Installing the Correct Underlayments for Your Metal Roof Project … “The Right Stuff”

Selecting the right underlayments for a lifetime metal roof should be a primary concern for anyone thinking about specifying or choosing to install a metal roof on their home. The quality and type of underlayments used on the project in the long term will affect the overall performance of the roof.

 

Taking Care of Your Building Envelope

The roof is an important element of the Building Envelope. A Building Envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer. It needs to be addressed with the utmost care and consideration. Be sure that the contractor you choose is trained in the best methods of installation. Having the “Right Stuff” and not having the skill to do the installation nullifies the benefits of its use.

First Two Underlayment Layers

On most projects, there are two types of underlayments needed. The first on the list would be the ice and water shield. This is a roof membrane underlayment made up of either a rubberized-asphalt or butyl-based adhesive with a polyethylene carrier sheet. It has an adhesive backing with a peel and stick feature. This membrane should be labeled “high temperature” to withstand the surface temperature of the roof on a hot summer day. The heat in hot areas of the country, even here in New England, can exceed the melting point of some of these products. The product should be a premium, high temperature ice and water shield for use with tile, metal, and other roofs with scrim reinforced top surface. It self-seals around nails and provides protection for roof areas prone to ice and water intrusion.

The product needs to be installed on the eave edges up the roof at least two feet inside the face of any exterior wall. The rakes on gable roofs also need treatment to the same standard. All valleys and hips need to be covered. Penetrations such as chimneys, skylights, solar tubes, and vent pipes need to be surrounded with ice and water shield, as well.

If the roof slope is below 3/12 pitch (see our previous post on how to determine a roof’s pitch), the entire roof should be covered, but only if the roof is conventionally vented (cold roof construction).

Once the high temperature ice and water shield has been laid down, you can now add the second underlayment layer. This underlayment layer should be of the highest quality. If the roof is designed to last a lifetime, why would anyone want anything but the best? We recommend a polypropylene scrim reinforced underlayment made up of multiple layers. This type of underlayment is critical to a quality roof installation. It serves as a moisture barrier, as well as a slip sheet. Make sure the product is designed to be used under metal. Once again, this is used on cold roof construction.

When hot roof construction is done – a roof with no ventilation – a breathable underlayment is recommended. This allows moisture that can become trapped in the roof deck to evaporate and escape. This will prevent damage to the deck, stemming from rot and mold, over the long term life of the structure.

Third Underlayment Layer

There is a third product that is sometimes used and specified in new construction: That product is a mesh that would go on top of and over the other underlayment used. It is akin to cedar breather. It provides trapped moisture from under the roof in low slope applications a path to drain. It also creates a thermal break to minimize heat transfer from the roof surface to the structure. This thermal break promotes energy efficiency by stopping conductive heat transfer much like the air space between multiple panes of glass in a thermal pane window. This same break also provides sound attenuation by disrupting sound waves into the structure when rain hits the roof surface. The end result is a cooler, quieter, and healthier building. This product is generally used in hot roof low slope applications with aluminum roofing. 

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