How to measure a gable or roof peak for lights

The short answer

This is one step of measuring a whole roofline for lights. To measure a roof peak for lights, measure how wide the gable is at its base, halve that number, and multiply it by the pitch factor from the table below. The result is the length of one rake, the sloped edge that climbs from the eave to the peak. Two rakes meet at every peak, so double it for the gable.

That one step fixes the most common mistake on a hand-measured roofline: adding the gable's width as if the light ran straight across its base. It does not. It runs up one slope and down the other, and that path is always longer than the width. On a shallow ranch gable the difference is a few feet. On a steep 16/12 gable, the kind often seen on prewar Tudors, it is about two thirds more than the flat width.

If the terms eave, rake and fascia are fuzzy, our guide to where permanent eave lights mount maps the whole roof edge. This post picks up where the level eave ends.

Why a rake is longer than it looks

Picture a gable from the street as a triangle standing on its base. The base is the width of the gable. The two sloped sides are the rakes. Each rake is the long side of a right triangle whose other two sides are the rise (how high the peak sits above the eave) and the run (half the gable's width). The long side of a right triangle is always longer than either short side, so a rake is always longer than the half-width under it.

How much longer depends only on the pitch. Roofers write pitch as inches of rise per 12 inches of run, so a 6/12 roof climbs six inches for every foot it travels inward. A flat-looking 4/12 garage gable barely stretches the number. A 12/12 gable rises as fast as it runs, at 45 degrees. Past that, on the tall narrow gables common on Tudor-style houses, the rakes get long quickly.

There is a second reason gables get undercounted. From the front of a side-gabled house, the rakes on the two end walls face the side yards. In a photo from the street they look like short diagonals at each corner, or disappear entirely behind the eave. If you want those ends lit, measure them from the side of the house.

Finding your roof pitch

You do not need to climb anything to find pitch. Use whichever method suits your house.

1. From a straight-on photo. Stand square to the gable and take a photo. On the screen, measure the height of the peak above the line where the two rakes meet the eave, then measure half the gable's width at that same line. Divide the height by the half-width and multiply by 12. A peak that rises exactly as far as the half-width is 12/12. One that rises a third more is about 16/12. This works because the gable wall faces the camera flat, so height and width shrink by the same amount on screen. Hold the phone upright rather than tilted back, and step far enough away that the whole gable fits without tilting.
2. With a level and a tape. On a low garage or porch gable you can reach from a short step, hold a carpenter's level with one end touching the underside of the rake board and the bubble centered, so the level sits dead horizontal. From the 12-inch mark on the level, measure straight up to the underside of the board. That number is your rise in 12. The same measurement works on a rafter in an unfinished attic, which is often the easier place to do it.
3. With your phone. The Measure app on an iPhone includes a level that reads degrees from level. Hold the phone's edge against a rafter in the attic or the underside of an accessible rake board and read the angle, then find the nearest degree figure in the table below.

Measure every gable you plan to light, not just the main one. The garage, porch and front gable can each be framed at a different pitch.

The pitch multiplier table

Multiply the run, which is half the gable's width, by the factor for your pitch to get one rake. The factors below match published roofing slope tables. The last column shows what 10 feet of run becomes along the slope.

If your pitch falls between two rows, use the steeper one. Rounding up a foot on a gable is harmless. Rounding down is how a run ends up short at the peak.

Adding up a gable

Here is the ground method from start to finish. You need a long tape and a helper.

1. Measure the span. Stand under the gable and run the tape along the ground from the outside edge of one rake overhang to the other. Sight up to the roof edge at each end so you catch the overhang, not just the wall.
2. Halve it. That is the run for each rake.
3. Multiply by the factor. Use the row for that gable's pitch.
4. Double it. Both rakes are the same length on a symmetrical gable. If one side drops lower than the other, measure each side's run separately.
5. Add it to the eaves. Leave out the width you just replaced. The gable's base is wall, not lit edge.

A worked example. Say a house front has 60 feet of level eave plus two front gables, one 24 feet wide and one 18 feet wide, both at 16/12. The wide gable's run is 12 feet, times 1.667, is 20 feet per rake, so 40 feet for the pair. The narrow one works out to 30 feet. Total lit roofline is 130 feet.

Had you added the gable widths instead, you would have written down 102 feet, call it 100. At $22–34 per lit foot, 100 feet reads as $2,200–$3,400 for permanent lighting. The real 130 feet is $2,900–$4,400. Birmingham's two-story colonials typically land at 120 to 180 feet of front roofline, which you can see priced on the Birmingham page.

Peaks and valleys: counting them

Footage sets the size of the job. Peaks and valleys set how much handwork goes into it, and they are why two houses with the same number of feet can sit at different points in the $22–34 band.

A peak is any point where the lit line climbs and then turns downhill: the top of a gable, a garage gable, a porch gable or a dormer. At each one the rigid channel has to be cut to the exact angle of that pitch on both sides and joined, with a ladder set high on the gable to do it. A steeper gable means a sharper joint and a longer climb.

A valley, for lighting purposes, is a low point where the line comes down one rake and starts up the next. It shows up wherever two gables sit side by side. Often the rake of one gable runs into the roof of the other instead of meeting it cleanly, so the run stops at one board and starts again at the next.

To count your own:

• Count one peak for every apex the lit line will go over, including dormers and garage gables you plan to light.
• Count one valley for every dip between two lit peaks.
• Leave out gables you are not lighting. Their peaks do not count.
• Leave out hip corners. The diagonal seams on a hip roof are shingled and never carry a channel.

Rochester Hills is the clearest local example. The hillside subdivisions there often stack three or more gables across one front, and a side-entry or three-car-garage colonial there runs 150 to 250 feet, as the Rochester Hills page shows. With that many peaks, those homes tend to sit higher in the band than a plain colonial of the same length.

How LightCast counts peaks

The LightCast measuring tool does most of this from one photo. Stand across the street, shoot the front of the house straight on, and tap something whose size you know. Garage doors make the best ruler: 16 feet wide for a double, 9 for a single.

• Rakes are measured along the slope. The tool traces the roofline where it actually runs, so a front-facing gable's rakes come through at full length with no multiplier needed.
• Peaks and valleys are counted from the trace. Every point where the line rises and falls is a peak, and every dip between two rises is a valley. The result shows both counts next to your footage.
• The price range covers the full band. The footage and peak count ride along with your estimate request, and they are how we place your house within the band.
• The photo gets processed in the browser and is not uploaded anywhere. Everything runs in the browser.

Two limits are worth knowing. A photo from the front cannot see side-facing rakes or gables at the back, so add those with the ground method. And if a tree hides a peak, manual mode lets you place the line by hand along that gable. For tips on getting a photo the tool reads cleanly, see our guide to measuring your roofline from a photo, and for the level eaves and overall totals, how many feet of lights your roofline needs. Staggered rooflines have their own post on lighting a split-level.

Once you have your number, request your estimate with the footage and peak count attached. We confirm within 24 hours, and every gable is measured again on site before any channel is ordered, so the channel is cut to the confirmed length, not the photo estimate.

Where this applies

Typical roofline lengths and priced examples for these communities are on their own pages.

• Birmingham
• Rochester Hills
• Royal Oak

Sources

1. TAMCO (Thompson Architectural Metals Company): Roof slope factors tamcometalroof.com
2. Roof Online: Roof pitch factor table and calculator roofonline.com
3. InterNACHI: Measuring roof slope and pitch nachi.org
4. Apple Support: Use iPhone as a level support.apple.com