How to Seal a Metal Roof Valley

How to Seal a Metal Roof Valley

Roof valleys are areas where slopes of different angles join together. With shingled roofs, you can have closed or open roof valleys. Closed valleys are completely covered by the shingles. But metal roofing panels don’t have the flexibility of shingles. Therefore, these valleys will be open. At these seams, the roofing material needs to be cut at the junction to accommodate for the valley. This leaves room for moisture and debris to enter the area beneath the seam, compromising the protective qualities of the roof itself.

Why Seal a Metal Roof Valley?

Water flows to the lowest point on a roof. Therefore, when it rains, the flow runs heavily down the valleys. In fact, valleys effectively direct rainwater toward the gutters, away from the peaks of the roof.

But roof valleys are especially vulnerable to water damage. Because they experience a heavier water flow than other parts of the roof, they’re susceptible to erosion. 

If they’re not sealed properly, the water can also penetrate into the underlayment. This part of the roof is made of porous materials, which soak up moisture. It doesn’t take much water to seep into a tiny crack and cause major damage. 

Metal Valley Flashing

If you picture a metal roof valley, which is shaped like a V, you can imagine the water gushing down both sides of the sloped roof to meet in the depression. Because the water flows at such a high speed, it may not settle into the trough of the V right away. If there is enough speed or volume of water, it can splash up the opposite side of the valley. If this area doesn’t have an adequate seal, the water will get underneath the metal roof, affecting the underlayment.

The material that is used to seal a metal roof valley is often referred to as metal valley trim or metal valley flashing. This is the panel that is placed in the valley to catch and direct water. You can use a W-valley or V-valley for this purpose. 

A W-valley is often more effective for combating heavy rain flow. It has a crimp in the middle, which prevents the water from flowing up the opposite slope. It also creates two separate channels to guide the water.

Offset Cleats

Offset cleats are long metal strips that are placed on either side of the metal valley flashing. They run parallel to the cleat in the center of the flashing and prevent water from flowing up the opposite slope or entering the area beneath the metal roof. You would use offset cleats on a standing seam. This is one of the best ways to create a durable, water-tight metal roof.

The following steps are necessary for attaching an offset cleat to a metal roof valley:

  • Pre-drill holes 4 inches apart in the cleat.
  • Clean any debris off of the cleat.
  • Prepare the area where the fasteners will go with butyl tape–this is a synthetic rubber adhesive that creates a seal and weather barrier, protecting the roof from wind, moisture, allergens and debris.
  • Measure and mark the location to install the cleat on the valley flashing panel.
  • Lay the cleat on the marked line.
  • Attach the fasteners starting at the eave end of the roof, using the pre-drilled holes. Make sure that the fasteners are installed straight and perpendicular to the slope of the roof.
  • At the eave, you may also need to install an offset cleat parallel to the roof edge. Where it meets the vertical cleat, allow the vertical cleat to overlap the one that’s parallel to the eave.


Where to Attach the Offset Cleat

The cleat should run parallel to the raised valley trim. But how do you know how far from the center of the valley it should be attached? The location of the offset cleat depends on the climate in your area. 

In areas with heavy precipitation, you’ll want to leave plenty of room between the offset cleat and the center trim. This creates more surface area to channel away water and snow. In these cases, the cleat should be attached about 6 inches from the center of the valley flashing. You may be able to install it closer to 4 inches away in drier areas.

Expanding Foam Sealant

Expanding foam metal roof sealants can be used in any areas that have gaps where water and debris can enter. Roofing materials contract and expand with temperature changes. This means that they may be tightly joined when it’s warm out but contract and separate when it’s cold. 

Adding a layer of foam metal roof sealant between the surfaces creates a flexible area that will remain sealed no matter what the temperature is. It also prevents wind-driven snow and rain from entering potentially vulnerable spots in the roof.

The best expanding foam sealants are UV-stable. They should have a soft feel and should not become hard or brittle when exposed to the elements. 

When you first unwind the sealant tape from the roll, it will be compressed and flat. The product expands up to about 2.5 inches over time. It also has an adhesive backing for easy installation.

Use the Right Underlayment

Metal roof valleys need extra protection because of their purpose and design. Therefore, traditional felt roofing underlayment may not be effective enough. Felt doesn’t create a seal against the roof. Therefore, it’s more likely than some other types of underlayment to shift or crack over the years.

Synthetic underlayments repel water better than felt. They also have a much longer lifespan. However, synthetic underlayments must be attached using fasteners. Any time the roofing material is punctured, you increase the chances of water seeping into the deck. This poses an increased hazard at roofing valleys because of the higher volume of water in those spots.

Peel and stick underlayment is usually the best option for metal roof valleys. Like other synthetic underlayments, this material is resistant to cracking and tearing and lasts a long time. What makes it different is that it sticks directly to the roof deck, sealing it without requiring you to drill any holes.

Because peel and stick underlayment is completely waterproof, it provides extra protection in valleys. Even if some water does seep underneath the metal roof, it can’t penetrate the wooden roof deck. 

You can also take a hybrid approach. To cut costs, some homeowners choose felt or synthetic underlayment in other areas of the roof and peel and stick underlayment along the eaves and valleys.

How to Maintain Metal Roof Valleys

Even with the proper installation, metal roof valleys can be vulnerable to moisture penetration if they’re not maintained properly. One of the most important ways to maintain a metal roof valley is to keep it clean. Prevent it from getting overloaded with debris, which can trap water and prevent it from flowing away from the roofing materials. 

It’s important to maintain a watertight seal at metal roof valleys. The best way to ensure proper installation and maximize the lifespan of your roof is to use a professional installer. At Presidio Roofing, we’re experienced with metal roofing and will use the best techniques for sealing metal roof valleys so that you can enjoy the look and protection that your new roof offers.

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