Who this guide is for
- Owners of buildings with parapets, valleys or concealed gutters
- Renovators altering a roof edge, a parapet or a roof covering
- Self-builders choosing a roof form that puts the collector inside the wall line
- Anyone who has found damp beneath a concealed gutter
- Readers who know eaves gutters and meet a concealed one for the first time
Same duty, opposite consequence
Both collectors do the same thing in normal use: they receive the sheet of water shed by the roof and hold it while it travels towards an outlet. The entry notes that the outlet, rather than the collector profile, usually governs how the collector behaves in heavy rain, and that is true in either position.
What differs is the failure. An eaves gutter that overtops sends water down the outside face of the wall, where it is visible and where the fabric is arranged to be wetted. A collector inside the building line sends it into the construction, where it is not.
The secondary route is promoted, not added
The entry lists an overflow or secondary outlet among the components of roof rainwater drainage and describes it as a designed release point fixing where water goes once the primary outlet is obstructed. On a collector inside the building line, it becomes the difference between visible spillage in a known place and water entering the construction unseen.
That is why the entry names it among the common misunderstandings: an overflow is treated as an optional refinement, when in this position it is the component deciding where a blockage announces itself.
What makes an overflow mean anything
An overflow only has meaning in relation to the outlet's assumed failure. It has to sit where water will actually reach it once the outlet is obstructed, and it has to discharge somewhere that reveals the problem rather than hides it.
The component entry adds the two conditions that are easiest to lose: it has to be independent of the primary drainage, so a blockage in one does not disable both, and it has to be set high enough not to operate normally and low enough to act before accumulation becomes a problem.
- Set at a level water can actually reach once the outlet is obstructed
- Independent of the primary route rather than draining into it
- Discharging where a person using the building would notice
- Checked despite being rarely used, because that is when blockages are found
A parapet turns a roof into a vessel
The junction record for an overflow through a parapet states the mechanism: once the enclosing wall is there, water the outlets are not removing has nowhere to go but upward, and water accumulating on a flat roof is a load rather than an inconvenience.
The opening through the parapet is what fixes the level at which water leaves regardless of what the primary route is doing. It is also a hole through a wall wetted on both faces and on top, so one junction penetrates the roof's weather line and the parapet's own.
Where the gutter lining and the upstand become one problem
The roof drainage entry names the parapet and box gutter upstand as a single continuous problem with the gutter lining. How far water can rise before it passes the upstand, and where it goes when it does, is settled at that junction rather than in the pipework.
So a concealed gutter is not a rainwater goods decision that happens to sit behind a wall. It is a junction between the roof covering, the collector, the upstand and the parapet, and the entries put each of those with a different owner in the documentation.
Reading it by what is absent
The junction record describes how a secondary route is actually read, and every observation is a negative one. Water standing on a roof after rain with nothing running down the external face. Staining, growth or a washed strip beneath an opening that is now dry, showing it ran once and no longer does. Water held above the level the opening was set to hold.
Where the discharge does run, what the building shows is at the other end of it: a wall face, a walkway or a terrace below taking the wash of every event the primary route has missed, in a position nobody chose.
Access, and what silts fastest
Everything in a rainwater route silts, and the parts hardest to reach silt fastest because nobody reaches them. A collector inside the building line, behind a parapet, at height, is close to the definition of hard to reach.
Debris interception moves the failure rather than preventing it: a guard keeps litter out of the closed pipe and concentrates it at the collector or the grating, converting an unreachable obstruction into a reachable one. It only helps if somebody can reach it, which makes access a decision taken with the layout.
What to establish, and who settles it
Whether a particular arrangement is arranged correctly, what level an overflow should be set at and what capacity it needs are determinations for the drainage designer, and overflow provision on roofs is regulated. Nothing here settles any of them.
What an owner or a designer can establish is the position: whether the collector sits inside or outside the building line, what the overflow arrangement assumes about that, where it discharges, how the route is reached, and whether the roof structure has been assessed for the water an overflow allows to stand.
Concealed gutter review checklist
- 1Establish whether each collector sits inside or outside the line of the wall
- 2Identify the primary outlet serving each length of collector
- 3Identify the secondary route and the level at which it operates
- 4Check that the secondary route does not drain into the primary one
- 5Check that it discharges where a building user would see it
- 6Look for staining, growth or a washed strip under an opening that is now dry
- 7Look for water standing above the level an overflow was set to hold
- 8Ask what is beneath the discharge point of every overflow
- 9Ask how the collector and outlet are reached for clearing, and by what means
- 10Ask how the roof waterproofing is taken through the parapet at each opening
- 11Ask whether the roof structure was assessed for the water an overflow permits
- 12Record the catchment assumed for each outlet and the position of every overflow
Common mistakes to avoid
- Treating a concealed gutter as an eaves gutter that happens to sit behind a wall
- Reading the overflow as a refinement rather than as the component that reports a blockage
- Taking the overflow into the same pipe the primary outlet uses
- Setting an overflow above the level a blocked collector can reach
- Concealing the discharge so the warning is never seen
- Enlarging the collector while leaving the outlet as it was
- Leaving no access to a collector that silts faster because it is hard to reach
- Never checking an overflow because it is not running
When to involve a professional
- Overflow provision on roofs is regulated and part of drainage design
- Ask whether the collector sits inside or outside the building line
- Ask what level the overflow sets and what that level was decided against
- Ask what sits beneath the point the overflow discharges to
- Ask whether the structure has been assessed for the water permitted to stand
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why is a parapet gutter treated differently from an eaves gutter?
Because of where it spills. An eaves gutter sits outside the line of the wall and overtops clear of it. A collector inside the building line discharges into the construction when it overtops, which is what promotes the secondary route from refinement to requirement.
My overflow never runs. Is that a good sign?
It may mean the primary route is working, and it may mean the overflow is obstructed. The entries note that overflows are rarely used and therefore rarely checked, and the junction record treats a dry opening beneath an old washed strip as a reading worth making.
Would a larger gutter stop a concealed gutter overflowing?
The entry lists this among its common misunderstandings. Water enters along the length of a collector and leaves at a point, so where the outlet is the restriction, enlarging the collector alone changes very little about how that length behaves in heavy rain.
Is damp below a parapet always a roof failure?
The parapet entry notes that damp beneath a parapet very frequently originates at the coping or at the base junction instead, and that parapet failures are commonly traced to those rather than to the wall. Establishing which is a matter for a professional inspection.
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