Acrylic or polycarbonate dome skylights come in various sizes today. Traditional ones, however, are square to rectangular in shape and are approximately 5″ tall. They can range in width and length from 12″ to 36″ and are 1/8″ thick.

These domes can be mounted to wood curbs (2×2″ to 2×6″), around which roof tiles are caulked or thin metal flashing is applied to prevent rain from seeping around. These domes can also be surface mounted directly to the tar paper and sheets of wood directly under the shingles.

In the latter case, the tiles surrounding the dome are placed directly on its flat outer flanges in a similar manner to how tiles are placed on the flat metal of roof vents, chimneys, sewer grates, etc. This installation minimizes the height of the dome.

Problem. Either way, or however domes are mounted, they can and will produce unwanted condensation and similar leaks, even if they have an internal plastic shield inside them. These domes are also vulnerable to cracking from large hail, fallen tree limbs, earthquakes, or careless roofers. When these events occur, the skylights will also filter rainwater in addition to forming moisture.

Solution. Build and install a 5-1/2″ tall shallow box-like hard transparent cover over the dome on the roof. This cover will act like a storm window by taking the brunt of temperature change and formation of most condensation For this article, an example will be used to cover a 20″ square dome (including a 1″ flange on each side of the 18″ square dome), 5″ high and mounted on the surface directly on the roof sheet.Similar covers can be built proportionately for other dome sizes.

Materials and tools (total cost of parts: $50-90, depending on availability of acrylic sheet).

• One 2x6x96″ pressure treated board ($5)
• One 22×22″ sheet of 0.22″ thick heavy acrylic plexiglass ($25-60, depending on available cut sizes at the store). If the part is to be cut from a larger sheet, leave the protective films unpeeled on both sides of the sheet until sawing is complete. Use a sharp, fine-toothed saw blade. Otherwise, let the outlet cut it to size.
• Two #12×3″ Philips screws
• 20 to 30 – #8×1″ Philips Drywall Screws (Broad Head)
• A small tube of clear silicone caulk ($4)
• Two tubes of silicone window/door caulking similar in color to roof shingles ($8)
• Four 2-1/2″ Perpendicularly Flat L-Brackets ($5)
• Eight #10×1″ wood screws (to attach L-brackets to deck frame)
• Eight #10×1-1/2 to 2″ wood screws (to attach frame L-brackets to roof through shingles)
• One quart of oil-based waterproof paint similar in color to the roof tiles ($4)
• Tools: electric circular saw, electric drill, drill bits, Philips screwdriver, caulking gun, putty knife, rat-tail file, brush

roof building.

1. Cut the 2×6″ board into four square corner pieces: 2 22″ long and 2 19″ long.

2. Assemble the 22″ square box frame by sealing/securing the four corners with the waterproof silicone window/door caulking and 3″ screws.

3. On what will be the bottom end of the bottom side, use the file to make at least three crescent-shaped slots for moisture drainage.

4. Paint both the inside and outside of the wood frame. Let dry.

5. Place the acrylic plexiglass flush on top of the frame.

6. Mark on the plexiglass where the screw holes will go, approximately 5″ apart around the top of the frame.

7. At each mark, drill a hole through the plexiglass (only) wider than the drywall screw.

8. Next, using a narrow bit, drill 1/2″ deep holes at the dead center of the wood frame through the plexiglass holes.

9. Remove the plexiglass from the frame, noting how it fits into the frame.

10. Brush any plastic or wood chips from the frame and plexiglass.

11. Using the silicone caulk, add a narrow bead along each side of the frame’s pre-drilled holes around it.

12. Carefully replace the plexiglass on top of the frame, lining it up with the original drilled holes where appropriate.

13. Install the screws with a medium tightening, so as not to break the plexiglass. The putty will flatten out and spread between the plexiglass and the frame, sealing it.

14. Add at least one flat L-bracket to each side of the frame flush with its underside using the 1″ wood screws.

15. Paint the supports.

installation cover (the easy way).

1. While using safety precautions, bring the entire canopy up to the roof, assuming the roof has a low slope and is safe to walk on.

2. Center it over the dome skylight, square by square.

3. Attach dome to ceiling through shingles with 1-1/2 to 2″ wood screws, depending on thickness of shingles.

4. Seal the bottom side of the top end and both sides of the cover with silicone door and window caulking. Seal the brackets and screws in the same way. Leave underside of bottom end unsealed for drainage. It’s a storm window.

5. If necessary, touch up the frame and supports with paint.

By installing the deck over the shingles, the plywood under the shingles is also protected from any moisture damage. This location also raises the cover above the dome for proper air space.

This cover will take over the formation of almost all of the condensation that would otherwise occur inside the dome, none of which will pass through the dome into the house or under the shingles. It will also protect the dome from hail, falling tree limbs and other damage. For more information on the types of skylights and their maintenance, check out these sites.