Craft Quantities
Plan & Takeoffs

What to check, and in what order

My gutter overflows when it rains hard

Where along the run it spills tells you which problem it is. Overflowing at one spot with the rest clear is a local blockage or a low point. Overflowing along a whole length, only in heavy rain, usually means the outlet or the capacity is the limit — and that is a sizing question rather than a cleaning one.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

A gutter overflows when water arrives faster than it can leave. That makes it a comparison between two rates, and everything below is a way one of them is wrong.

The most informative observation is WHERE. A gutter that spills at one point while the rest of the run carries water is telling you about that point — a blockage, a sag, a joint that has dropped. A gutter that fills and spills along a whole length is telling you about its exit: the outlet, the downpipe, or the capacity of the section itself.

The second is WHEN. Overflowing in every shower points at a blockage or a fall, because the capacity is barely being used. Overflowing only in heavy rain points at capacity or outlet size, because everything works until the rate rises.

Capacity is where people are most often surprised, and it is genuinely a calculation rather than a judgement. The flow a gutter must carry is the plan AREA of roof draining into it multiplied by a design rainfall intensity, and the capacity available depends on the section, the fall, and crucially how many outlets there are and where. A long run with a single outlet at one end carries far less than the same gutter with an outlet at each end — which is why an extension or a converted roof that added area to an existing gutter often starts overflowing without anything having failed.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. A blockage at or near the outlet

    Most common

    Water standing in the gutter after the rain stops, spilling near the outlet, and often visible debris or plant growth at that point.

    How to check: Look into the gutter at the outlet once it is dry. Standing water is conclusive: a clear gutter empties.

  2. Wrong fall, or a sagged run

    Most common

    Water standing at a point away from the outlet, a visible dip when sighting along the run, and spilling at that dip rather than at the ends.

    How to check: Sight along the gutter's front edge against the roof line, and look for standing water when dry. Both show a sag immediately.

  3. Water overshooting the gutter from the roof

    Common

    Spilling on the OUTSIDE edge in heavy rain with the gutter itself not full, usually on steeper roofs, and worst where a valley discharges.

    How to check: Watch during heavy rain. Water arcing over the front of a gutter that is not full is an interception problem, not a capacity one.

  4. The outlet or downpipe is too small

    Common

    The gutter filling along its length in heavy rain only, with water backing up from the outlet rather than standing at a dip.

    How to check: Note whether the water is deepest AT the outlet. An outlet that cannot pass the flow backs the run up toward itself.

  5. More roof draining into it than it was sized for

    Common

    Overflowing that began after an extension, a roof alteration, or a valley being redirected — with nothing having broken.

    How to check: Work out the plan area now draining into that gutter and compare it with what it served before. A change in area is a change in required capacity.

  6. A joint or union that has failed

    Less common

    Dripping or running at a specific joint in all conditions rather than spilling over the edge in heavy rain — a leak rather than an overflow.

    How to check: Whether water is coming from a joint or over the lip. They look similar from the ground and are different problems.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    One point versus the whole length separates a local obstruction from a capacity or outlet problem, and no other observation does that as cleanly.

  2. 2

    Standing water locates a sag or a blockage precisely, because a correctly falling, clear gutter holds none.

  3. 3

    A dropped bracket or a sagged run is visible from the ground and explains an overflow that cleaning will not fix.

  4. 4

    Overshooting is an interception problem — the water never entered the gutter — while backing up is a capacity problem. The remedies have nothing in common.

  5. 5

    Capacity is a calculation, and an overflow that started after an alteration usually traces to area being added without an outlet being added.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Gutter Calculator →Gutter Hanger Spacing Calculator →Roof Squares Calculator →

When to stop and get somebody in

  • Where overflowing water is running down a wall, since prolonged wetting of masonry is how penetrating damp starts and the wall is the expensive part.
  • Where the gutter or its brackets have pulled away from the fascia, which usually means the fascia itself is decayed behind them.
  • Anything on a roof that cannot be reached safely from the ground or a short ladder — gutter work is a common source of falls and the view from below is usually enough to diagnose it.
  • Where the answer turns out to be capacity on a building with a valley, a parapet or an internal gutter, since those overflow inward rather than harmlessly outward.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.