Install a Skylight 4 - Preparing the Openings Install a Skylight 4 - Preparing the Openings

Installing a skylight in a room that has a finished ceiling with an attic or crawl space above it involves planning both a ceiling opening and a roof opening. On the other hand, you'll need to lay out only one opening if the area you're working in has an open beam ceiling.

Regardless of the type of construction, the size of the roof opening will be determined by the size of the skylight you're installing. Generally, the dimensions of the roof opening in a house with finished ceilings are the same as the inside dimensions of the skylight or of the curb if it's a curb-mounted unit.

Most manufacturers provide the necessary dimensions along with installation instructions. If a manufacturer gives the curb's outside dimensions, deduct twice the curb's thickness from the length and width to determine the size of the opening.

Contractors differ on the methods they use to determine the location of roof and ceiling openings in homes with attics or crawl spaces. Some contractors cut the opening in the ceiling before cutting the roof opening. Others cut the roof opening first, and this is the method described below; it allows you to adjust to the conditions in your home.

Planning and Marking the Ceiling Opening

Whether your desire is for moon-beams on your bed, diffused light for your art projects, or sunlight splashing across your dining room table, the location of your ceiling opening depends on where you want the light and what kind of light you need.

Even after you've identified the ideal location, you may want to make some adjustments if you discover some structural impediment when you explore your attic.

The size of the ceiling opening depends not only on the skylight's size, but also on the amount of light you want to bring in. The light enters the room through a light shaft, which can be straight, angled, or splayed.

An angled or splayed light shaft allows you to offset the roof opening from the ceiling opening. If you want to maximize the amount of light coming into the room, make the ceiling opening larger than the roof opening and connect them with a splayed light shaft; you can splay any or all shaft walls.

After you've decided on the location and size of the ceiling opening, mark the four corners and the center of the proposed opening. Drive nails deeply enough through these five points so you can find them in the attic. If any of the nails hits solid wood, you may want to move the proposed opening or adjust its size to avoid the obstruction.

Planning and Marking the Roof-opening

Much of the work of installing the skylight is done from the attic or crawl space. If you don't have convenient access to your attic or your attic is not roomy enough to work in, you'll need to cut a hole big enough to climb through or work through (from a ladder or platform) within the proposed opening, at one side of the center. Then, after you've marked the center of the roof opening, you can cut the ceiling opening for access.

If your attic is insulated, wear clothing that will protect you against insulation material: gloves, a long-sleeved shirt, long pants, a painter's mask, and goggles.

Checking for Obstructions

Clear away any insulation material covering the area of your proposed ceiling opening. Locate the nails that were driven through from the ceiling below. Look for obstructions-wires, pipes, or heating or cooling ducts-within the area of the proposed opening. If you find any and don't want to move or adjust the size of the opening, you'll have to move the obstruction.

To save yourself work when you're framing the ceiling opening, try to arrange the opening so two opposite sides butt up against the facing sides of two joists.

Measuring the Opening

To locate the center of the roof opening, hang a plumb bob from the underside of the roof with the point of the bob over the center nail in the ceiling opening. Mark this point clearly on the underside of the roof.

With your tape, square, and straightedge, mark the manufacturer's recommended dimensions for the roof opening on the underside of the roof.

You can simplify the framing of the opening by locating at least one edge (preferably two) against a rafter.

Check the area of the roof opening for obstructions. If you find wires, pipes, or heating or cooling ducts, you can either move the opening or move the obstruction. If the roof ridge or a purlin (a structural member positioned at right angles to the rafters) crosses the opening, you'll have to relocate the opening and use an angled or splayed light shaft between the roof and the ceiling.

Marking the Corners

You'll want to be able to find the corners of the opening when you go up on the roof. Drill a hole at each corner and drive a 16-penny nail (20-penny for a shake roof) through each hole.

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