How to read a roof diagram

The first roof diagram most homeowners see comes attached to a quote or an aerial measurement report. It can look technical. It contains a small set of elements, though, and nearly every diagram shows the same ones even when the drawing styles differ.

Start with the view

A roof diagram is a plan view: you are looking straight down on the house. A sloping roof looks smaller in plan than it is, because the slope is hidden. The shapes on the page show the footprint of each section of roof, while the areas written inside them usually give the true surface, with the slope included.

Most diagrams include a north arrow and a scale. The north arrow shows which way is north. If you know which way your house faces, you can work out which side of the drawing is the front, the back and each end. The scale tells you how many feet each inch on the page represents, though the lengths that matter are normally written on the drawing.

Planes

A plane is one flat section of roof surface. A simple gable roof has two. A hipped roof on a rectangular house has four. A house with dormers, a porch and an attached garage can have twenty or more.

Each plane usually carries an identifier, a pitch and an area. A label reading E, 7/12, 412 sq ft means plane E rises seven inches for every twelve inches of horizontal run and has 412 square feet of surface. When a report or quote mentions plane E, that label shows you where to look.

The lines

The lines between planes carry most of the information. Each type is usually drawn in its own color or line style and listed in a legend.

• Ridges are the level lines along the top, where two planes meet at a peak.
• Hips are sloping lines on outside corners, where two planes meet and water runs away from the join.
• Valleys are sloping lines on inside corners, where two planes meet and water runs toward the join.
• Eaves are the lower edges of the roof, usually where the gutters are.
• Rakes are the sloping edges at the ends of a gable.
• Wall lines mark where a roof plane runs into a vertical wall, such as the side of a dormer or a second story.

Each type has a total in linear feet, usually in a table beside the drawing. Estimators use those totals to order drip edge, starter strip, ridge cap and flashing. You can use them to spot a quote that includes far more valley metal than your roof has valley length.

Penetrations and flashing points

A penetration is anything that passes through the roof: plumbing vents, bathroom and kitchen fans, skylights and chimneys. Many diagrams number them and mark their positions, although reports drawn from imagery can miss small pipes.

Some diagrams, especially those drawn from on-site measurements, also mark flashing points. These are the places where the roof meets a wall, a chimney, a skylight or another roof. Leaks often start at these joins, as explained in what flashing is, and why leaks often start there.

The totals table

Next to the drawing there is usually a table. It gives the total area in square feet and in squares, where one square is 100 square feet, along with linear totals for each type of line and a count of penetrations. Many reports also show the area with several waste percentages added.

If you look at only one number, make it the total squares, and check whether it is before or after waste. That is the number a roofing quote is built on. How pitch turns a footprint into squares is covered in roof pitch and roofing squares explained.

A sample diagram

The drawing below is an example made for learning. It uses the same footprint, 50 feet by 34 feet including overhangs, and the same 6/12 pitch as the worked example in roof pitch and roofing squares explained. Here the ridge runs north to south, and each end of the roof is cut back by a small triangular plane, a shape often called a clipped gable. That lets one drawing show a ridge, hips, eaves and rakes. It has no valleys.

Each plane's surface area is its footprint multiplied by 1.118, the factor for 6/12. Planes B and D each cover 814 square feet of footprint, and 814 multiplied by 1.118 is 910.1 square feet. Planes A and C each cover 36 square feet of footprint, or 40.2 square feet of surface. The four planes add up to 1,900.6 square feet, which rounds to the same 1,901 square feet and 19.0 squares as the worked example.

Totals for the sample roof

Planes B and D	814 sq ft of footprint each, 910.1 sq ft of surface each
Planes A and C	36 sq ft of footprint each, 40.2 sq ft of surface each
Total footprint	1,700 sq ft
Total surface	1,900.6 sq ft, about 1,901 sq ft
Squares	19.0, or 20.9 with 10 percent waste
Ridge	38.0 ft
Hips	36.0 ft, 4 at 9.0 ft
Eaves	124.0 ft, 2 at 50.0 ft and 2 at 12.0 ft
Rakes	49.2 ft, 4 at 12.3 ft
Valleys	None

A five-minute reading order

1. Find the north arrow and turn the drawing until its north arrow points north.
2. Match the largest planes to the roof you can see from the street.
3. Read the pitch of each plane, and note any that differ from the rest.
4. Find the total squares and the waste percentage, if one is shown.
5. Check how and when the roof was measured, and which structures are included.

What a diagram cannot tell you

A diagram shows the shape and size of the roof surface. It does not show what is underneath. It cannot tell you whether the decking is soft, how many layers of shingles are on the roof, or whether the underlayment was ever replaced. A professional can judge some of that at the roof edge or from inside the attic, and some of it only shows up when the old roof comes off.

Condition is the job of an inspection or condition report, described in insurance and pre-purchase roof inspection reports.

Reading your own diagram safely

Read your diagram from the ground or from an upstairs window. Stand near each corner of the house, find the eaves, valleys and chimneys you can see, and match them to the plan. Binoculars help with vents and skylights.

Stay off the roof itself, since everything on the drawing can be matched from below. If a number looks wrong, ask whoever produced the diagram how it was measured.

Holding a quote against the drawing

When a quote arrives, find three numbers on it: the roof area in squares, the length of any valley or flashing work, and the number of vents or pipe boots. Compare each one with the diagram. Small differences are normal, because contractors allow for waste in different ways. A difference of several squares, or a line item for a chimney you do not have, is worth a polite question.