Two catchments, one line
A valley is the intersection of two roof planes that both slope toward it. Every drop landing on either plane travels across the shingles until it reaches the valley line, and then it travels down the valley. By the time it gets to the eave it is the combined runoff of both planes moving in a channel a few inches wide, and it arrives at a single point on the gutter.
The arithmetic is the same as anywhere else on a roof — plan area times intensity — and the only thing that makes a valley different is geometry. A 24 by 14 plane and an 18 by 12 plane are 552 square feet together. At four inches an hour that is 184 cubic feet an hour, 1,376 gallons an hour, about 23 gallons a minute, all of it arriving at one place. Spread along thirty feet of eave that would be 0.76 gallons per minute per foot of gutter. Delivered into the three feet under the valley outlet it is 7.6, ten times the load.
The gutter fails locally, not globally
Gutter sizing is done on total flow: how much water the whole run has to carry to its downspouts. That calculation can come out perfectly and the gutter can still overflow every storm, because a gutter has to accept water as well as carry it, and acceptance happens at a point.
Water leaving a valley is not dribbling over the drip edge. It has velocity, it has been travelling in a confined channel down a slope, and when it reaches the eave it launches. Below a certain flow the trough catches it. Above that, the stream crosses the width of the gutter and goes over the front lip, and the whole run downhill of that point stays dry while the ground below the valley takes everything.
| Symptom | Usually about |
|---|---|
| Gutter overflows at one spot only, in heavy rain | Local concentration at a valley or a long run falling to one point |
| Gutter overflows along its whole length | Total capacity, downspout count, or a blockage |
| Overflows only after leaves fall | Debris, and valleys collect it first |
| Water behind the gutter, against the fascia | Overshoot or a drip edge issue rather than volume |
Valleys also collect debris, because everything that lands on either plane funnels into the same line. A valley that is clear in July and full of needles in November concentrates the flow further by narrowing the channel, and the point where the valley meets the gutter is where the debris finally stops. The gutter cleaning estimate calculator is a different question — how long the job takes — but valleys are the part of it that has to be done properly.
Why the ground below a valley is always the worst ground
Follow the water past the gutter. Whatever overtops lands on a narrow strip below the valley, and that strip receives the runoff of hundreds of square feet of roof concentrated into a few square feet of soil. Run the equivalent depth figure and it comes out at ten, twenty, forty times the rainfall depth of the storm, over and over, in exactly the same place.
What follows is mechanical rather than mysterious. Bare soil erodes because the impact energy is concentrated. Mulch floats away. Planting fails because roots are alternately scoured and waterlogged. The soil compacts and stops absorbing, so the next storm runs off it instead, usually toward the wall. And the backfill zone below, which is looser than the undisturbed soil around it, takes the infiltrated share down beside the foundation.
This is the mechanism behind a very common complaint: one corner of a basement that takes water when the rest never does. The corner is under a valley. Working out how much water that is turns an argument into a number, and the downspout discharge calculator and the basement water entry organizer pick up from there.
What to do with the number
Three interventions follow from a high concentration figure and they escalate in cost. A downspout placed at or immediately beside the valley outlet, rather than at the end of the run, removes the water before it has to travel sideways through the trough. A larger gutter section or a deeper profile under the valley gives the arriving stream somewhere to land. And below the roof, a solid apron, a gravel band or a piped connection under the discharge point takes the concentration off the soil rather than asking the soil to cope with it.
The one thing worth avoiding is treating it as a gutter capacity problem and buying a bigger gutter for the whole house. The rest of the run was never the issue.
Where the water is allowed to end up
Where roof water and site water are allowed to go is set locally and it varies enormously. Discharging onto a neighbouring property, into a street gutter, into a storm sewer, into a sanitary sewer, or into a watercourse may be required, permitted, permitted only with approval, or prohibited outright depending on where you are, and some jurisdictions also set minimum setbacks and grading requirements at the building. None of that is stated on this page as fact, because none of it can be. Your local building department or stormwater authority is the source, and it is worth a phone call before anything is dug, because water arriving on someone else's land is one of the more common ways a drainage project turns into a dispute.
Before digging anywhere on the property, have the underground utilities located. In much of North America that is a free call-before-you-dig service and using it is a legal requirement, not a courtesy. Separately: excavating alongside a foundation can undermine it. Soil next to a footing is carrying load, a trench cut beside or below the footing can let that soil move, and the failure does not always show up on the day. This page gives no excavation procedure and none should be inferred from it. Work close to a foundation, below the footing line, or in soil that will not hold a face belongs with a contractor or an engineer who is standing on the site.
Questions people ask
Why does my gutter only overflow in one place?
Almost always because water is arriving there faster than the trough can accept it, and the usual reason for that is a valley discharging at that point. A gutter is sized on the total flow it carries, but it fails where flow enters. Water leaving a valley has velocity and arrives concentrated into a few feet of trough, and above a certain rate it crosses the gutter and goes over the front. The rest of the run stays comfortably within capacity, which is why the overflow looks like a defect in one small section.
How much water does a roof valley carry?
The plan area of both planes feeding it, times the rainfall intensity. Two planes totalling 550 square feet at four inches an hour deliver about 23 gallons a minute at the bottom of the valley. The rate builds along the valley, roughly linearly, from nothing at the head to the full figure at the eave, which is why the lower third of a valley is where flashing and debris problems show up and the top rarely does.
Should I put a downspout under the valley?
Placing an outlet at or immediately beside the valley discharge is a common and effective response, because it takes the concentrated flow out of the gutter at the point it arrives rather than asking it to travel sideways along a trough that is already full. What it does not do by itself is deal with what happens on the ground, since the same concentration now leaves through a pipe at one spot. The downspout discharge calculator on this site puts a volume on that.
Why does nothing grow under my roof valley?
Because that patch of ground is receiving many times the rainfall the rest of the garden gets, delivered as a concentrated stream rather than as rain, repeatedly in the same place. Soil there is alternately scoured and saturated, fines wash out, the surface compacts, and mulch does not stay. It is a water management problem before it is a planting problem, and changing the plant seldom helps. Taking the concentration off the spot with an apron, a gravel band or a piped outlet is the part that works.
Does the roof pitch change how much water a valley carries?
Not the volume. Rain falls vertically, so what a roof collects depends on its plan area and nothing else, and two roofs of different pitch over the same footprint gather identical amounts. Pitch changes velocity, and in a valley velocity is what causes the trouble: water leaving a steep valley arrives at the eave moving fast enough to overshoot a gutter that would have caught the same flow arriving slowly. That is why gutter capacity calculations apply a pitch factor even though the volume figure does not.