Detailed Guide to Installing & Flashing Around Skylights
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Skylights: choosing, installing, design issues, energy efficiency
How to install a skylight
How to construct the skylight well, framing, opening
Sealing the underlayment around skylights
Skylight flashing details
Ice dam protection for skylights
How to install a skylight on a low-slope roof
How to install flashing around a skylight
Questions & answers about roof skylight flashing installation, troubleshooting, repair

Skylight flashing details: here we provide details about framing the skylight opening, constructing the skylight well or chute, sealing the skylight, flashing around skylights to avoid leaks, and protecting from ice dams and ice dam leaks at skylights. We also discuss how to install a skylight on low slope roofs and on very steep roofs.

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Skylight Installation Procedures

This article group discusses the selection and installation of skylights, including choosing a skylight, how to install the skylight, constructing the skylight well or chute, skylight controls, and skylight shades or screens. We also discuss skylight condensation and special skylight products such as light tubes.

In this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants. We review the proper installation details for windows and doors, and we compare the durability of different window and door materials and types.

See WINDOWS & DOORS our home page for window and door information, and also see WINDOW TYPES - Photo Guide for a photographic guide to window and door types and architectural styles.

According to Best Practices Guide to Residential Construction Chapter 3, BEST PRACTICES GUIDE: WINDOWS & DOORS:

Skylight Opening Framing

Take care when framing skylight openings to make the opening as accurate and square as possible. T

his will simplify both the installation and trim, since most skylight designs leave little margin for error where the interior finish meets the frame. Unless using a skylight designed to fit between 16- or 24-inch on-center framing, you will need to head off the opening and use double trimmers on either side.

For truss roofs, consult the truss designer or an engineer before modifying the truss plan or cutting an existing truss. In new construction, trusses are typically doubled on either side of the opening with 2x cross pieces framed in “ladder” fashion.

The Skylight Window Well: the Light Well

There are many ways to frame a light well or "skylight chute" as some builders call them. In a cathedral ceiling, the rafters and headers create the well. Framing the upper header square and lower header plumb splays the opening.

Our photo (left) shows a wood-framed skylight well (the builder called them "shoots", meaning "chutes") in a truss-framed roof in Minnesota -DF. To frame a perfect skylight well the builder actually constructed the shell on the floor below then lifted and nailed it into place below the skylight. We took care to assure that the sides of the skylight well were insulated.

With an attic, the top side of the well is typically framed perpendicular to the roof plane and the bottom and sides are framed plumb.

For small skylight wells, the opening can be created with 3/4-inch plywood while larger openings are typically built like stud walls. Make sure the well is sealed tightly against air infiltration and well-insulated to guard against condensation on the skylight or sides of the well.

It is also possible to splay the two sides of a skylight well in addition to the top and bottom sides, although the framing and drywall are a lot more complicated.

To keep the four sides of the skylight window well flat, the bottom of the opening must be shaped like a trapezoid, wider on the high side of the opening. You can just see that feature in our skylight photo above.

To keep the opening rectangular at the ceiling plane, the two sides become curved like twisted strips of plastic. This requires framing with angled struts and bending the drywall. Either approach makes for a complicated framing job.

Also see Skylight Light Tubes.

Sealing the Underlayment at Skylights

To ensure a leak-free installation with any type of roofing, use peel-and-stick membrane to seal the skylight to the sheathing and underlayment, creating a primary barrier to water entry below the finished roofing.

Any water that manages to penetrate the flashing will be stopped by the membrane. Hold the roofing felt back about 6 inches from the opening so the peel-and stick membrane can bond directly to the roof sheathing.

After the skylight frame is in place, install 9- to 12-inch-wide strips of membrane around all four sides of the curb, starting with the bottom, and working uphill so the upper pieces always overlap the lower pieces (Figure 3-21).

Detail corners carefully using patches of moldable flashing (see FLASHING MEMBRANES PEEL & STICK) if necessary. Tuck the top piece of membrane under the roofing felt above, and lap the lower piece over the roofing felt below.

With ganged window units, use a single strip of membrane between each unit, running down one skylight frame, across the mullion, and up the next skylight frame.

Ice Dam Protection for Skylights

Snow can build up above skylights and ice can build up below them as snowmelt from the glass area refreezes.

As extra protection against leaks in areas subject to heavy snow and ice damming, some contractors run a wide band of the peel-and-stick membrane around the sides and top of the skylight and all the way down to the roof eaves (see Figure 2-5 at left.)

On low-slope roofs, the membrane often covers the entire roof surface and continues up and around any skylight curbs for complete protection.

Extensive details about ice dam prevention are at ICE DAM PREVENTION. Also see SKYLIGHT VENTILATION DETAILS.

Skylight Flashing Details

Older-style skylights (photo at left) were notorious for leaking (see SKYLIGHT LEAK DIAGNOSIS & REPAIR), but today’s pre-formed skylight flashing kits combined with bituminous peel-and-stick membranes have made leaky skylights mostly a thing of the past (see FLASHING MEMBRANES PEEL & STICK).

Most major skylight manufacturers provide flashing kits for asphalt shingles as well as specialty flashings for tile and metal roofing, typically with a wraparound head flashing and moldable apron flashing that conforms to the roofing profile.

With asphalt shingles, avoid skylights that provide only a continuous side flashing, which relies on roofing cement for a waterproof seal. Step flashing is much more reliable.

With metal roofing, use the manufacturer supplied kit or have the roofing contractor fabricate a matching flashing from the metal roofing material. While a custom flashing can create a more attractive installation, make sure the corners of the head flashing are properly sealed with solder or a high-performance caulk.

Also see
  SKYLIGHT LEAK DIAGNOSIS & REPAIR
    Diagnose & Repair Skylight Leaks
    Rooftop Skylight Leaks
    Water Testing Skylights
    Repairing Skylight Leaks
    Avoiding or Preventing Skylight Leaks

Skylight Installation on Low Slope Roofs

For roof slopes between 2:12 and 4:12, most skylight manufacturers will not guarantee a waterproof installation unless the skylight angle is raised with a built-up curb or a special inclined flashing kit (Figure 3-22).

The raised skylight curb should be sealed on all sides with peel-and-stick membrane carefully sealed at all corners, as in Figure 3-21.

Figure 3-21

Skylight Installations on Steep Slopes

For very steep slopes, such as mansards, skylight manufactures typically recommend a water deflector or Z-flashing to safely divert water around the unit. Check with the manufacturer for steep-slope requirements.

Skylight Manufacturers & Product Sources

Andersen Windows and Doors www.andersenwindows.com Skylights and roof windows with exterior sash clad with glass-fiber-reinforced material

Milgard Windows and Doors www.milgard.com Skylights with aluminum frames (thermal break optional) with vinyl subframes on operable models; optional motorized controls with rain sensor

Pella Windows and Doors www.pella.com Wood interior, aluminum exterior, optional motorized controls, and manual or motorized fabric-pleated shades

Roto Frank of America www.roofwindows.com Wood interior, aluminum exterior, optional motorized controls, and manual or motorized fabric-pleated shades; Sweet16 model fits 16 in. o.c. framing

Velux America Inc. www.velux.com Skylights and roof windows with wood interior and aluminum-clad exterior. Options include insect screens, blinds, motorized controls and shades with rain sensor, electrochromatic glass, and flashing kits for metal and tile roofs and mulled units

Skylight Light Tube Manufacturers & Sources

SolaTube www.solatube.com Light tubes from 10 to 21 in. in diameter; options include electrical lighting, daylight dimmer, and integral bath fan

Sun-Tek Skylights www.sun-tek.com Light tubes from 10 to 21 in. in diameter; options include electrical lighting and multitube Spyder skylight

Velux America Inc. www.velux.com Sun Tunnel light tubes from 14 to 22 in. in diameter with flexible or rigid tubes

Industry Associations for Windows & Doors

American Architectural Manufacturers Association (AAMA) www.aamanet.org

Efficient Windows Collaborative www.efficientwindows.org

National Fenestration Rating Council (NFRC) www.nfrc.org Sustainable by Design www.susdesign.com

Shareware calculators for sun angles, solar heat gain, and shading

Window and Door Manufacturers Association (WDMA) www.wdma.com

-- Adapted and paraphrased, edited, and supplemented, with permission from Best Practices Guide to Residential Construction.

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InspectAPedia.com^® - Daniel Friedman - Publisher & Editor.
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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  SKYLIGHTS, GUIDE
  SKYLIGHT CONDENSATION PROBLEMS
  SKYLIGHT DESIGN ISSUES
  SKYLIGHT ENERGY EFFICIENCY
  SKYLIGHT INSTALLATION Procedures
    Skylight Opening Framing
    The Skylight Window Well
    Sealing the Underlayment at Skylights
    Ice Dam Protection for Skylights
    Skylight Flashing Details
    Skylight Installation on Low Slope Roofs
    Skylight Installations on Steep Slopes
  SKYLIGHT LEAK DIAGNOSIS & REPAIR
  SKYLIGHT VENTILATION DETAILS

Basic Housing Inspection, US DHEW, S 352.75 U48, p.144, out of print, but is available in most state libraries; New York State version, ca 1955, source of our window parts and window repair sketches.
Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspect ion services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
The Journal of Light Construction has generously given reprint permission to InspectAPedia.com for adaptations, quotations, or reproductions used at this website. All rights and contents of the JLC material are ©Journal of Light Construction and may not be reproduced in any form.
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Skylight Installation Procedures

Skylight Opening Framing

The Skylight Window Well: the Light Well

Sealing the Underlayment at Skylights

Ice Dam Protection for Skylights

Skylight Flashing Details

Skylight Installation on Low Slope Roofs

Skylight Installations on Steep Slopes

Skylight Manufacturers & Product Sources

Industry Associations for Windows & Doors

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair