How to measure your roofline for Christmas lights

Short answer: stand in the driveway, list every roof edge you intend to light, measure each one with a laser measure or a tape, and add them. The only edge that needs arithmetic rather than a direct measurement is a gable rake, and even that you can estimate accurately from the ground.

Decide which edges get lights first

This sounds obvious and it is where the number goes wrong. People measure the front gutter, buy for that, and then decide on install day that the gable should be lit too.

Walk to the end of the driveway and look at the house as a stranger would. The edges that read from the street are the ones worth lighting: the main gutter, any front-facing gable rakes, the garage, and the porch. Edges that face a side yard nobody sees are usually not worth the sets.

Measuring horizontal runs

A gutter line is a horizontal run and it is the easy case. Three methods, in order of preference:

1. Laser measure from the ground. Stand at one end of the run, aim along the gutter at the far end, press once. Ten seconds, no ladder. Overcast days are better than bright ones because the dot is easier to see.
2. Tape and a helper. Measure along the ground directly beneath the gutter, which for a horizontal run is the same length.
3. Pacing. Walk it heel to toe with a known stride length. Accurate to within a few percent, which for buying light sets is fine.

Measuring gable rakes, the part people get wrong

A gable rake is the sloping edge that runs from the gutter up to the ridge. It is the hypotenuse of a right triangle, so it is always longer than the horizontal distance underneath it, and estimating it as that horizontal distance is the most common source of a short order.

Two ways to get it:

• Measure it directly with a laser from a position at the base of the rake, aiming up the slope at the ridge. This is the accurate method and it takes seconds.
• Calculate it. Measure the horizontal half-width of the gable and the vertical rise to the ridge, then take the square root of (half-width squared plus rise squared). A gable 12 feet wide at the half and 6 feet tall gives a rake of about 13.4 feet.

This table is straight Pythagoras — the rake is the hypotenuse of a triangle whose base is 12 inches and whose height is the pitch rise. On a common 6:12 roof, a rake is about 12 percent longer than the horizontal distance it spans. On a steep 12:12, it is about 41 percent longer.

Dormers, garages and porches

Dormers are small gables and measure the same way. Two rakes each, usually 4 to 8 feet apiece. Three dormers can quietly add 40 feet to an order.

Attached garages usually have their own gutter and often their own gable. Measure them as a separate structure and add.

Porch roofs are typically short, low and the easiest edge to light — and because they are at eye level, the one where sloppy spacing is most visible.

Write it down properly

Keep the per-edge numbers, not just the total. A sketch of the front elevation with a figure against each edge is worth twenty minutes next November, and it is what lets you extend the display next year without remeasuring the whole house.

With the footage recorded, convert it to sets in how many Christmas lights do I need, and to clips in how many clips do I need.