Wood shingle roof, Wappingers Falls NY (C) Daniel Friedman Wood Roof Sheathing & Underlayment Requirements

  • WOOD ROOF SHEATHING, UNDERLAYMENT - CONTENTS: Roof decking needed under wood shingles/shakes: roof sheathing requirements. Underlayment (roofing felt) requirements under wood shingles & shakes. Using a shingle course felt interlayment on wood roofs. Using battens to provide ventilation for wood shingles & shakes. Using a ventilating underlayment for wood shingle & shake roofs
  • POST a QUESTION or READ FAQs about the requirement for sheathing & underlayment (felt) beneath wood shingles or shakes

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Wood roof sheathing & felt requirements:

This article discusses the requirements & specifications for roof sheathing (nailers or plywood or OSB) and underlayment (roofing felt) or interlayment (between shingle courses) for wood shingle & wood shake roofs.

This article series discusses best practices in the selection and installation of residential roofing. Our page top photo shows a wood shingle roof on the historic Mesier Homestead in Wappingers Falls, NY.

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Sheathing or Nail-Base Specification for Wood Shingle & Shake Roofs

Figure 2-46: Wood Roof Shingles Installed over Spaced Sheathing (C) J Wiley, S Bliss

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Other than selecting a durable wood, the most important factor in determining a wood roof’s longevity is its ability to dry out from both top and bottom when wet.

While this was a natural feature of traditional installations over spaced sheathing, new methods and products are required for installation over solid sheathing.

[Click to enlarge any image]

The two main approaches are:

Specifications for Spaced Sheathing or "Skip Sheathing" for Wood Shingle & Shake Roofs

Wood shingle roof, Wappingers Falls NY (C) Daniel FriedmanThe traditional way to lay wood shakes and shingles on spaced sheathing was ideal for wood roof longevity, but it has largely fallen by the wayside.

Spaced sheathing is especially beneficial in warm, high-moisture climates, since the gaps in the substrate allow the shakes or shingles to dry out from both sides.

Installing wood shingles or shakes over skip sheathing is not recommended in areas of windblown snow and not always permitted structurally. Where allowed, spaced sheathing typically uses nominal 1x4s for shingles or 1x6s for shakes.

Building Codes for roofing reqiure a minimum 1x4, and the spaces between battens should not exceed 3 1/2 inches (Figure 2-46 - wood shingles over spaced sheathing, and Figure 2-47 wood shakes installed over wood sheathing are illustrated below).

[Click any table or image for an enlarged version with more detail.]

The boards are spaced on centers equal to the weather exposure of the shakes or shingles, and they are lined up so the nailing falls in the center of each board. In areas where the average daily temperature in January is 25°F or less, solid sheathing is required on the lower section of the roof to support an eaves membrane. The eaves membrane should extend into the house 24 inches past the interior face of the outside wall.

Figure 2-47, wood shakes over spaced sheathing is shown below.

Figure 2-46: Wood Roof Shakes Installed over Spaced Sheathing (C) J Wiley, S Bliss

Solid Sheathing Required for Wood Roofs in High Wind Areas

This is required in areas of high wind or seismic activity and wherever else a solid roof diaphragm is required by code. Solid sheathing is also recommended in areas subject to windblown snow.

Because of their irregular surface, rustic-style shakes are partially self-ventilating and may perform adequately on solid sheathing in relatively dry climates.

Pressure-treated shingles or shakes can also be installed over solid sheathing. Shingles or smooth-surface (taper-sawn) shakes, however, are more prone to moisture buildup over solid sheathing, so a batten system or a ventilating underlayment is recommended, as described below.

Watch out: having inspected quite a few wood shingle roofs, we find that wood shingle roofs nailed directly to solid plywood or OSB sheathing (that is with no ventilation, battens, etc), while a permitted practice, produces much shorter wood shingle life, sometimes showing severe deterioration: splits, curls, cracks, lost shingle fragments, in as few as four years after new construction - Ed.

Battens Over Solid Sheathing Improve Wood Roof Venting

This provides the full benefit of spaced sheathing on top of a solid roof deck. After laying down No. 30 felt underlayment, install vertical 2x battens lined up with the rafters beneath for solid nailing. Next, place horizontal 1x4 or 1x6 battens (see “Spaced Sheathing,” above) and nail into the vertical battens (Figure 2-48 below).

Figure 2-46: Wood Roof Shingle installation detail (C) J Wiley, S Bliss

At the upper and lower edges of the roof, use insect screening or matrix-style roof vent material to block the entry of insects and other pests. Shake and shingle installation proceeds as for spaced sheathing.

Wood Shake or Shingle Roof Installation Specifications: Felt Interlay or Felt Underlayment

Underlayment Specifications for Wood Shingle & Shake Roofs

Wood Roof Shingles: Over solid sheathing, use minimum No. 30 felt lapped at least 3 inches horizontally and 6 inches at end laps. Over spaced sheathing, no underlayment is used except at the eaves if eaves flashing is required.

Wood Roof Shakes: Over solid or spaced sheathing, use 18-inchwide “interlayment” strips of No. 30 felt installed between shakes, as described below (Shake Installation, below).

Wood Roof Interlayment Detailed Requirements

Whether installed over spaced or solid sheathing, shakes should always be interlaid with 18-inch-wide strips of No. 30 roofing felt. The felt strips acts as baffles to keep windblown snow and other debris from penetrating the roof system during extreme weather.

The felt “interlayment” also helps shed water to the surface of the roof. It is important to locate each felt strip above the butt of the shake it is placed on by a distance equal to twice the weather exposure (Figure 2-51).

Figure 2-51: Wood Roof Shingles Installed felt interlayment (C) J Wiley, S Bliss

Placed higher, the felt strips will be ineffective. Placed too low, they will be visible in the keyways and will wick up water, leading to premature failure of the shakes. In addition, follow these guidelines:

Ventilating Underlayments Used with Wood Shingle/Shake Roofs

Many installers are shifting to a ventilating underlayment such as Cedar Breather (Benjamin Obdyke), which is easy to install and only adds about 10% to the cost of a wood roof. Cedar Breather is three-dimensional nylon matrix with dimples on the bottom and a smooth top surface.

It lays over the felt paper and is tacked in place. It creates a continuous air space below the roofing, helping the shingles to dry out more rapidly and evenly.

Although the air space is only about 1/4 inch, contractors report that it reduces cupping and splitting. And by speeding up drying time, the air space should also help reduce the growth of decay fungi.

However, ventilating underlayments are too new to draw conclusions about long-term performance. Installation details are shown in Figure 2-49. [Click any table or image for an enlarged version with more detail.]

Figure 2-49: Wood Roof Shingles with ventilating underlayment (C) J Wiley, S Bliss Figure 2-49: Wood Roof Shingles with ventilating underlayment (C) J Wiley, S Bliss

Eaves Flashing Details for Wood Shingle & Shake Roofs

Apply eaves flashing to either spaced or solid sheathing in regions with an average daily temperature of less than 25°F (under the IRC) or in other areas prone to ice and snow buildup. The eaves flashing should extend up the roof to a point 24 inches inside the building. Where eaves flashing is required with spaced sheathing, install solid sheathing along the bottom section of the roof to support the eaves flashing.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Wood Roof Venting Underlayments

More Information about Wood Shake & Shingle Roofing Materials, Methods, Standards

-- Adapted with permission from Best Practices Guide to Residential Construction.


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