How to measure your roofline without getting on the roof

Measure every roof edge you intend to light, including both sloping sides of each gable. The method below estimates the run from ground-level measurements; confirm uncertain dimensions before ordering.

Take the printable roofline worksheet outside with you. For worked kit-size decisions, see whether 100 ft is enough.

What you need

A twenty-five foot tape measure, a helper to hold the other end, and about ten minutes. A phone is useful for one step. A ladder is not required and, for this, not recommended — every measurement here is taken at ground level or read off a photograph.

Write the numbers down as you go. There are only four of them.

Measure the front eave

The eave is the horizontal edge of the roof, the one the gutter sits on. That is where a permanent run goes.

Stand at one end of the house and run the tape along the ground, following the line of the roof edge above you rather than the line of the wall. Those are not the same: the roof overhangs the wall, usually by twelve to eighteen inches at each end. Look up, pick the point directly under where the roof edge stops, and measure to there.

If the front is broken up — main house, then a garage set back, then a porch — measure each straight horizontal run separately and add them. A house with seventy feet of frontage often has two eave sections of forty-four and twenty-six feet with a gable between them.

Do not include anything you do not intend to light. Most people light the front eave, both rakes of the front gable, and the porch. Sides and back are usually neither visible nor worth the cable.

Get the gable width from the ground

A gable is the triangle of wall under the peak. Its width is the horizontal distance between the two bottom corners of that triangle — again following the roof edge, not the wall.

Measure it on the ground, directly underneath. If obstacles prevent a reliable measurement, use building plans or ask an installer to confirm it. Siding and brick dimensions vary; do not use a generic course size as a tape measure.

Estimate the pitch from the ground

Pitch is written as rise in twelve: a 6/12 roof climbs six inches vertically for every twelve inches horizontally. You need it because it tells you how much longer the sloping rake is than the flat distance underneath it.

Stand square to the gable — not at an angle, directly in front of it — and take one photograph with the whole triangle in frame. Then, on the photo:

1. Measure the width of the triangle on screen, in whatever units your fingers or a ruler give you. Call it W.
2. Measure the height of the triangle, from the eave line up to the peak. Call it H.
3. Pitch in twelfths is H ÷ (W ÷ 2) × 12.

So a gable that measures 90 mm wide and 22 mm tall on screen is 22 ÷ 45 × 12 = 5.9, which is a 6/12 roof. The photo does not need a scale, because you are measuring a ratio.

A ground-level photograph is an estimate: perspective can distort the ratio. If the pitch is uncertain, confirm it from plans or an installer. Do not assume 6/12 is conservative for a steeper roof.

The rake arithmetic

The rake is the sloping edge of the roof running from the eave up to the peak. There are two of them per gable, and they are the part everyone leaves out.

Each rake is longer than the flat distance under it by a factor that depends only on the pitch:

pitch factor = √(1 + (rise ÷ 12)²)

So the rake run for one gable is:

(gable width ÷ 2) × pitch factor × 2

Half the width, because each rake covers half the span. Times the factor, because it slopes. Times two, because there are two of them.

A thirty-foot gable at 6/12: half is fifteen feet, times 1.118 is 16.77 feet per rake, times two is 33.5 feet. That is thirty-three and a half feet of run that is invisible on a tape laid across the driveway.

Put it together

Take the example house: seventy feet of eave in two sections, one thirty-foot gable at 6/12, and a twelve-foot porch edge.

Round up to the foot and you are buying for 122 feet.

A hundred-foot kit does not cover this house. That is the discovery that would otherwise have happened on a Saturday afternoon with the packaging already open.

The calculator does exactly this arithmetic, with the same pitch factor, if you would rather type four numbers than do it on the back of an envelope.

Why the five per cent allowance exists

It is not waste in the offcut sense. It is the difference between a tape lying flat on a driveway and a strip of light fastened to a real roof edge.

The strip has to turn corners, and a turn eats an inch or two of slack. It has to run down from the end of the eave to reach the outlet. It has to get past a downpipe bracket, a satellite mount, a bit of fascia that is not straight. And your own measurement has an error bar on it, because you were looking up at a roof edge and estimating where the ground point below it was.

Five per cent covers all of that on a normal house. On a fussy front with several returns, dormers, and a bay, use ten.

What people get wrong

Measuring the wall instead of the roof edge. The overhang is real and it is usually two to three feet in total across the front.

Forgetting the rakes, or counting only one. This is the single most common error and it is worth thirty feet on an ordinary house.

Measuring the rake as the straight line from eave to peak on the ground. That distance is the run, not the rake. The rake is longer, by the pitch factor. That is what the factor is for.

Rounding down. Kits come in fixed lengths. If your number is 121 feet, you are not buying 121 feet of anything.

Assuming a run can jump a gap. A permanent run is a continuous strip. Two eave sections separated by a gable are one continuous path over the gable, not two independent pieces you can total loosely.

Buying to the exact number. See the allowance above. Being three feet short is not a three-foot problem, it is a returns problem.

Then go and look at it

The number tells you what to buy. It does not tell you whether twenty-inch spacing is what you want on your particular fascia, or whether the gable reads well lit at all — some do not.

That takes a picture of it lit, which is why every kit we cover is rendered on the same three houses. Related reading: what permanent lighting actually costs against string lights.

Check different gables separately

The calculator repeats one width and pitch for every gable entered. If yours differ, work out each gable separately on the worksheet and add the results. Mark cable gaps separately, then check the kit’s extension rules; a footage allowance does not prove that a cable route is supported.

Price tiers are our own rough budget bands, not live prices. Check the listing for today’s price.