How to Measure a Roof for Shingles

You can estimate a whole roof of shingles from the ground. The trick is that the footprint - the flat outline the roof covers - plus the pitch is all you need; the pitch’s slope factor turns the flat area into the real sloped surface.

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If you already have your footprint and pitch, the roof shingle calculator does the slope factor, squares and bundles for you. Read on for how to take the measurements.

Step 1: measure the footprint

Measure the building outline the roof sits over - the length and width of each rectangular section, taken at ground level, and include the eave overhangs on each side. Multiply length by width for each section and add them up. That total, in square feet, is your plan (footprint) area.

For an L-shaped or complex roof, break it into rectangles and add them. A satellite measuring tool gives the same footprint if you trace the roof edges.

Step 2: find the pitch

The footprint ignores steepness, so you need the pitch to add it back. Measure it safely from the attic or with a level against a rafter - see how to measure roof pitch. You only need it as a x:12 ratio or an angle.

Step 3: add the pitch back with the slope factor

Every pitch has a slope factor - the amount the sloped area exceeds the footprint:

slope factor = sqrt(1 + (rise ÷ run)²)
sloped area  = footprint area × slope factor

A 4:12 roof is about 1.054 times its footprint, a 6:12 about 1.118, and a 9:12 about 1.25. The roof pitch chart lists them all. Multiply your footprint by the factor for your pitch.

Step 4: convert to roofing squares

Divide the sloped square feet by 100:

squares = sloped area (sq ft) ÷ 100

So a 1600 sq ft footprint at a 6:12 pitch is 1600 × 1.118 = 1789 sq ft, which is 17.9 squares. From there it is a short hop to bundles - see how many bundles of shingles do I need.

Hip, gable and cut-up roofs

The method is the same for any shape, because the footprint already captures the outline:

• Gable - one footprint rectangle (or a few), low waste.
• Hip - same footprint, but add waste for the diagonal cuts and add the hip lengths to your ridge-cap run.
• Gambrel / mansard - two pitches per side, so measure each pitch band separately and add them, since the slope factor differs.
• Dormers and valleys - measure their footprints in, and step the waste up to 15%.

Eaves and ridges for starter and ridge cap

While you have the plan out, total two more runs:

• Eaves (the lower edges) for the starter strip.
• Ridges and hips (the top edges and diagonals) for the ridge cap.

Enter those linear feet in the shingle calculator and it counts the starter and ridge-cap bundles alongside the field shingles.