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Roof Valley Leaks: Open Metal, Closed-Cut and Woven Valleys, Why Yours Leaks, and What a Rebuild Costs
Roofing
Valleys: Where Two Roofs Pour Into One Channel, and the Five Reasons It Leaks
A valley carries the water from two roof planes at once, so it sees two or three times the flow of any other part of the roof, holds ice longer in winter, and catches every leaf and shingle granule that slides down. It is the second most common leak on a sound roof, after the flashings, and the fix is usually a rebuild of the valley, not a patch. Here is how valleys are built, which way is right for an Ontario roof, and why yours is leaking.
The three ways to build a valley
Open metal valley
A strip of metal (galvanized, aluminum or prefinished steel, 24 in. wide, with a centre rib on the good ones) laid down the valley over ice and water membrane, with the shingles stopped 3 to 4 in. short of the centre on each side, so a channel of metal is exposed. Sheds water, ice and debris fastest; the easiest to inspect; the one that lasts. The right valley for Ontario snow and the one most manufacturers prefer on steep or long valleys.
Closed-cut valley
Shingles from one side run across the valley; shingles from the other side are cut in a straight line 2 in. off the centre. Looks clean, no metal showing, common on new subdivisions because it is fast. Relies entirely on the membrane underneath and on the cut being kept off the centre line and the nails kept well back. Fine when done right; the one that leaks when done fast.
Woven valley
Shingles from both sides alternate across the valley, woven together. No cut, no metal. Only works with flexible 3-tab shingles; the thick laminated shingles most people buy now will not lie flat woven, and manufacturers say not to. If a roofer proposes weaving architectural shingles, that is a roofer to question.
The five reasons a valley leaks
- No membrane underneath. Ice and water shield, 36 in. wide, centred on the valley, under everything, is what actually keeps the water out when ice backs it up or wind pushes it sideways. Older roofs have felt or nothing. Everything else on this list is survivable with the membrane and fatal without it.
- Shingles cut on the centre line. On a closed-cut valley the cut edge must stay 2 in. off centre, with the corner of each cut shingle clipped so water cannot catch it. Cut on the centre, the water follows the edge under the shingle.
- Nails in the valley. No nail within 6 in. of the centre line, ever. A nail in the channel is a hole in the busiest place on the roof.
- Debris and ice. Leaves, needles and granules collect in the valley, hold water against the shingle edges, and freeze. A shaded valley under trees needs clearing every fall. Snow lingers in valleys and the ice dam that forms at the bottom of one backs water under the shingles for 6 ft.
- The metal. On an open valley, plain galvanized rusts through in 20 to 30 years at the bottom where it stays wet; narrow metal (under 20 in.) lets fast water jump the edge; metal without a centre rib lets water from the steeper side cross and run under the shingles on the shallower side. Prefinished steel or aluminum, 24 in., with a rib.
What a correct valley looks like
- Membrane first, 36 in. wide, centred, run from the eave past the top of the valley and lapped under the underlayment on each side.
- Open valley: 24 in. prefinished metal with a centre rib, nailed only at the outer edges, lapped 8 in. where pieces join, lower piece under upper. Shingles stopped 3 in. off centre at the top widening to 4 in. at the bottom, corners clipped, a bead of sealant under the shingle edge on the metal.
- Closed-cut: the first side’s shingles run 12 in. past the centre line; the second side’s are cut 2 in. off centre in a straight line, corners clipped, no nails within 6 in. of centre.
- At the bottom, the valley discharges into the eavestrough, not onto a lower roof or against a wall without flashing.
Finding it and fixing it
Valley leaks show up on the ceiling under the valley or a few feet downhill, after heavy rain or in a thaw, and in the attic as a stain along the line of the valley rafter. The hose test on the leak diagnosis page confirms it in ten minutes. The repair is almost always a rebuild: shingles out 3 ft each side, old metal and whatever was underneath out, membrane in, new metal, shingles back, $1,000 to $2,500 for a typical 15 to 25 ft valley depending on pitch and height. A patch of roofing cement in a valley is a leak postponed by a season. If the roof is over 20 years old and the valley is leaking, it is usually the roof’s way of saying the rest is due; the repair vs. replacement page has the test.
Questions people ask
Why does my roof leak at the valley?
No ice and water membrane under it, shingles cut on the centre line, a nail in the channel, debris or ice holding water against the shingle edges, or metal that has rusted through or is too narrow. Usually two of those together.
Which is better, open or closed valley?
Open metal for Ontario: it sheds snow, ice and debris and you can see its condition. Closed-cut is fine when done precisely over membrane, and it is the one that fails when done fast. Woven is for 3-tab shingles only.
How much does it cost to repair a roof valley?
$1,000 to $2,500 to rebuild a typical valley with new membrane and metal, before HST. More on a steep two-storey. A cement patch is $150 and does not last.
Should ice and water shield be used in valleys?
Yes, always, 36 in. wide and centred, under the metal or the shingles. It is not strictly required by code in every case but no competent roofer skips it, and most manufacturers require it for their warranty.
What metal is used for roof valleys?
Prefinished steel or aluminum, 24 in. wide, ideally with a centre rib. Plain galvanized rusts through at the bottom in 20 to 30 years; copper is lovely and expensive.
Membrane, metal, shingles kept back, no nails
Four things, and a valley lasts the life of the roof. Clear the leaves out of it every fall and it lasts longer than that.
Sources and further reading
- Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
- IKO: Canadian-made shingles; the warranty and the technical data sheets are on the site.
