How to Prep a Roof Valley Before the Metal or Shingles Go On
A clean, sound deck and a wide strip of membrane centred on the valley. The layer that does the real waterproofing.
- Advanced
- 1 to 2 hours per valley
- Updated
The short answer
To prep a roof valley, strip the old roofing, repair any soft deck, and sweep the valley clean. Lay self-adhered ice and water shield centred on the valley, commonly 36 inches wide with 18 inches on each side, pressed tight into the corner. Lap sections at least 6 inches with the upper piece over the lower, run the field underlayment over its edges, and keep every nail at least 6 inches from the centreline.
No valley method, whether open metal, closed-cut or woven, is better than the layer under it. The valley metal or shingle edge sheds most of the water, but the self-adhered membrane beneath is the barrier that catches what gets through: wind-driven rain, ice damming and the occasional puncture. If the prep is wrong the best metal in the world will not keep the valley dry.
Prep has four parts: a sound and clean deck, a wide membrane strip centred on the valley, correct laps, and a nail-free zone down the middle. This guide covers all four and applies to any valley type.
Tools
- Hook blade utility knife
- Tape measure
- Chalk line
- Roofing hammer
- Stiff broom
- Hand roller or a rolled towel for pressing membrane
- Pry bar and roofing shovel
Materials
- Self-adhered ice and water shield, 36 inch wide roll
- Synthetic or felt underlayment for the field
- Roofing nails, 1-1/4 inch
- Plywood scraps for deck repair
- Cap nails or plastic cap fasteners for the underlayment edges
Safety first
- Work tied off. The valley is at the low point between two planes, but a slip on wet needles sends you down either plane toward the eave. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Peel-and-stick membrane is hard to reposition. Two people handling the roll reduces folded, wrinkled and stuck-to-your-boots mistakes.
- Cold weather stiffens membrane and makes the adhesive slow to grab. In the Fraser Valley winter, follow the manufacturer's minimum application temperature, often around 5 degrees C.
Step by Step
Step 1: Strip the valley to bare deck
On a reroof the valley is the first place to look for trouble. Old valleys are usually lined with layers of shingles, roofing cement and rusty nails. Pull every nail and scrape the tar. Leave no ridge of old cement that will telegraph through the new membrane.
Look closely at the deck in the valley centre. A valley that has leaked for years is often dark, stained or soft along the joint, and that is where wet rot starts.
Step 2: Inspect and repair the deck
Sound sheathing resists a screwdriver. Soft, spongy or crumbling plywood must be cut back to the nearest rafters and replaced with the same thickness, and the new panel nailed to framing at the valley. Plank decks need any rotted boards replaced and loose boards re-nailed. See how to replace roof sheathing for the method.
The valley joint should be tight, not open. A gap of 1/4 inch or more at the centre should be closed with a strip of plywood or the membrane will bridge it and tear.
Watch out: Do not cover rotted sheathing. New membrane and metal laid over wet or rotted wood trap moisture and the rot continues.
Step 3: Sweep the deck and snap the centreline
Membrane adhesive will not bond to dust, needles, frost or damp wood. Sweep the valley and let any moisture dry. A dry, clean deck is also what the self-adhered membrane manufacturer specifies.
Snap a chalk line along the valley centre. Every later measurement, from membrane width to metal position to shingle cutline, is taken from this line.
Step 4: Cut and fold the ice and water shield
A common specification is a strip 36 inches wide, which puts 18 inches on each side of the centreline. Some manufacturers allow a minimum of 24 inches, and some call for more on low slopes or snowy regions, so check the membrane and shingle instructions. A full-width roll is convenient because it needs no trimming.
Cut the strip about 3 to 4 inches longer than the valley at the bottom so it hangs over the eave and can be trimmed to the drip edge later. For the general technique of handling the product, see how to install ice and water shield.
Step 5: Adhere the membrane centred on the valley
Start at the bottom and work up so any wrinkles can be pushed out ahead of you. Press the membrane firmly into the corner first, then out across both planes. If it bridges the corner, it will tear the first time someone steps there or a nail is driven nearby.
Press with a hand roller or a rolled towel along the full length, paying extra attention to the valley centre and the edges. Air pockets under the membrane become the first leak path.
Tip: Cut the backing in two along its length so you can peel and set one side at a time. It keeps the membrane from curling onto itself.
Step 6: Lap sections correctly
Always start at the eave and work upward, so each higher piece overlaps the one below, never the other way round. A backwards lap makes a small dam at every joint. Many manufacturers ask for 6 inches of end lap, and some ask for more, so check the roll.
At the top of the valley, extend the membrane at least 6 inches up the ridge or onto the adjoining plane, following the detail for where the valley starts, so water cannot back up under it.
Step 7: Lap the field underlayment over the membrane edges
The field underlayment should finish over the valley membrane, not under it, following the rule that the upper layer always laps over the lower one. Fasten it near the upper edge only, well away from the valley centre.
If the roof uses a full ice and water shield membrane across the eaves, it simply ties into the valley strip at the bottom. See how to install synthetic underlayment for laps and nailing.
Step 8: Keep the centre of the valley free of nails
Every nail near the valley is a hole in the most heavily watered part of the roof. Membrane edges hold by adhesive, not by nails. The only fasteners that should appear are those that later hold valley metal edges or shingle ends, each located well away from the centreline.
Tell the rest of the crew the nail-free zone exists. The most common valley leak starts with a nail driven into the membrane centre by someone who did not know.
Watch out: If a nail goes through the valley centre, pull it and patch the hole with a square of membrane. Do not leave it.
Cold Weather and Wet Decks
Self-adhered membrane needs a dry, clean surface and a minimum temperature, usually around 5 degrees C, to bond properly. In Fraser Valley winters, frost and damp mornings are common. Wait until the deck is dry, or follow the manufacturer's cold-weather method with a primer. Membrane applied in cold damp conditions can look stuck and still lift a few weeks later.
If the valley cannot be completed before rain, cover it with a temporary tarp and do not leave exposed membrane in sunlight for more than the manufacturer's stated time, because ultraviolet exposure degrades it.
What About Two Layers?
Some valleys, especially on low slopes, long valleys with large planes draining in, or in snow country like Hope and the canyon, are built with a second layer of membrane centred over the first, or with a wider strip. Follow the shingle manufacturer's instructions. More membrane is cheap compared with a valley leak.
Common Mistakes
- Skipping the deck check. Membrane and metal laid over soft wood trap moisture and the valley fails from underneath.
- Bridging the corner. Membrane that spans the valley without being pressed into the angle tears under a footstep or a nail.
- Lapping the wrong way. An upper piece tucked under a lower one makes a dam and pushes water into the joint.
- Nailing through the centre. Even one nail driven through the valley membrane is a permanent leak site.
- Using felt alone. Felt does not seal around nails the way self-adhered membrane does, so a felt-only valley depends entirely on the top layer.
- Applying membrane to a dusty or damp deck. The adhesive does not grip and the membrane lifts, leaving air channels.
When to Call a Professional
If the deck in the valley is soft, stained or open at the joint, the repair involves framing and sheathing and should be done before any valley is built. The same goes for a valley on a steep roof where a slide down either plane ends at the eave.
A roofer will also check where the valley meets the eave, a wall or a lower roof, which is where most valleys actually leak, and build the membrane and metal to suit.
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 Prep a Roof Valley: Questions
How wide should ice and water shield be in a valley?
A common specification is 36 inches wide, centred on the valley so 18 inches lies on each side of the centreline. Twenty-four inches is a typical minimum, and some manufacturers ask for a second layer on low slopes or in heavy-snow regions. Follow the membrane and shingle manufacturer's installation instructions for the method you are building.
Do you put underlayment under or over valley flashing?
Self-adhered ice and water shield goes on the deck first, centred in the valley. The metal valley or shingles then go over it. The field underlayment, synthetic or felt, laps over the edges of the membrane rather than under it, so water flows from the field onto the membrane and down the valley without finding a seam.
How far from the valley centre should nails be?
Keep roofing nails at least 6 inches from the valley centreline, and use the distance given in the shingle manufacturer's instructions if it is larger. For an open valley the metal is fastened at its outer edges only, and shingle nails stay clear of the metal. A nail in the valley centre is one of the most common valley leaks.
Can you use felt paper instead of ice and water shield in a valley?
Some installations use felt layers, but self-adhered membrane is the stronger choice. It seals around nails, resists backed-up water and protects against wind-driven rain and ice damming. Felt relies entirely on the shingles or metal above it being perfect. In wet or snowy regions a self-adhered strip is the standard under valleys.
Does a valley need new membrane during a reroof?
Yes. The valley is the most heavily watered part of the roof, and old membrane or felt under it is usually degraded. A reroof exposes the valley deck, so it is the time to inspect the sheathing, replace anything soft and install a fresh, wide strip of membrane before the new valley goes on.
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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