Start with the roof, not the gutter

Rainwater goods are bought last. The cladding is chosen, the frame is priced, and gutters and downpipes go onto the schedule as accessories. The order is backwards: the gutter is the one item there sized by a number the buyer does not control, rainfall intensity in the local code times the plan area that drains into it.

Take 100 mm an hour, a common design figure from Southeast Asia to the Gulf: 0.0278 litres a second on every square metre, so a 1,000 m² shed delivers 27.8 litres a second to the eaves. At 150 mm an hour the same roof delivers 42. No profile changes that volume; it only decides whether the water reaches a downpipe or runs down the wall.

One outlet takes less than the pipe suggests

A 100 mm downpipe running full at 1 m/s passes 7.85 litres a second, which looks generous. On a gutter the head is low, the pipe is not full at the top and the velocity is nearer 0.7 m/s, so the useful figure is 5 to 6 litres. On the 27.8 litre roof that is five outlets, not the two usually drawn, about 12 m apart on a 60 m building.

Outlet count drives everything behind it: each is a hole, a seal and brackets, and each can leak. An extra outlet at design stage is cheaper than a wider gutter later, when the fall is already set.

The gutter is an open channel

A 150 mm half-round on a 1:200 fall carries about 6 litres a second with the water 75 mm deep, which is the figure the outlet count is built on. Beyond that it is a deeper profile or more outlets. What does not work is a 150 mm gutter on a 1,000 m² roof, two outlets and a fall set by eye.

Fall is where gutters fail. A run fixed level because the fascia looked straight holds water, and the water sits at the low end against the stop end, where a bead of sealant is asked to do the work of a weld. Set the fall from a string line before the first bracket, keep brackets at 600 mm, and check the line at the far end.

The coil behind the profile

Almost every gutter is roll-formed from Z275 galvanized coil: 0.12 to 2.0 mm thick and 600 to 2,000 mm wide, zinc 30 to 275 g/m², with thickness held to 0.01 mm and width to 2 mm. Z275 means 275 g/m² across both faces, about 19 microns a side, the coating that must survive ladders, roof grit and twenty years of wet and dry. The SECCN2 sheet carries rainwater gutter on its own application list, next to roof tiles and garage doors.

Gauges run 0.5 to 0.7 mm, and water does not choose between them. The ladder that leans on the gutter does, and so does snow leaving a metal roof in one sheet. A 0.5 mm gutter that dents holds a pond, and a pond is a leak with a date on it.

Cut edges and the aluminium argument

Where the gutter has to match the wall the coil is pre-painted: TDC51D, 0.12-2 mm thick and painted to RAL colours, with rainwater gutter on its application list a few lines below roofing and walling. The cut edge is then the weak point. Paint stops short of the steel, so every end and outlet is bare, and on galvanized sheet the zinc beside it protects that strip by sacrificing itself.

Aluminium-zinc coated coil behaves differently. The 55 per cent aluminium coating is a barrier rather than a sacrificial layer, and the comparison between the two coatings matters most at this detail: the cut edge has no reserve behind it and depends on the paint film. On a coastal run that argument belongs before the coil is ordered, not after the first rust stain under an outlet.

Joints, movement and the stop end

A steel gutter moves. Twenty metres through a 40 K swing from a cold night to a sunny afternoon grows about 10 mm, and a run fixed hard at both ends puts that into the sealant. Slip joints at 6 to 8 m, or a bracket slot that lets the gutter slide, take it out of the sealant line.

The joint that leaks first is rarely the slip joint. It is the stop end, a flat plate sealed by hand against a curve. Rivets set under a bead of neutral-cure sealant rather than through it, and a bracket within 150 mm of the end, is the detail that holds.

Stainless for the coast, and the pipe that follows

Near the sea, or above a plant that vents chloride, galvanized is the wrong metal to sacrifice. 304 coil from 0.2 to 18 mm covers inland work; 316L, with molybdenum between 2 and 3 per cent, is the first step where the air carries salt; 2205 at 22 per cent chromium and 3 per cent molybdenum, and 254 SMO carrying molybdenum at 6.0-6.5 per cent, are for a deck that sees chloride daily.

The downpipe follows the same ladder, and stainless pipe, 3 mm to 2,200 mm OD, covers a 100 mm spout or a 300 mm collector main. Stainless gutters are welded at the corners, the weld is where tea staining begins, so pickling and passivation belong in the order rather than in a conversation six months later.

Brackets, and the roof that feeds it

Brackets are the cheapest part of the package and the first to fail. A folded bracket in 1.5 mm galvanized sheet is enough above a 0.5 mm gutter on a sheltered site; anything with snow or a long cantilever needs one cut from Q235 plate, bent to the profile and hot-dip galvanized after fabrication, so the holes are coated rather than bare. A spot-welded bracket on pre-galvanized strip rusts at the weld in the first winter.

A corrugated galvanized roof delivers water at the same rate as any other, with grit and loose zinc in it, and that load settles at the outlet.

The blocked case, and what goes on the order

Every gutter above a tree is sized for clean water and spends its life half blocked. A leaf guard halves the open area of the outlet, which is why the overflow matters more than the strainer: a scupper at the stop end, 20 mm below the lip, sends the excess where it does no harm instead of back under the eaves.

The order is short once the numbers are fixed: 200 m of 0.6 mm gutter at 400 mm developed width is 1.9 kg a metre, 377 kg in total, plus 335 brackets, five outlets, two stop ends and five downpipes. Put intensity, plan area and outlet count on the enquiry and the profile can be checked against them. Leave them off and the only thing compared is price per metre, which is how a 1,000 m² roof ends up with two downpipes and a wet wall.