Is my guttering big enough for my roof?
Capacity depends on the roof area draining into the run, the profile of the gutter and how many outlets it has — and the commonest fix is another downpipe rather than a bigger gutter.
- Difficulty
- intermediate
- Time
- 20 min
- Read
- 2 min
Short answer
Rule out blockages and sags first, because they mimic undersizing exactly. Genuine undersizing shows as overflow along the whole length in heavy rain only. More outlets usually buys more capacity than a wider gutter does, because a run drains through its outlets rather than along its length.
This question normally arrives after an overflow diagnosis has ruled out the easy causes. It is worth being clear that changing gutter size is the last option considered rather than the first, because the same symptom is produced far more often by a partial blockage, a lost fall or a single outlet doing too much work.
Step by step
- Rule out the cheap causes first, properly.A partial blockage, a sagged section holding water, a restricted outlet or a blocked gully all produce overflow that looks like a capacity problem. Undersizing is the diagnosis of exclusion.
- Note whether it only overflows in heavy rain.A system that copes with ordinary rain and fails in a downpour is behaving like one at capacity. One that overflows in any rain almost certainly has an obstruction.
- Work out what area actually drains into the run.Capacity is driven by the plan area of roof feeding that gutter, and the pitch matters too because a steep roof delivers water faster. An extension, a re-roof with heavier tiles, or a roof extended over a new area all change this.
- Count the outlets and look at where they are.A long run with one outlet at one end is the classic constraint. Water has to travel the whole length to leave, and the far end runs deepest. Adding a second outlet often solves it outright.
- Look at the profile, not just the width.Deep-flow and square profiles carry more than a shallow half-round of the same nominal size. Changing profile can gain capacity without changing everything else.
- Get the sizing done properly if you are changing anything.There are published methods relating roof area, rainfall intensity, gutter profile and outlet position to capacity. This is a calculation rather than a guess, and merchants and manufacturers provide the figures.
- Check the downpipe and the drain can take the extra.Increasing gutter capacity moves the constraint downstream. A larger flow arriving at an already marginal gully or soakaway simply relocates the problem to ground level.
- Consider whether the rain has changed rather than the roof.Short intense downpours have become more common. A system that was adequate for decades can be overwhelmed by rainfall it was never sized for, without anything having failed.
Questions people ask
Is a bigger gutter or another downpipe better?
Usually another outlet, because a run drains through its outlets. It is also far less disruptive than replacing the whole run and its brackets.
Do I need to replace matching gutter on the rest of the house?
Only for appearance. Capacity is decided per run, so upgrading the one elevation that overflows is legitimate — though a visible profile change on a front elevation is worth thinking about.