The short answer
Measure every roof edge you want lit, in feet, and add them up. Flat, horizontal edges can be measured on the ground right below them. Sloped edges, like the sides of a gable, are longer than they look from the ground, so you measure the width and height of the gable and work out the slope with a simple formula below.
You don't need to climb onto the roof for any of this. A tape measure or measuring wheel, a phone camera and a few minutes on a map will get you a number that's close enough to buy lights or compare quotes.
What to measure
Start by deciding which edges you actually want lit. Most people start with the parts you can see from the street, so walk to the sidewalk and look at the house the way your neighbours will. Then list each edge:
- Eaves: the flat, horizontal edges along the bottom of the roof.
- Gables and rakes: the sloped edges that run up to a peak. Each gable has two of them.
- Peaks and ridges: the top line of the roof, if you want it outlined too.
- Garage roofs, porches and dormers: smaller rooflines that are easy to forget but add up quickly.
Measure the roof edge itself, where the lights will hang, not the wall below it. The roof usually overhangs the wall, so a measurement taken along the wall needs a little added at each end. Write each edge down separately; it makes it easier to change your mind about what to light later.
Measuring flat edges from the ground
For horizontal eaves, the edge directly above you is the same length as the ground below it. Walk the line under the eave with a tape measure or a measuring wheel, from one corner to the other.
- Stand under each corner and look straight up to see where the roof edge actually ends, then measure to that point rather than to the corner of the wall.
- Measure each straight run on its own and note it down, rather than trying to measure around a corner in one go.
- Skip edges you won't light, such as the back of the house or a side that faces a fence.
Measuring sloped gables without a ladder
A sloped edge is always longer than the flat width under it, so measuring along the ground will come up short. You can work it out from two numbers: the width of the gable at its base and how tall the gable is from that base to the peak.
Measure the width on the ground under the gable. For the height, take a photo of the gable from straight in front, then compare the height to the width on your screen. If the gable is 24 ft wide and its height looks like a quarter of the width, it's about 6 ft tall.
Each sloped side is then the square root of (half the width squared + the height squared). For that 24 ft wide, 6 ft tall gable: half the width is 12 ft, 12² + 6² = 180, and the square root of 180 is about 13.4 ft. Both sides together come to about 26.8 ft, almost 3 ft more than the 24 ft you'd get measuring along the ground.
If you know your roof pitch (written as rise over 12, such as 6/12), you can instead multiply the flat width by these factors:
- 4/12 pitch: about 1.05
- 6/12 pitch: about 1.12
- 8/12 pitch: about 1.20
- 10/12 pitch: about 1.30
- 12/12 pitch: about 1.41
Steeper gables add more footage, which is why a house with several front-facing peaks needs noticeably more lights than its width suggests.
Measuring your roofline on a map
If you'd rather not walk around with a tape, a map works for a first estimate. Google Maps has a Measure distance tool: on a computer, right-click your starting point, choose Measure distance, then click along the roof edge to add points. It shows the total distance as you go.
Switch to satellite view and trace each edge you want lit. Keep two things in mind:
- A satellite image looks down from above, so sloped gable edges show up at their flat width. Measure the gable's width on the map, then use the formula or pitch factors above to get the real length.
- Trees, shadows and the angle of the photo can hide or shift the roof edge, so treat a map measurement as an estimate and check one or two runs with a tape if you're buying lights to the foot.
If you try a phone measuring app instead, check its result against a tape measure on one run before trusting it for the whole house.
A worked example
Say the front of a house has a main eave of 46 ft, a garage eave of 20 ft and two front-facing gables, each 24 ft wide and about 6 ft tall.
- Main eave: 46 ft.
- Garage eave: 20 ft.
- Two gables at about 26.8 ft each (both sloped sides): about 53.7 ft.
- Total: about 120 ft of roofline.
That's the kind of number our cost guide uses for a bungalow (about 120 ft); it puts a typical two-storey home at about 180 ft and a large home at about 300 ft. If your total is far off those, it's worth re-checking your gables.
Turning feet into how many lights you need
Once you have your total, check the box for the lit length of each string, which is often shorter than the full cord length once you take off the lead and plug ends. Divide your roofline total by the lit length and round up. With 120 ft of roofline and strings that light 25 ft each, you'd need 5 strings.
Plan where the power comes from too. Measure from your nearest outdoor outlet to where the lights start; that gap is covered by an outdoor-rated extension cord, not by more lights. The Electrical Safety Authority advises following the manufacturer's instructions and connecting no more than three light strings together, so plan how a long roofline will be split into shorter runs from your power source.
The ESA also advises using lights rated for outdoor use with a Canadian certification mark from an accredited agency, GFCI protection for outdoor connections, and proper clips rather than staples or nails, which can damage cords.
Stay safe while you measure
Measuring is one of the few parts of a Christmas light job you can do entirely from the ground, so keep it that way where you can. If you do need a ladder to check an edge:
- Check for overhead power lines first and stay at least 3 metres away, as the ESA advises.
- Set the base 1 ft out from the wall for every 4 ft of height, and extend the ladder at least 1 m (3 ft) past the edge you're leaning it on, following CCOHS ladder guidance.
- Keep three points of contact, don't work from the top three rungs, and move the ladder instead of overreaching.
Skip the tape measure
Our instant price tool does the measuring for you. Enter your address, it finds your house on the map, measures the roofline and shows a year-one estimate right away, with the lights included. It's an estimate only: the final price is confirmed with a free measurement before any work starts, and taxes are extra.
Home Guardians prices Christmas light installation by the linear foot. Year one starts at $8 per foot for warm or cool white and $11 for RGB colour, with the lights included and yours to keep, and that covers installation, mid-season fixes, January takedown and off-season storage. Following years are half the year-one price ($4 per foot for white, $5.50 for RGB, $200 minimum). We install across Vaughan, Maple, Woodbridge, Kleinburg, Concord and Thornhill, plus Richmond Hill, King City and nearby areas. See the Christmas light installation page for the full service.
Install slots fill up by mid-November. If you're still deciding whether to do it yourself, our guide to hiring a pro or doing it yourself walks through what each involves.