Do you need wood strips under metal roofing?

Installing metal roofing can be a great way to protect your home or building. Metal roofs are durable, fire resistant, and energy efficient. When installing metal roofing, one decision you’ll need to make is whether or not to use wood strips underneath.

What are wood strips?

Wood strips, also called roofing slats or battens, are narrow pieces of wood that are installed horizontally across the roof deck before the metal roofing goes on. The strips raise the metal panels off the surface of the roof deck slightly, creating an air gap underneath.

Typically, 1×3 or 1×4 lumber is used for the strips. They are spaced out in regular intervals, often 16 inches or 24 inches apart. The gap created by the wood strips is usually around 1 inch high.

Why are wood strips used?

There are a few key reasons why wood strips may be used under metal roofing:

• They create airflow under the panels, which helps prevent condensation buildup.
• They provide a more even surface for attaching and supporting the metal panels.
• They create a thermal break, interrupting heat transfer through the roof.
• They allow the metal to expand and contract without distortion.
• They can help minimize noise from rain or hail hitting the metal roof.

Essentially, the wood strips act as furring strips, creating a small ventilated air space between the metal and the solid roof deck. This air space helps manage moisture, prevents water trapping, and provides other benefits.

Are wood strips always necessary?

Whether or not you need wood strips under metal roofing depends on a few factors:

• Roof slope – On steeper sloped roofs over 6:12 pitch, wood strips are usually recommended. The steeper slope increases rain noise and condensation risks which the strips help mitigate. On lower sloped roofs, they are sometimes less critical.
• Climate conditions – In hot, humid climates, controlling condensation is very important so strips may be more essential. In dry climates, their moisture control benefit is less needed.
• Roof deck material – If installing over solid wood decking, strips are often not needed. For roof decks made of plywood, OSB, or insulation panels, they are more beneficial.
• Type of metal panels – Some interlocking, snap-together metal panel systems integrate features to provide airflow and don’t require an additional air space. But for systems without built-in ventilation, strips are recommended.
• Manufacturer’s instructions – The installation guidelines from the metal panel manufacturer will indicate if wood strips are required or recommended for their product.

Are there alternatives to wood strips?

In some cases, alternatives to wood strips may be used. Some options include:

• Synthetic plastic battens – Strips made from plastic polymers that provide a moisture barrier, insulation, and airflow space.
• Foam spacers – Closed cell foam strips that lift the panels off the roof deck.
• Integrated panel design – Interlocking panels engineered to create built-in air channels between panels.
• Counter battens – Narrow wood strips running perpendicular to the main rafters, providing fastening points between the rafters.
• Raised seam roofing – Panels with raised seams that hold the panels up off the roof surface.

Pros of using wood strips under metal roofing

Here are some of the benefits of using wood strips beneath metal roofing panels:

• Provide airflow to prevent condensation and moisture buildup
• Create a thermal break to reduce conduction of heat and cold
• Allow for expansion and contraction without distortion
• Smooth out small deck irregularities for even attachment
• Hide fastener screws from view underneath panels
• Reduce noise from rain, hail, wildlife, and other sources
• Inexpensive and lightweight option for lifting metal panels
• Familiar material easy for roofers to install

Cons of using wood strips under metal roofing

There are also some potential drawbacks to using wood strips, such as:

• Added cost for material purchase and labor to install strips
• Required maintenance over time as strips deteriorate
• Can react to moisture and warp, twist, or rot over time
• May be difficult to anchor and secure properly
• Can create unevenness if strips are wavy or uneven
• Wood can be susceptible to mold growth if moisture accumulates
• May allow increased dust buildup compared to direct fastening methods
• Can be damaged during metal panel installation

Installation tips for wood strips under metal roofing

Here are some best practices to follow when installing wood strips beneath metal roofing:

• Use preservative-treated lumber rated for outdoor use. Pressure-treated strips will hold up better than untreated wood.
• Anchor into roof decking securely using roofing nails. This keeps strips from moving.
• Space strips evenly according to manufacturer recommendations for the roofing.
• Install along straight lines across roof for appearance and performance.
• Use quality, dry lumber without splits or checking to prevent stripping or roof noise.
• Allow for 1-2 inches of overhang space at roof edges for proper runoff.
• Consider breathable underlayment beneath strips to allow moisture escape.
• Flash and seal any penetrations through the strips to prevent leaks.
• Install metal panels as soon as possible after installing strips.

Properly installed wood strips will last for decades and provide enhanced performance for a metal roofing system. Taking steps to create a strong, durable installation will maximize benefits.

Conclusion

Wood strips can provide useful benefits like ventilation and condensation control when installed under metal roofing. They are most recommended on steeper sloped roofs, in humid climates, and over solid decking. Wood strips do have some drawbacks, mainly related to long term durability and maintenance. However, overall they are an inexpensive way to enhance the performance of a metal roof. Following best practices for installation and using preservative-treated lumber will help maximize the lifespan of wood strips.