Eavestroughs and gutters on a metal roof

The eavestrough is the part of the job nobody asks about until it fails. People will spend an hour on panel width and seam profile, then wave a hand at the trough as if it were a gutter-shaped afterthought bolted on at the end.

It is not. On a metal roof the trough is doing a harder job than it did under shingles, and if it is sized, hung and detailed the way it was under shingles, it will not keep up.

Metal delivers water differently

Asphalt shingles are rough, absorbent and full of edges. Rain hitting them slows down, spreads sideways, and arrives at the eave as a fairly even sheet.

A standing seam panel is smooth, sealed and continuous from ridge to eave. Water moves fast, and the seams channel it into lanes. In a heavy summer downpour, the kind that comes off Georgian Bay in July and drops 25 mm in twenty minutes, the water arrives at the eave with real velocity, concentrated between the seams.

Two things follow from that. First, water can overshoot a shallow trough entirely, which is why homeowners phone in the first year saying the new roof “throws water past the gutter” and assume the roof is wrong. Second, the same volume that trickled into the trough under shingles now arrives all at once, so the outlet capacity, not the trough capacity, becomes the limit.

Neither is a defect in the roof. Both are a trough that was never re-thought.

Sizing: when 5 inch is fine and when it is not

Most Ontario houses have 5 inch K-style eavestrough with 2x3 inch downspouts. That is the default because it is what the trough machines on most trucks roll, not because someone did a calculation for your roof.

Five inch is genuinely fine on a lot of houses: modest roof area per downspout, moderate pitch, no long unbroken runs, no valleys dumping onto a short section of eave.

Go to 6 inch trough with 3x4 inch downspouts when any of these are true:

• A steep roof. Steeper slope means faster water and a flatter trajectory off the edge. A 10/12 gable throws water much further out than a 5/12.
• A valley discharging near the eave. A valley collects two roof planes and empties them into perhaps a metre of trough. That one metre carries the load of a large section of roof.
• Long runs to a single outlet. Water has to travel the length of the trough before it can leave.
• Big single planes. Rural houses, additions with shed roofs, anything where one uninterrupted plane runs 12 metres or more to the eave.

The upgrade to 6 inch is a modest add on a re-roof, and the extra capacity is bought once. On a house that needs it, it is one of the least glamorous and highest-value decisions on the quote.

That said, do not let anyone talk you into oversized trough on a small, low-slope, well-broken-up roof. Six inch trough is visually heavier, it sits lower on the fascia, and on a modest bungalow it reads as commercial. If the roof does not need it, the trough is just bigger.

The outlets matter more than the trough

Adding capacity to the trough while leaving the same two undersized outlets is a common half-measure. Water backs up at the drop, rises, and pours over the front lip exactly as before, only now from a bigger channel.

More downspouts almost always beat a bigger trough. A run with an outlet at each end drains twice as fast as the same run with one, and it halves the distance water has to travel. On long elevations we would rather add a third downspout than go up a size.

Check where the downspouts discharge while you are at it. A great many Simcoe County basements are wet because the spout dumps against the foundation. Extensions and splash blocks are cheap; excavating a wet basement is not.

Snow is what actually kills eavestrough on metal roofs

This is the part that catches people.

A metal roof releases snow in slabs. It sits until a thin melt layer forms under it, then the whole mass lets go at once. If that mass catches the back edge of the trough on its way down, it will tear the trough off the fascia, bend the front lip flat, or shear the hangers straight through the aluminum.

We see it every spring. The roof is perfect and the eavestrough looks like it lost an argument with a truck.

The fixes are all detailing decisions made at install time, not products bought afterward:

Panel overhang and kick. The panel edge should project past the fascia and over the trough far enough that released snow clears the back edge, with a hemmed drip that directs water into the trough rather than behind it. Get this wrong in either direction and you either soak the fascia or feed the trough from the wrong side.

Hidden hangers, screwed, at proper spacing. Spike-and-ferrule is not adequate under a metal roof. Hidden hangers with screws into solid fascia, spaced closer than the standard 900 mm, at least on elevations that will take snow. On high-load elevations we tighten that further.

Snow retention above the areas that matter. Seam-mounted retention holds the slab on the roof so it melts off gradually instead of coming down in one piece. We cover placement in detail in our article on snow guards and ice dams, and the short version is that retention belongs where release causes a problem, not blanketed across every plane.

Or accept that the trough will be sacrificial. On some Muskoka and Kawarthas cottages, the honest recommendation is no eavestrough on the elevations that shed hard. Put a drainage bed of clear stone along the drip line, run trough only where you need it (over entries, or to feed a cistern), and stop replacing aluminum every second spring. Not every eave needs a trough. It needs the water controlled, and stone does that without anything to tear off.

Ice in the trough, and the honest word on heat cable

Ice forms in troughs on metal roofs. Less than it did under shingles, because less snow sits at the eave to feed the melt-refreeze cycle, but it still happens on shaded north elevations and on any house with heat escaping into the attic.

Heat cable is sold as the fix. It is a symptom treatment. It keeps a channel open so meltwater has somewhere to go, which is worth something on a problem eave, and it costs money to run all winter, it has a service life measured in a handful of years, and it does nothing about the cause.

The cause is nearly always warm air leaking into the attic. Air-seal the attic floor, get the insulation to depth without blocking the soffit intake, and confirm the ventilation is actually balanced before you spend anything on cable. If the eave still ices after that, then cable it, with your eyes open about what it is doing.

Box gutters and built-in troughs

Older homes in Barrie, Collingwood and Orillia sometimes have a box gutter: a trough built into the roof structure itself at the eave, lined rather than hung. Many farmhouses and century homes have them, and plenty of owners do not know it until a ceiling stains.

A box gutter has no forgiveness. It is inside the building envelope, so when the liner fails, water goes into the soffit, the wall, or the ceiling below rather than down the outside of the house. The old ones were lined in soldered metal, then patched with roofing tar by somebody in the 1980s, and the tar is usually what is failing now.

Two honest options at re-roof time. Reline it properly, which is metal fabrication work: a new lined trough formed to the existing box with proper expansion allowance, outlets and overflow scuppers. Or abandon the box, close it up as part of the roof plane, and hang conventional trough outside the wall. The second is cheaper and more reliable. The first preserves the look of the house, which on a designated heritage property may not be optional.

Either way, a box gutter needs to be on the table before the quote is written, not discovered on day two. If you have one, say so when you call.

Sequence and colour

Trough comes off before the roof goes on and gets rehung after, because the panel edge and drip detail have to be set first. Anyone planning to work around existing trough is planning to compromise the eave detail.

On colour: matching the trough to the roof is not always the right call. On a dark roof with light fascia, a trough that matches the fascia disappears. A trough that matches a bold roof colour becomes a stripe across the elevation. We would rather see the trough vanish into the trim than announce itself.

What to ask for

When you get quotes for standing seam in Barrie, Simcoe County and the surrounding region, the eavestrough scope should be written down, not assumed: trough size, downspout size and count, hanger type and spacing, what happens at valleys, and whether snow retention protects the runs that need it. If the quote says “reinstall existing eavestrough” and nothing else, ask the questions above.

We would rather spend ten minutes on it in your driveway than have you calling next April about a trough lying in the garden. Call 705-816-5282 or get in touch.