Gutter installation cost

Ask how to size a gutter and you will be told five inches, or six if the house is big. Neither answer mentions rain, which is the only thing a gutter has to cope with.

Drainage is sized against design rainfall intensity: the short-duration peak, in inches per hour, that a downpour delivers. Not the annual total — Seattle gets far more rain a year than Houston and needs much smaller gutters, because Seattle rain arrives gently over days and Houston rain arrives all at once.

That spread is more than three to one, and it lands directly on the size of gutter you need. A 5 inch K-style carries a comfortable 2,200 square feet of roof in the Pacific north-west and barely 600 on the Gulf coast — and the same national default gets fitted in both places.

It is worth being clear about what failure looks like. An undersized gutter does not gradually underperform: it works fine for most of the year and then, during the storm that matters, water sheets over the front edge and lands in a line along the foundation. That is the exact outcome gutters exist to prevent.

A gutter is a holding trough. What actually moves the water is the downspout, and it is usually the limiting part of the system — a 2 × 3 downspout has a cross-section smaller than a fist, and it is asked to drain half a house.

Two rules apply at once, and you take whichever gives more. By flow: a 2 × 3 carries about 2,320 square feet of roof per inch-per-hour of rainfall, and a 3 × 4 about twice that. By spacing: at least one every 35 feet of run whatever the arithmetic says, because water has to travel along the trough to reach one and a long run with a single outlet overflows at the far end.

A gutter is not level. It slopes towards its outlet at about 0.25 of an inch every ten feet — enough to keep water moving, little enough that the eye does not catch it against the fascia.

Too little fall and water stands in the trough: it breeds mosquitoes, it holds debris that would otherwise wash through, and in winter it freezes as a solid block that the hangers have to carry.

Too much and it shows. A fascia board is level and a sloping gutter under it reads as a mistake — and the further it runs, the more obvious it gets. Fifty feet at a quarter inch per ten is an inch and a quarter of drop, which is visible from the street on a light coloured house.

That is why runs over about 35 feet are sloped from a high point in the middle down to a downspout at each end. Same fall, half the distance, half the visible drop — and two outlets instead of one, which the flow calculation usually wanted anyway.

Four realistic options, and the choice is mostly decided by whether someone else is fitting them.

The thing worth understanding about seamless aluminium is that it is not a product you can buy. It is rolled on site from a coil of flat stock by a machine on the back of a van, so a forty-foot run arrives as one continuous piece. That is why it dominates professional work and why it cannot be a DIY option: the machine is the trade.

Sectional vinyl exists to fill that gap. It clicks together in ten-foot lengths, which means a joint every ten feet, and every joint is a future leak. For a garage or a shed that is a reasonable trade. For a house that will stand for thirty years, less so.

Gutter guards are sold as the end of cleaning gutters and they are not that. What they genuinely do is keep leaves and twigs out of the trough, which on a house under trees is a real improvement — it can take an annual chore down to a two-yearly one.

What they do not do is stop fine debris. Shingle grit, seed pods, pine needles and windblown dirt all pass through or accumulate on top, and a guard that nobody ever looks at can hide a blockage for years while the homeowner assumes the problem is solved.

They also add cost across the whole run, and the good ones are not cheap. The honest arithmetic is: guards where trees overhang the roof and getting on a ladder is genuinely difficult; skip them on a clear roof where a yearly clean is a half-hour job.

About $28 a foot of gutter run for seamless aluminium — $2,250 for the 80 ft of eave this page opens with. That figure is higher than the numbers quoted around, and the reason is what is inside it: the same 80 ft also needs four downspouts and 80 ft of downspout pipe, forty hangers, end caps, a dozen elbows, and the old gutter taken down and carted away.

A quote of “twelve dollars a foot” is usually the gutter alone. Neither figure is dishonest; they are answers to different questions, and comparing them directly is how people end up surprised by an invoice.

The thing that genuinely moves the price is material, not length. Aluminium is the default; copper is several times it and lasts a century; steel sits between and rusts at the cut ends. Guards are a separate decision and a separate line, and on a house with no trees over it they buy very little.

Sizing follows the sheet metal industry method: capacity per size divided by design rainfall intensity gives the roof area a gutter or downspout can carry. Rainfall intensities are compiled by state at the higher end of each state’s range, because undersizing is the expensive failure. The pitch factor accounts for wind-driven rain on a steeper roof. All of it is on the sources page, dated.

Two simplifications worth stating. The capacity figures are for K-style profile at standard slope — half-round carries less for the same nominal size. And the roof area draining into each run is something you supply: the calculator cannot see your roof, and on anything more complex than a simple gable, splitting the area between eaves is a judgement worth making carefully.

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Design rainfall intensity is the one that decides the size. Everything else is profile and hardware.