Flat Roof Drainage Outlet: Why One Alone Needs an Overflow

A flat roof drainage outlet is the smallest part of the roof and the one that decides whether it survives a blocked week in November. On most domestic extensions and garages, all the water from the roof leaves through one outlet or one gap in the upstand. When a handful of leaves, a moss clump or a tennis ball sits over that hole, the roof stops being a roof and starts being a shallow tank, held in by the very upstands that were built to keep water out.

This page explains how outlets are sized, what the regulations and the drainage standard actually say, why a roof that drains to a single point needs a second way out, and what changes when a new warm deck lifts the roof surface by 150mm. It is written for homeowners reading a quote, not for drainage engineers, but the numbers are the real ones.

How water leaves a flat roof

A “flat” roof is never truly flat. It is laid to falls so the water runs to one of four places:

• An external gutter at the low edge. The commonest domestic arrangement: the roof falls to one side and discharges over a drip into an ordinary eaves gutter.
• A chute or scupper through a parapet. A rectangular opening in the upstand, discharging into a hopper head and down a pipe. Common on roofs with a parapet wall on the low side.
• An outlet at a low point in the roof. A round outlet, usually with a leaf guard or gravel guard, feeding a pipe that runs down inside or outside the building.
• A gutter formed within the roof itself, such as a valley between two roof areas, which then drains to outlets.

The HR Wallingford manual for the design of roof drainage systems, the standard guide to BS EN 12056-3, describes the last three. Each has a different failure mode, but they share one: when the single exit blocks, water has nowhere else to go.

What the rules actually say

Approved Document H, the Building Regulations guidance for England, covers rainwater in its section H3. Three paragraphs matter here:

• Where a design includes drainage from flat roofs, parapet gutters or valley gutters, and over-topping would have particularly high consequences, such as water entering the building or wetting insulation, it should be designed to BS EN 12056.
• Gutters should be laid so that any overflow beyond their design capacity, caused by above-normal rainfall, discharges clear of the building, reducing the risk of water entering it or of structural overload.
• On flat roofs, valley gutters and parapet gutters, additional outlets may be necessary.

The text is in paragraphs 1.2 and 1.7 of Approved Document H.

BS EN 12056-3 is the drainage standard those paragraphs point to. Its UK National Annex recommends that flat roofs are normally designed for a two-minute storm with a one-year return period, and notes that a maximum design water depth of 35mm at outlets has traditionally been accepted for nominally flat roofs laid to falls.

Manufacturers go further. Marley Alutec, a UK maker of aluminium rainwater systems and roof outlets, states that emergency overflows should be included in the design of all flat roofs and balconies with perimeter upstands greater than 50mm. That describes almost every domestic flat roof with an upstand, because BS 6229:2025, the flat roofing code of practice, asks for the waterproofing to turn up at least 150mm above the finished roof surface at abutments.

Put together, the guidance points one way: if water can be trapped by an upstand, a single outlet on its own is not a complete design.

Sizing an outlet: the arithmetic

Flow off a roof is simply area times rainfall intensity. A flat roof counts at its plan area.

The table in Approved Document H for eaves gutters gives a sense of scale. It allows a 115mm half-round gutter with a 63mm outlet to drain 53 square metres of flat roof at 1.11 litres per second, which works out at about 0.021 litres per second for every square metre. On that basis:

Those flows are small, and a single outlet can carry them with room to spare when it is clear. The capacity of a roof outlet depends on how much water can build up over it, which is why outlets set into a sump, a small recess in the deck, carry much more than outlets set flush.

For a chute through a parapet, the HR Wallingford manual gives a formula for the width of opening needed:

Width (mm) = 24,000 × flow (l/s) ÷ head (mm)^1.5

where the head is the depth of water allowed at the opening. Taking the traditional 35mm design depth, a 20 m² roof needs a chute about 50mm wide and a 40 m² roof about 100mm wide. The manual adds that the opening should be at least 25mm taller than the design water depth. The same formula sizes an overflow weir.

Why one outlet is never enough

The flow figures above assume the outlet is clear. The realistic risk on a domestic roof is not a storm that exceeds the design, it is a blockage during ordinary rain: leaves in autumn, moss washed off an adjacent pitched roof, gravel from a ballasted finish, a nest or a lost ball.

Once the outlet blocks, the depth on the roof rises until it reaches the lowest point it can escape from. On a roof with 150mm upstands and no overflow, that lowest point may be the top of the upstand, the flashing at a wall, or the threshold of a door onto a balcony.

Water weighs a kilogram per litre, so the load adds up quickly:

• 35mm of water, the traditional design depth, is 35kg on every square metre.
• 100mm is 100kg per square metre. On a 20 m² garage roof, that is two tonnes.
• 150mm, water up to the top of a standard upstand, is 150kg per square metre, or three tonnes on the same roof.

That is far beyond the load a small timber flat roof carries on a normal day, and it bears down hardest at mid-span, where joists deflect, which deepens the pond, which adds more load. Before any structural issue, the water finds laps, poorly dressed upstands and door thresholds. A flat roof that only leaks after a wet spell is often a blocked outlet with no overflow.

Where the overflow goes

An overflow is a second exit set higher than the primary outlet, so it only runs when the primary cannot cope. The HR Wallingford manual gives three reasons to fit one, and a good domestic design gets all three:

1. A warning. An overflow that discharges visibly, through the face of a parapet or over an edge, tells you the main outlet has blocked. Water pouring from a spout that is normally dry is an unmistakable prompt to go and clear it.
2. Protection. It stops the water rising to the top of the upstand or a door threshold.
3. Capacity for rare storms, so the main system does not have to be sized for the worst downpour in decades.

In practice that means:

• Set it above the primary outlet but well below the lowest weak point, which is usually a door threshold or the top of the lowest upstand.
• Put it through the upstand or parapet as a rectangular weir or a round spout, sized with the same formula as a chute.
• Make it discharge clear of the building, as Approved Document H asks, not down the face of the wall or onto a path people use.
• Do not connect it to the same downpipe as the primary outlet. A blockage lower in the shared pipe then disables both.

When a warm deck raises the roof by 150mm

Re-roofing as a warm deck, with insulation laid over the deck and the waterproofing on top, is now the normal way to bring an old flat roof up to modern thermal standards. It also lifts the finished surface by around 150mm, sometimes more with a tapered scheme, and the drainage has to follow. Our warm roof vs cold roof guide explains why the build-up goes above the deck.

Four things change:

• The outlets have to rise with the surface. An outlet left at the old deck level becomes a hole at the bottom of a 150mm pit that the new membrane cannot dress into properly. The roofer should fit outlets designed for the new build-up, with the membrane bonded to the outlet at the finished surface.
• Chutes through parapets have to be re-cut. A scupper that was at roof level is now 150mm below the new surface, and nothing will drain through it. It needs forming again at the new level.
• The 150mm upstands are measured from the new surface. BS 6229 requires the upturn to finish at least 150mm above the finished roof, so a warm deck can run out of wall below a window sill or a door. At an accessible threshold that needs step-free access, the code allows 75mm, but that still has to be designed.
• Falls have to reach the outlet. BS 6229:2025 now asks for a minimum completed fall of 1:80, and for small roofs of 50 m² or less it asks designers to aim for 1:60. If the old joists fall the wrong way, a tapered insulation scheme can correct it; see our tapered insulation guide.

The overflow is the item most often left out in a warm deck quote, because the old roof never had one. It should be on the drawing.

Keeping outlets clear

• Check outlets and leaf guards in late autumn, after the leaves are down, and again in spring.
• Clear the gutter or hopper the outlet feeds as well; a blocked downpipe backs up to the roof. Our gutter cleaning cost page covers what a clear-out involves.
• On gravel-ballasted roofs, keep the gravel guard in place and the stones clear of the outlet grate.
• After heavy rain, look for water that is still standing on the roof two days later, and for water marks on the upstands. Both point to a fall or outlet problem. Our guide to flat roof problems and repairs covers what to do next.

Questions to ask your roofer

1. How many outlets does the design have, and where is the overflow?
2. What level is the overflow set at, relative to the outlet and the door threshold?
3. Are the outlets being replaced to suit the new build-up, or reused?
4. Will the finished fall to each outlet meet BS 6229:2025?
5. Where does each outlet and the overflow discharge?

If the answer to the second question is “there isn’t one”, the quote is incomplete. For the rest of a flat roof quote, see how to read a roofing quote.

Frequently asked questions

Does a flat roof need an overflow? Where the roof is enclosed by upstands or a parapet and drains to one outlet, yes in practice. Approved Document H asks for overflow to discharge clear of the building and notes that flat roofs may need additional outlets, and outlet makers such as Marley Alutec specify emergency overflows on all flat roofs with upstands above 50mm.

How many outlets does a flat roof need? Enough to carry the design flow, which on a domestic roof is usually one, plus an overflow as the second exit. The design flow is roughly the roof area times 0.021 litres per second per square metre, so a 20 m² roof produces about 0.42 litres per second.

What size should a flat roof outlet be? It depends on the roof area and the depth of water allowed over the outlet. Approved Document H’s gutter table pairs a 63mm outlet with up to 53 square metres of flat roof; purpose-made flat roof outlets publish their own capacities, and setting an outlet in a sump increases its capacity considerably.

Why is water pooling on my flat roof? Usually because the fall is too shallow or the deck has sagged, or because the outlet is partly blocked. BS 6229:2025 asks for a minimum finished fall of 1:80, and 1:60 on small roofs. Water that stays for days after rain should be investigated before it finds a lap or a seam.

Where should a flat roof overflow discharge? Clear of the building, through the face of an upstand or parapet where it can be seen. It should not run down the wall, onto a path, or into the same downpipe as the main outlet.

Do I need to change the outlets when I insulate my flat roof? Almost always. A warm deck raises the roof surface by around 150mm, so the outlets must be replaced or extended to the new level and any chutes through parapets must be re-formed. It is also the right moment to add an overflow if the roof never had one.