Overflow when outlets block

Every outlet eventually blocks. A leaf mat, a dead bird, a tennis ball, hail packed into a sump, a plastic bag blown onto the roof: none of them appears in the design storm, and all of them happen. When an outlet blocks, the water still arrives, and it will leave by the lowest available path. The only question is whether that path was chosen by the designer or found by the water. A drainage system without designed overflow is one that will, sooner or later, put water into the building.

This chapter designs W-40’s overflow paths. It sets out the principle and the storm they are sized for, makes the rear eaves gutter spill outwards over its front edge, gives each front rainhead an overflow for a blocked downpipe, adds an independent overflow through the parapet for a blocked opening, and makes each overflow visible so that it warns as well as protects. The failure case is a row of overflow slots that a painter sealed, thinking they were defects.

The principle and the storm

The ABCB Housing Provisions require eaves gutter overflow measures for houses to be designed for the 1% AEP storm, and their explanatory information describes the aim: water must be directed away from the building, so that it does not pond against, enter or damage it, even if the drainage system is blocked. W-40 is not a house, but the principle is universal, and this book applies it to every overflow on the building.

The design case for overflow is simple and severe: assume the outlet is completely blocked, and the full 1% AEP flow arrives. Every litre must then leave by the overflow, outside the building line, without the water rising high enough to find another way in.

Figure 1What happens when a rear downpipe blocks in a heavy storm

1. 1Outlet blocksthe 5.6 L/s arriving at that downpipe has nowhere to go
2. 2Gutter fillswater rises along the 6.7 m of gutter draining to the blocked outlet
3. 3Water reaches the front edgethe front edge, 15 mm below the back, is the lowest point
4. 4Gutter spills outwardswater pours over the front edge, away from the fascia and wall
5. 5Neighbouring outlets helpsome water flows past the high points to the next outlets
6. 6Someone noticesthe spill is visible from the ground and prompts a clean

Make the eaves gutter spill outwards

W-40’s rear gutter has its front edge set 15 mm lower than its back, as Chapter 3 described. When the gutter fills, water spills over the front, away from the building, before it can reach the back edge and run behind the gutter into the fascia, wall or roof space.

How much head does the front edge need to pass the overflow? If a downpipe is completely blocked, its 6.7 m of gutter must spill the full 5.6 L/s at 1% AEP. Treated as a long weir, a 6.7 m front edge passes that flow with only about 6 mm of water over it. Even if the spill were concentrated over just 2 m of the front edge near the outlet, the head would be about 14 mm, still below the 15 mm difference between front and back. The front edge works as an overflow, provided it really is lower than the back along its whole length.

That proviso matters. A gutter that has rotated on its brackets, or been set level front to back for appearance, or that sits under a roof whose sheets end lower than the gutter’s back, can overflow backwards. The 15 mm difference is set at installation with a level on every bracket and checked again after the gutter has been loaded with water, as Chapter 8 describes.

Where a building has lined eaves, the overflow must also stay clear of the lining, because water spilling into an eaves space soaks the lining and runs into the wall. Gutter makers offer slotted fronts, raised back brackets and other overflow measures for these cases, and the Housing Provisions list acceptable overflow measures for houses. The principle is always the same: a lower, designed path to the outside.

Rainhead overflow for a blocked downpipe

At the front, each box gutter end discharges into a rainhead. If the rainhead’s downpipe blocks, the rainhead fills. Without an overflow, it would back up through the opening into the box gutter, which would then fill towards its roof-side edge.

Each W-40 rainhead therefore has an overflow lip along its front face, 500 mm long, set 100 mm below the sill of the parapet opening. When the downpipe blocks, water rises in the rainhead and spills over the lip, out from the face of the building and down to the paving below.

Figure 2Head over the rainhead's overflow lip with the downpipe blocked

Q = 1.77 × L × h^1.5, so h = (Q ÷ (1.77 × L))^(2/3)

Flow with the downpipe blocked, 1% AEP

17.4 L/s = 0.0174 m³/s

Overflow lip length L

0.5 m

1. h = (0.0174 ÷ (1.77 × 0.5))^(2/3) ≈ 0.073 m
2. water stands about 73 mm above the lip, 27 mm below the opening's sill

The water in the rainhead stays below the opening’s sill, so the box gutter continues to drain freely into the rainhead even with the downpipe blocked. The spill falls in front of the building’s street face, onto paving that falls away from the wall, where it is plainly visible.

Independent overflow for a blocked opening

A rainhead overflow protects against a blocked downpipe. It does nothing if the opening through the parapet blocks, for example with a mat of leaves and roof debris caught across it. Then the box gutter’s water cannot reach the rainhead at all.

Each end of W-40’s box gutter therefore has a second, independent overflow: a separate opening through the parapet beside the main one, 600 mm long, with its sill set 120 mm above the gutter’s sole at that point, above the highest normal water level of about 84 mm. In normal storms it stays dry. If the main opening blocks, the water rises past 120 mm and spills through the overflow opening into a short spout, often called a spitter, that throws the water clear of the face of the building.

With the full 17.4 L/s spilling through it, a 600 mm overflow needs about 64 mm of water over its sill, so the water in the box gutter rises to about 184 mm above the sole at the end. The gutter is 250 mm deep there, leaving about 66 mm of freeboard below the roof-side edge, above the 50 mm teaching figure from Chapter 5.

Figure 3W-40's overflow paths

	Blockage it protects against	Where the water goes	Sized for
Rear gutter front edge, 15 mm below the back	a blocked rear downpipe	over the front, clear of the rear wall	5.6 L/s per downpipe at about 6 mm head
Rainhead front lip, 500 mm long	a blocked rainhead downpipe	over the rainhead front, onto the street paving	17.4 L/s at about 73 mm head
Parapet overflow opening, 600 mm long, sill 120 mm up	a blocked parapet opening	through a spitter clear of the facade	17.4 L/s at about 64 mm head, 66 mm freeboard

Overflow above doorways

W-40’s rear wall has two roller doors, where forklifts and staff pass constantly. A gutter spilling over its front edge above a doorway drops a curtain of water onto the people and goods passing through it, and onto the threshold, which then floods the floor inside. The designer moved the rear downpipes, as Chapter 3 noted, so that none sits directly above a door, and set a short length of gutter above each door with a slightly higher front edge, so that overflow occurs either side of the opening rather than over it. A small rain diverter above each door head catches any wall run-off and sends it sideways. The rear overflow still works; it simply chooses where to spill.

Hail and debris

Hail is the most common reason a well-designed system overflows. Hailstones pack into gutters and sumps, then melt slowly, and while they melt they behave as a blockage. Box gutters are especially vulnerable, because hail drifts and settles in them. The overflow design, sized for a fully blocked outlet, covers this case. What it cannot cover is hail that blocks the overflow too. W-40’s parapet overflows are deliberately long and open, with no grilles, so that hail and debris pass through them rather than lodging, and their sills are clear of the main opening, so that one drift of hail is unlikely to block both.

Prove the overflow works

Overflows are tested at completion, not assumed. With the roof plumber, the designer plugged one rear outlet with a test plug and flooded the gutter from a hose, watching water rise and spill over the front edge, away from the wall. The same was done for each rainhead, with its downpipe plugged, and for each box gutter end, with the main opening temporarily blocked by a board. In each test, the water left by the intended path and did not approach the roof-side edge of the box gutter. The tests took a morning and were photographed and filmed for the building’s records, with the date, the outlet tested and what was seen. A hose cannot reproduce the 1% storm’s flow, but it shows the path the water takes, which is what matters most.

Keep overflows independent

The most important rule of overflow is that it must not depend on the thing it backs up. An overflow pipe that joins the same downpipe below the blockage is useless if the blockage is in the downpipe. An overflow slot in a gutter that has a leaf guard laid over it is useless when the leaf guard is the problem. W-40’s paths are independent: the rainhead lip does not depend on the downpipe, and the parapet overflow does not depend on the main opening or the rainhead.

Make overflow visible

An overflow that operates silently can hide a blockage for months. Each of W-40’s overflows is visible from the ground: the rear gutter spills in a sheet along the rear wall, the rainhead lip spills onto the street paving, and the parapet spitters throw a stream that anyone passing would notice. The building’s maintenance plan, in Chapter 8, tells staff that any water from an overflow means an outlet is blocked and must be reported the same day. A visible overflow turns a silent hazard into a warning. Staff were shown each overflow during the building’s handover, so they would recognise it running and know whom to call.

Overflow water must land somewhere sensible. The rear spill lands on a concrete apron that falls away from the wall. The street spills land on paving that falls to the kerb. None land on doorways, walkways or in front of electrical equipment, which the designer checked against the site plan and the electrical layout before the drawings were issued.

A failure case: the sealed slots

This is an illustration. On a house in the owner’s family, the eaves gutters had been fitted with a slotted front, a row of small slots along the top of the front face designed to let water spill outwards before it could reach the back of the gutter, because the eaves were lined and the gutter’s front could not be set lower.

When the house was repainted, the painter saw the slots, assumed they were rust holes or manufacturing faults, and filled them with sealant before painting the gutters. They looked perfect afterwards.

The next heavy storm, with an outlet partly blocked by leaves, filled the gutter. With the slots sealed, the lowest path was over the back of the gutter, into the eaves lining. The lining sagged and stained, and water ran down the inside of a wall cavity.

The detection came from the ceiling stain. The investigation found the sealed slots within minutes. The correction was to cut the sealant out of every slot, clean the outlet and add a note to the house’s maintenance file, and a small label inside each gutter end, explaining that the slots are deliberate overflow devices and must never be sealed or painted over.

How this applies to a small Australian business

Assume every outlet will block and design for where the water goes when it does. Size overflows for the rarer storm, the 1% AEP intensity used by the Housing Provisions, with the outlet fully blocked. Make eaves gutters spill outwards, with the front edge lower than the back or an equivalent overflow measure. Give rainheads their own overflow lips, and give box gutters a separate overflow through the parapet that does not rely on the main opening. Keep every overflow independent of what it protects, visible from the ground and discharging where the water will do no harm. And make sure anyone working on the building, from painters to roofers, knows the overflows are deliberate and must be left open.

Exercise: a shorter overflow opening

The architect would prefer W-40’s parapet overflow openings to be 400 mm long rather than 600 mm. With the full 17.4 L/s spilling through, about how deep is the water over the sill? With the sill 120 mm above the sole and the gutter 250 mm deep at the end, what freeboard remains? Is it acceptable against the 50 mm teaching figure, and what could restore the margin?

Worked answer. The head is (0.0174 ÷ (1.77 × 0.4))^(2/3) ≈ 0.084 m, about 84 mm. The water rises to about 120 + 84 = 204 mm above the sole, leaving 250 − 204 = 46 mm of freeboard, just below the 50 mm teaching figure. The 400 mm opening is not acceptable on these numbers. The margin could be restored by keeping the 600 mm length, by lowering the overflow sill slightly, though not so far that it runs in normal storms when the water is about 84 mm deep, or by making the gutter deeper at its ends.