How to Find and Repair a Leaking Roof Valley
Nine valley leaks in ten come from a nail, a debris dam, worn metal or an unsealed edge. Here is how to tell which.
- Advanced
- 1 to 6 hours
- Updated
The short answer
To repair a leaking valley, find the source first: clear debris, then look for nails or screws in or near the centre, rusted or pinholed metal, lifted or unsealed shingle edges and cracked shingles. Test with a hose from the top down. Small faults can be patched with a membrane or metal repair. A worn valley, open deck or stained sheathing means stripping and rebuilding the valley on the existing roof.
Valleys carry more water than any other part of the roof, and they are the place where leaks most often start after a flashing or shingle edge has worn. Because the water is concentrated, even a small fault leaks heavily. A nail hole that would never show up on an open roof surface can soak a ceiling when it sits in the valley centre.
The trick is to find the real source before repairing. The stain on the ceiling is rarely under the leak, because water travels along the deck and rafters. This guide covers how to diagnose a valley leak, the repairs that work, and when to rebuild. For the wider process see how to trace a leak from the attic.
Tools
- Garden hose with a spray nozzle
- Soft broom
- Flat bar
- Hook blade utility knife
- Tin snips (straight cut)
- Roofing hammer
- Caulking gun
- Flashlight for the attic
Materials
- Ice and water shield
- W-valley metal or a section of replacement metal
- Roofing nails, 1-1/4 inch
- Asphalt roof cement
- Polyurethane or hybrid polymer sealant for metal joints
- Replacement shingles
Safety first
- Work tied off. A wet valley is the slipperiest spot on the roof. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Do not test with water while standing in the valley. Stand above and to the side, and have a helper watch inside.
- Wear gloves. Old valley metal has rust and sharp edges, and lifting shingles reveals old nails.
Step by Step
Step 1: Confirm the leak is at the valley
Go up with a flashlight after a heavy rain. Look at the deck on both sides of the valley line and along the rafters it crosses. Dark streaks, damp insulation and water beads on nail tips all point to the valley. Follow the stain uphill, because water runs down the deck before it drips.
If the stain is farther from the valley than expected, check for other sources such as a chimney or vent upslope. See how to trace a leak from the attic.
Step 2: Clear the valley of debris
A debris dam is the most common cause of a valley leak in the Fraser Valley. Needles and leaves mat together, slow the flow and push water sideways under the shingle edges. Sweep the whole valley and check for a mound near the bottom or where two valleys meet.
Do not use a pressure washer, which drives water under shingles and strips granules. Use a broom and a hose. See how to clean roof debris.
Tip: If the valley is clear after one sweep and the leak returns only in heavy rain, suspect a hidden fault rather than debris.
Step 3: Look for nails and fasteners in or near the centre
An exposed nail head in the valley channel, or a nail within about 6 inches of the centreline, is a direct hole to the deck. Look for small rust stains on the metal or a dimple where a nail was driven through. Also check for staples, cable clips and dish bracket screws from satellite installers, which often go through valleys.
Pull an offending nail, then patch the hole with a square of ice and water shield or a metal patch lapped properly. A dab of roof cement alone is a short-term fix at best. See how to fix popped nails.
Step 4: Inspect the metal and shingle edges
Worn or rusted valley metal shows orange streaks and pinholes, especially at the laps and in the lowest foot. A lap joint lapped the wrong way shows a dark stain along its line. Buckled or dented metal, often from foot traffic, holds water. On a closed-cut valley, lift the cut edge gently and check for cement and nails under it.
Shingle edges that have lifted or curled let wind-driven rain in. Check that the clipped corners are still bedded. See how to reseal lifted shingles.
Step 5: Water test from the top down
Start above the suspected spot and run water for 5 to 10 minutes in each section before moving down. Start at the top of the valley, then each side, then the edges. The first section where water appears inside is where the leak starts.
Do not spray up under shingle edges. Use a gentle flow that mimics rain, not a jet.
Step 6: Make a targeted repair
For a nail hole, pull the nail and patch with membrane or a lapped metal patch. For a rusted lap, cut out the damaged metal and replace it with a new section lapped over the lower piece. For a lifted shingle edge, lift the edge, apply a band of roof cement about 3 inches wide and press it down. For a cracked shingle, replace it. See how to replace a damaged shingle.
After each repair, re-run the hose test in that section.
Watch out: Do not bury a leaking valley in roof cement. Thick cement traps water and debris and hides the real fault.
Step 7: Rebuild the valley when the patch will not hold
Cut back the shingles along both sides of the valley, remove the old metal and membrane, and inspect the deck. Replace soft sheathing, then rebuild: wide membrane, new open metal valley and reshingled sides with new cutlines. On an existing roof an open valley is often the best choice, because the new shingle edges can be tied into the old courses without weaving.
If the valley dies into a wall or flat area, see how to fix a dead valley.
The Usual Causes, in Order
- Debris dam: needles, leaves and moss pile up and push water under the edges. Common under firs and cedars.
- Nails in or near the centre: from the original install, from a repair or from a satellite dish or cable clip.
- Worn metal: rust-through and pinholes at the laps and in the lowest section.
- Unsealed or lifted cut edges: closed-cut valleys where the cement failed.
- Cracked shingles: from foot traffic or bridging the corner.
- A valley too narrow for the volume: water jumping the channel in heavy rain.
Prevention
Sweep valleys at least once a year, and again after a windstorm. Keep trees trimmed back from the roof edge. Never walk in the valley, and ask any installer working on the roof to keep fasteners out of it. See how to check your roof after a windstorm for the routine.
Common Mistakes
- Fixing the stain instead of the source. The ceiling stain is rarely under the leak. Trace it uphill.
- Smearing roof cement everywhere. Thick cement hides the fault, traps debris and fails again.
- Using a pressure washer to clean. It drives water under shingle edges and strips granules.
- Patching rusted metal. Rust-through spreads. Replace the section.
- Ignoring the deck. Soft or stained sheathing means the valley has been leaking for a long time.
- Leaving the real cause in place. A debris-prone valley will leak again unless it is cleared and kept clear.
When to Call a Professional
If the valley is on a steep or high roof, the leak is over finished living space, the metal is worn through or the deck is soft, have a roofer look at it. Valley rebuilds involve cutting back shingles on both planes and reworking the membrane, which is hard to do on an existing roof without damaging the courses around it.
If you have patched a valley more than once, the problem is likely structural or a wrong method for the roof, and a roofer can tell you whether it needs a new valley.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Repair a Leaking Valley: Questions
Why is my roof valley leaking?
The most common causes are debris dams of needles and moss that push water under the edges, nails or fasteners in or near the valley centre, rusted or pinholed metal, lifted or unsealed shingle edges and cracked shingles. A valley also leaks if it is too narrow for the volume of water. A hose test from the top down finds which one it is.
Can you repair a roof valley without replacing it?
Yes, if the fault is small: a nail hole patched with membrane, a rusted section of metal replaced, or a lifted edge rebedded and sealed. If the metal is worn through in several places, the deck is soft or the valley has leaked repeatedly, stripping and rebuilding the valley is the lasting fix.
How do you find a leak in a roof valley?
Check the attic first for wet sheathing and stained rafters under the valley, then sweep the valley clean and inspect for nails, rust and lifted edges. Finish with a hose test: run water at the top of the valley for 5 to 10 minutes and move down in sections while a helper watches inside. The first section that shows water is the source.
Is it okay to seal a valley with roof cement?
A small bead under a lifted shingle edge is fine, but covering a valley in cement is not a repair. Thick cement traps water and debris and hides the real fault, and it cracks in sun and cold. Find the cause, patch the nail hole or replace the metal, and use cement sparingly where the detail calls for it.
How often should roof valleys be cleaned?
In the Fraser Valley, valleys under firs, cedars and maples should be swept at least once a year, and again after storms. Needles and leaves hold moisture and push water under shingle edges. Use a soft broom and a gentle hose, never a pressure washer, and do not walk in the valley because it damages the metal and shingles.
Related Guides
All Roof Valleys guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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