = footage/10 rounded up
  5. Flashing: Add up the footage of valleys or places where the roof meets a wall. $1 to $2 per foot is plenty.
  6. Felt underlayment: $7 per square is close enough. One roll = 4 Squares
  7. Shingles: Prices vary widely, but $90 to $135 per square gives plenty of choices.
  8. Fasteners: Lots of variation here, but $2 per square should cover it nicely.
  9. Vents: Too many options. Ridge vents are popular. Add total feet of ridge and make a phone call. $3 per foot is possible.
  10. Ridge caps: Plain caps will cost about $1.20 per foot. Special air-space caps (sold in boxes) are about twice that.

Total: About $250/square, plus labor

Does your head hurt yet? Remember, we’re Estimating here. You need a roofing budget, and this bit of math will get you there. You’ll still have to nail down the actual numbers if you proceed on your own. Notice that the cost of removing and re-sheeting your old roof is significant. Add labor for the rest of the work, if you hiring a pro. Doing it yourself? Be sure to count the cost of tools and refreshments. And, please – be careful up there!