Gutter overflowing in heavy rain? Watch one storm and write it down

Able Roofing, a gutter and roofing contractor, puts it in one line: gutters overflow because of too much rain, too few gutters and downspouts, or clogged gutters and downspouts. More than one can happen at once. This Old House lists the sizing mistakes behind the second reason: the roof area was underestimated, the pitch was ignored, local rainfall wasn't considered, or there were too few downspouts.

Source: Able Roofing, why gutters overflow, checked October 16, 2026; This Old House gutter and downspout sizing guide, checked October 16, 2026.

You don't need to know which one it is. The installer will work that out. What helps is a clear note of what you saw, plus the run's numbers, so the installer starts from what actually happened.

1. Pick your window

   Watch from inside, or from a covered porch, while the rain is at its heaviest. Stay off ladders and the roof.

2. Name the run

   Use the names on your sketch or gutter log: front, back, garage, porch.

3. Name the spot

   Is it spilling next to the downspout, partway along the run, or at an inside corner under a valley where two roof slopes meet? Give it a letter: A, B or C.

4. Take a short video

   Ten seconds through the window, with something in frame that shows which part of the house it is.

5. Write one line

   Date, how hard it was raining and for how long, the run, the spot, and where the water landed.

Take the run that spilled and put it through the gutter tool. You need its length, how far the roof reaches back, the rise, and any upper roof that drains onto it. The measuring guide shows how to take all of those from the ground.

This Old House rates a 2 by 3 inch downspout at about 600 sq ft of roof and a 3 by 4 inch at up to 1,200. Say the back run is 40 ft long under a 6 in 12 roof that reaches back 15 ft, and a 300 sq ft piece of upper roof spills onto it. That's 600 + 300 = 900 sq ft, times 1.1 for the pitch is 990 sq ft. One 2 by 3 inch downspout covers about 600 of that. The tool says two 2 by 3 or one 3 by 4. If the run has a single 2 by 3, that goes on the note.

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

• Count the downspouts on the run from the ground and write the number down.
• Note their size if you can tell: a 2 by 3 inch downspout is visibly narrower than a 3 by 4.
• A run of 60 ft or more should have at least two outlets, according to This Old House. One outlet on a long run is worth a line on the note.
• Look at the roof above the spot. A valley, or an upper roof dropping water there, means more water arrives at that spot than the rest of the run gets.

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

Able Roofing recommends cleaning gutters at least twice a year, and more often with trees nearby or in a rainy climate. Find the last cleaning date on your gutter log or service log. If there isn't one, or it was over six months ago, book a cleaning before you book anything bigger, and say on the note when the last one was.

Source: Able Roofing, why gutters overflow, checked October 16, 2026.

Able Roofing points to larger gutters and downspouts in rainy areas, and to adding downspouts. Which one your house needs is the installer's call. Your note helps them decide. Give them:

Source: Able Roofing, why gutters overflow, checked October 16, 2026.

• The storm line: date, how heavy, which run, which spot, where the water went.
• The video.
• The run's numbers from the tool: length, roof area after the pitch factor, downspouts it calls for, downspouts it has.
• The last cleaning date, and who did it.

When the work is done, add a line to the gutter log: what was done, who did it, the date and the invoice number. Then watch the next heavy storm and write down what you see. That tells you whether it worked.

• Tip 01

  Watch the heaviest half hour

  Overflow shows up when the rain is heaviest, so check the forecast and be at the window when the downpour peaks.

• Tip 02

  One spot, one letter

  Letters keep the note short, and they're easy to circle on a printed photo of the house for the installer.

• Tip 03

  Watch the same spot twice

  If one spot overflows in two storms a few weeks apart, write down both dates. A pattern is more useful to the installer than a single storm.

• Tip 04

  Book cleanings by the calendar

  Put two gutter cleanings a year on the calendar, and more if trees hang over the roof, so the clog question has an answer before the storm.

  Source: Able Roofing, why gutters overflow, checked October 16, 2026.

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

• SystemGutter and downspout recordOne page for your gutters: measure each run from the ground, check the size and downspout count, book spring and fall cleanings, note every spill.8 steps · 5 parts · 1 page to printOpen the system

• Chapter 01Gutter and downspout sizingMeasure a gutter run and its roof from the ground, apply the pitch factor, and get the gutter size, downspout count and slope, plus a printable gutter log.Open the tool
• ProblemNo size printed on my air filter: how to find itThe print on the frame is gone or the slot is empty. Measure the opening, round to the nearest inch, and know when you need a custom size.Read the guide
• Practical noteWhat to change on move-in day: locks, garage opener and codesNew house, old keys. Here's the order to work in on move-in week: count what you got, change the locks, clear the garage opener, reset codes, then cut copies.Read the guide
• Chapter 01All records toolsPrintable home records with a calculator behind each one: appliance and warranty log, HVAC filter sizes and change dates.Open the chapter

• The Most Common Reasons Gutters Overflow, Able Roofing. Checked October 16, 2026.
• How To Size Gutters and Downspouts, This Old House. Checked October 16, 2026.

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.