Plain House Manual

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Chapter 01Records · Know-how

How to measure a gutter run and the roof above it from the ground

Quick answer

You need three numbers for each gutter run, and you can take all of them on the ground. Tape the run between ground marks under its two ends. At the gable end, tape the width and halve it: that is how far the roof reaches back to the ridge. Count siding rows from the eaves to the peak for the rise. Pitch is rise ÷ depth × 12, so 7.5 ft over 15 ft is 6 in 12, and that pitch multiplies the area by 1.1. A 40 ft run under that roof drains 600 sq ft, or 660 after the pitch factor.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026; Christmas Light Emporium, rooflines and gables, checked October 9, 2026.

Example5-inch K-style, 1 downspout a run40 ft runs, 3 by 4 inch spouts, 6 in 12 roof

What you're after, and what you need

This Old House works out drainage area by measuring the length and width of each roof section that feeds a gutter run and multiplying them. Then it applies a factor for roof pitch, since a steeper roof catches more rain. You can get all three numbers without a ladder. The Christmas Light Emporium describes measuring rooflines and gables from the ground by counting siding courses, and the same trick works for gutters.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026; Christmas Light Emporium, rooflines and gables, checked October 9, 2026.

  • A 25 ft tape or longer, and someone to hold the other end.
  • Something to mark the ground: stones, stakes or chalk.
  • A pencil and a rough sketch of the house from above, with each gutter run drawn and named.

Tape the run along the ground

  1. Find each end of the run

    Stand under one end of the gutter and look straight up. A run ends at a house corner or wherever the gutter stops, like a porch or a step in the roof.

  2. Mark the ground below it

    Set a stone or a stake right under that end, then do the same at the other end.

  3. Tape mark to mark

    Have your helper hold the tape at one mark and read it at the other, to the nearest half foot.

  4. Write it on the sketch

    Write the length on that run's line. A gutter that turns a corner counts as two runs, one per side.

Source: Christmas Light Emporium, rooflines and gables, checked October 9, 2026.

Three numbers for one gutter run, taken from the ground A two-story gable house seen from the long side and from the gable end. A 40 foot tape line runs between ground marks under the gutter ends. The gable end is 30 feet wide, split into two 15 foot halves for the depth. Twenty siding rows of 4.5 inches make a 7.5 foot rise. Pitch is 7.5 divided by 15 times 12, which is 6 in 12. LONG SIDE: THE RUNGABLE END run 40 ft Ground marks under each end of the gutter, taped mark to mark. rise 7.5 ft20 rows x 4.5 in depth 15 ft15 ft Gable width 30 ft, halved: the depth. 7.5 ÷ 15 × 12 = 6, so 6 in 12
Fig. 1One gable house, three numbers. The run is taped along the ground under the eaves, the depth is half the gable width, and the rise comes from counting siding rows.

Find how far the roof reaches back

The depth is the level distance from the gutter back to the ridge, the way you'd measure it on a floor plan, not along the slope. The pitch factor adds the slope back in later. On a plain gable roof the ridge runs down the middle, so the depth is half the width of the gable end. Mark the ground under both lower corners of the gable, tape between them, and halve it. A gable end 30 ft wide gives a depth of 15 ft for the front run and 15 ft for the back.

Eaves overhang the wall, so tape from your ground marks under the gutter, not from the wall. A foot of overhang on each side changes a 30 ft wall into a 32 ft roof.

Count siding rows for the rise

  1. Measure one row

    At the bottom of the gable wall, where you can reach, measure one row of siding from the bottom edge of one board to the bottom edge of the next.

  2. Count up to the peak

    Stand back from the gable end and count the rows from the eaves line to the peak. Binoculars help on a tall house.

  3. Multiply

    Rows times one row is the rise. 20 rows of 4.5 inches is 90 inches, which is 7.5 ft.

  4. Turn it into a pitch

    Divide the rise by the depth and multiply by 12. 7.5 ÷ 15 × 12 = 6, so the roof is 6 in 12: it rises 6 inches for every 12 inches it reaches back.

  • Brick or stone instead of siding: count courses the same way, and measure a few at the bottom of the wall to get one course.
  • No rows to count: take a photo straight across from the gable, measure the width and the rise on the photo with a ruler, and scale. A 30 ft gable that measures 120 mm wide and 30 mm high in the photo rises 30 × 30 ÷ 120 = 7.5 ft.
  • Old house papers or a roofing invoice that names the pitch, such as 6 in 12: use that and skip the counting.

The pitch only needs to land in the right row

This Old House's table goes up in steps: flat to 3 in 12 is 1, 4 to 5 in 12 is 1.05, 6 to 8 is 1.1, 9 to 11 is 1.2, and 12 in 12 or steeper is 1.3. A rise that's off by half a foot usually keeps you in the same row, so the area doesn't change.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026.

Which roof drains to which gutter Three roof plans seen from above. A gable roof splits along the ridge into a front and a back plane, each 40 by 15 feet, each draining to its own gutter. A hip roof has two long planes and two triangle ends, and each triangle drains to a short gutter. A two-story house has an upper roof plane with an arrow onto the porch roof: add the upper area to the porch run. GABLEHIPUPPER ROOF ONTO PORCH back 40 x 15front 40 x 15 Ridge down the middle.Two planes, two runs. endend Four planes, four runs.Each end is a triangle:half its base times its depth. upper roof porch roof Water from the upper planelands on the porch roof.Add its area to the porch run. Measure every plane level, length x depth. The pitch factor adds the slope later.
Fig. 2Seen from above, each roof plane drains to the gutter on its low edge. A plain gable has two, a hip roof has four, and an upper roof can spill onto a lower one.

Roofs that aren't a plain gable

  • Hip roof: all four sides slope down to a gutter. Each long side drains to its own run. Each end is a triangle: its area is half its base times its depth, entered in the tool as extra roof.
  • A second-story roof that spills onto a porch or garage roof: measure the upper plane the same way, length times depth, and add its area to the lower run as extra roof.
  • A dormer or a small roof over a bay window: length times depth again, added to whichever run it drains to.
Let the tool do the arithmeticGutter and downspout sizingEnter the run length, the depth and the rise (or the pitch). The tool works out the pitch factor, the gutter size, how many downspouts and the slope, and gives you a printable gutter log.Example: 5-inch K-style, 1 downspout a run (40 ft runs, 3 by 4 inch spouts, 6 in 12 roof)Open the tool
A gable end seen from the yard. The siding rows give the rise and the width at the eaves gives the depth, with no ladder.

What the three numbers give you

Using the tool's worked example: a 40 ft run with a 15 ft depth drains 600 sq ft. A 6 in 12 roof multiplies that by 1.1, so the run has 660 sq ft to carry. A 5-inch K-style gutter is rated for 5,520 sq ft, so that's nowhere near the limit. The downspouts are what you size: one 3 by 4 inch downspout is rated for about 1,200 sq ft and one 2 by 3 inch for about 600, so this run needs one 3 by 4 or two 2 by 3.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026.

Measure from the ground, hire out the ladder work

This page covers the numbers only. Hanging, cleaning or adjusting gutters means ladder and roof-edge work. If that isn't a job you do, a gutter installer can do it from your numbers, and renters can hand the sketch to the landlord.

Checklist to print

Take this outside with a tape, a helper and the sketch:

  • House sketched from above, each gutter run drawn and named
  • Ground marked under both ends of each run
  • Each run taped mark to mark
  • Each gable width taped at its lower corners and halved for the depth
  • One siding row measured at the bottom of the wall
  • Rows counted from the eaves to the peak
  • Rise worked out, then pitch = rise ÷ depth × 12
  • Upper roofs, dormers and hip ends noted as extra roof on the run they drain to
  • Numbers entered in the gutter tool and written on the gutter log

Open the printable checklist

Measure a gutter run from the ground

Tips and tricks

  • Tip 01

    Name the runs once

    Front, back, garage, porch: use the same names on the sketch, the gutter log and anything you hand an installer, so everyone talks about the same stretch.

  • Tip 02

    Measure both gable ends

    If the two ends of the house look the same, tape both widths anyway. If they match, you've confirmed the depth; if they don't, the ridge isn't centered and the front and back runs get different depths.

  • Tip 03

    Leave the marks down for an hour

    Keep the stones under the gutter ends until you've entered the numbers. If one looks odd, you can tape it again without starting over.

  • Tip 04

    Steeper means a bigger number

    If you can't decide between two pitch rows, take the steeper one. It's what the tool does too, and it errs toward enough downspouts.

    Source: This Old House gutter and downspout sizing guide, checked October 16, 2026.

What we would do

Three numbers per run, then let the tool do the rest

Sketch the runs, tape each one along the ground, halve the gable width for the depth, and count siding rows for the rise. Put the numbers in the gutter tool and print the gutter log, so the next person who works on the gutters starts from your measurements.

Questions

How do I measure for gutters without getting on a ladder?

Mark the ground under each end of the run and tape between the marks. For the roof behind it, tape the gable width on the ground and halve it, and count siding rows for the rise. That gives the length, depth and pitch.

Source: Christmas Light Emporium, rooflines and gables, checked October 9, 2026.

How do I find my roof pitch from the ground?

Divide the rise by the depth and multiply by 12. Count siding rows for the rise and halve the gable width for the depth: a 7.5 ft rise over 15 ft is 6 in 12.

Do I measure the roof along the slope?

No. Measure it level, the way it looks on a floor plan, and let the pitch factor add the slope back in. This Old House multiplies by 1.1 for a 6 to 8 in 12 roof.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026.

What counts as the roof area for one gutter?

Every roof surface whose water ends up in that gutter: the plane right above it plus any upper roof, dormer or hip end that drains onto it. This Old House measures each section that feeds a run and multiplies its length by its width.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026.

What if my count of siding rows is a little off?

It usually doesn't matter. The pitch factor goes up in steps, so a small miss tends to stay in the same row. When you're between two rows, take the steeper one.

Source: This Old House gutter and downspout sizing guide, checked October 16, 2026.

Part of a system

This page is one part of a bigger job. The system page puts the parts in order.

The tool and more guides

Sources

Every outside number on this page names its source and the date we checked it. See them all on the sources page. Spotted an error? Tell us.

Photos: Spacejoy / Unsplash, Kelly Sikkema / Unsplash, benjamin lehman / Unsplash