Building components · Roof Carcass & Edges
Back Gutter
Explains the back gutter as an interception channel within a roof slope, and separates it sharply from the eaves gutter that collects water leaving the roof entirely.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a back gutter is
Where anything passes through a sloping roof — a chimney, a rooflight kerb, a wall abutment — water running down the slope arrives at the obstruction's upper face and has nowhere to go. The back gutter is the channel formed across that face to catch the water and lead it sideways past the obstruction, where it rejoins the covering and continues down the slope.
It is one of the few places on a pitched roof where water is deliberately turned across the slope rather than allowed to run down it. That makes it an exception to the way the rest of the roof works, and it sits in the most sheltered position on the roof, which is also the position where debris collects and where nobody looks.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a gutter in the eaves sense. The published gutter entry describes a channel at the edge of a roof that collects water once it has left the covering and takes it out of the roof into a downpipe. A back gutter sits within the slope, upstream of any edge, and gives the water back to the covering rather than removing it from the roof.
- It is not a valley. A valley is formed where two roof slopes meet and carries water down the line of their intersection; a back gutter runs across the slope at the high face of an obstruction and discharges at both of its ends.
- It is not a flashing. A flashing sheds water across a junction from above and holds nothing; a back gutter is a channel that collects water along its length and conveys it. Flashings and soakers commonly close and lap into a back gutter, which is a different role from being one.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- chimney back gutter
- abutment gutter
- rear gutter
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Having somewhere to discharge at each end, since a channel that catches water and does not release it is a reservoir against the obstruction.
- Whether it falls toward its discharge points or is formed level, since a channel in this position is rarely visible enough for standing water to be noticed.
- How it laps with the covering above it and below it, because the covering has to deliver water into it and then receive it again.
- Being kept clear, given that its position on the sheltered upslope face of an obstruction is precisely where leaves, moss and grit accumulate.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The channel concentrates water from the whole slope above the obstruction into a short length, which is why this is one of the higher-flow positions on an otherwise unremarkable roof.
Water in the channel is close to the junction with the obstruction, so the way the channel is turned up against that face and closed above is what keeps the water in the channel rather than behind it.
It is invisible from the ground, it collects debris by virtue of where it sits, and reaching it means access onto the roof — so it is commonly first seen when there is already a stain on a ceiling.
It is formed early, under the covering around it, by a trade working at a junction that will be covered before anyone else sees it.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Clarifications
Commonly misunderstood
- That flashing a chimney means treating four sides the same way. The lower face sheds water away and the sides carry it down the slope, but the upper face has water arriving at it, and that face needs a channel rather than a cover.
- That it clears itself in heavy rain. The sheltered upslope face of an obstruction is where debris settles, and the channel's shape is what holds that debris in place once it arrives.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Where does the back gutter discharge at each end, and what does the water meet when it gets there?
- How is the channel turned up against the obstruction and closed at the top so water cannot pass behind it?
- How do the coverings above and below the obstruction lap into and out of the channel?
- How is the back gutter reached for inspection and clearing, and how often is that expected to happen?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes an element and what it does. It states no width, depth, fall, lap, upstand height or material gauge, and none can be inferred from it.
- Work at a chimney or abutment involves working at height on an existing roof and commonly touches the roof covering, structure and any flue; it is work for qualified trades rather than something a reference entry prepares anyone to carry out.
Roof Carcass & Edges
The battens, boards and edge elements that finish a roof structure and prepare it to receive a covering.
Browse all roof carcass & edges entries →