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
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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.
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.
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
Create a system of spaced sheathing above the solid
sheathing using vertical and horizontal battens; or
Use a breathable underlayment applied over the
Specifications for Spaced Sheathing or "Skip Sheathing" for Wood Shingle & Shake Roofs
The 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.
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.
of their irregular surface, rustic-style shakes are partially
self-ventilating and may perform adequately on solid
sheathing in relatively dry climates.
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).
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,
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).
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:
For the starter course, use either a single layer of shakes
or two layers separated by a strip of felt interlayment
(installed up from the eaves by a distance equal to the
weather exposure). Fifteen-inch shakes are available
for the bottom layer of a double starter course
Each shake gets two nails about
inch in from each end
inches above the butt line of the overlaying shake.
The first course should overhang the fascia by
All courses should overhang the rake trim by about
Leave a gap between adjacent shakes of
expansion when wet.
Offset joints in successive courses by at least 1
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
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.
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.]
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.
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Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237, If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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