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Skylight well (C) Daniel FriedmanHow to Install a Skylight
Detailed Guide to Installing & Flashing Around Skylights

Skylight flashing installation 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 describe: How to construct the skylight well, framing, opening, Sealing the underlayment around skylights, 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, and Skylight Installations on Steep Slopes.

We also discuss how to install a skylight on low slope roofs and on very steep roofs.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Skylight Installation Procedures

(C) J Wiley, S Bliss

This skylight installation and repair article series 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.

Article Contents

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 (Steve Bliss, J Wiley & Sons) 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

Skylight well (C) Daniel Friedman

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

(C) J Wiley, S Bliss

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

(C) J Wiley, S Bliss

Snow can build up above skylights and ice can build up below them as snow melt 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 ROOF ICE DAM LEAKS. Also see SKYLIGHT VENTILATION DETAILS.

Skylight Flashing Details

Bubble type leaky skylight (C) Daniel FriedmanOlder-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

Skylight Installation on Low Slope Roofs

(C) J Wiley, S Bliss

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.

(C) J Wiley, S Bliss

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 Installation Q&A

Reader Question: how do I install a skylight across several rafters?

I am a member of Church of God in Christ Mennonite. My husband is an air force veteran from 73 to 77. We are building a new house in Deridder,Louisiana. We are planing soon to install a skylight for over the kitchen island. Our kitchen faces directly South.

We have an 8 in 12 roof. The house has 8 ft 9 inch ceilings and 8 inch walls. The island runs north and south and is 7 foot by 4 foot. Our skylight is a Velux (Velux skylight model # GGL P10 3073).

We want to install the skylight longways with the island in a position that the average sun angle perimeter hits the north most edge of the countertop.

The island will be 36 inches tall with approx. 1 inch soapstone countertop. There should be a simple enough program for us to use to help us calculate the skylight placement. Thank-you for your help. - Anon

Reply:

(C) J Wiley, S Bliss

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with framing, building plans, or an existing building that is to be modified - seeing things that are not of course included in a person's email question nor answer.

That said,  I'm not sure what needs to be calculated. If you want the skylight to be placed over the island then you know the exact location of the skylight in the first place so you're not calculating location.

And running a skylight along the slope of the roof so that one or more rafters need to be cut and interrupted is a standard roof framing problem solved by adequately sized headers running parallel to the eaves and ridge, and possibly, doubling the rafters at either end.

For large roofs where the loads are greater, engineered wood beams may be more efficient for that framing location.

Typically, the top side of the skylight well runs at 90 degrees to the roof slope, and the bottom side, if splayed, is framed plumb (see Figure 3-20). On very steep roofs where the top of the skylight is near eye level, a horizontal angle works well for the top side of the well.

The sides are usually left plumb to simplify the framing (see the "light well" at The Skylight Window Well. Also see Skylight Light Tubes.)

Watch out:  However you are asking for trouble unless you consult with the skylight manufacturer and obtain a properly designed and flashed unit.

That's because you want to run the skylight across rather than along the slope of the roof, and because typically the flashing kit that comes with a factory-made skylight presumes that the upper end of the skylight is predefined and is in the narrow width of the unit.

Skylight Types

Using their terminology, Velux offers three types of residential skylights for applications where a building egress is not required through that opening: [adapted from information provided by Velux [3].]

  1. Deck-mounted skylights. A deck mounted skylight combines an insulated glass lens, pre-finished white wood frame and protective aluminum or copper cladding into one unit that mounts directly to the roof deck.
  2. Curb-mounted skylights. A curb-mounted skylight combines an insulated glass lens and maintenance free frame that is designed to mount directly to a site-built curb.
  3. Pan-flashed skylights. A pan-flashed skylight combines an insulated glass lens and an all-in-one frame and flashing that is designed to mount directly to the roof deck using roofing sealant.

Reader followup:

We purchased the step flashing kits from Velux along with the skylights.

The island as well as the skylight will run north and south. The ridge will be north of the skylight and  the eaves south which means the skylight is running on the slope.  Sorry, I wasn't clear on that.  The gables face east and west. 

We obtained some information from NOAA weather saying that our sun angles run from 36 degrees to 84 degrees in our area at noon throughout the year. 

Considering our allowable attic space, we were able to run the top of the north wall of the skylight well on a 65 degree angle down to the ceiling point directly over the north end of the counter. 

Not being used to installing skylights, we will see how this works when we cut the hole and install the skylight.

My carpenter has the installation instructions that came with the skylight and I plan to copy the instructions from your website as well for him.  Thank-you very much. Please leave my name anonymous.  Thank-you again. 

P.S.  Our goal is to light the large kitchen island as well as possible for the most amount of the year as possible.

Reply:

Sounds good, Just make sure that your flashing kit has a proper HEAD FLASHING along the up roof side of the skylight. What I intended to warn before is that step flashing won't work correctly along the up-roof or uppermost edge of the skylight. Velux and other skylight manufacturers provide specific head flashing for that location.

Watch out: Also, if you purchased an operable (openable) skylight whose uppermost end is in its narrow axis and if you then rotate it to install it with a long axis uppermost, the window may not open, vent, nor function correctly.

You can avoid all of these concerns by making sure you, your builder, and the skylight manufacturer/provider are all on the same page.

The skylight series you cited, (Velux skylight model # GGL P10 3073), is a 94cm x 160cm operable (can be opened) skylight that includes a center pivot and a pine finish vent opening across the 94cm axis and that is intended to be mounted in the uppermost position. This skylight is designed for roof slopes from 15o to 90o.

Contact information for Velux Corporation in the U.S. is

The company is happy to provide detailed installation instructions for their products and for the various flashing kits used with their skylights.

Skylight Manufacturers & Product Sources

Industry Associations for Windows & Doors

-- Portions of this article were adapted and paraphrased, edited, and supplemented, with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

Thanks to ASHI home inspector John Craynor for editing help with this article. - Ed.


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