Acrylic or polycarbonate dome skylights come is several sizes today. The traditional ones, however, are square to rectangular in shape, and run about 5″ in height. Their width and length can be anywhere from 12″ to 36″, and are 1/8″ thick.
These domes can be mounted onto wooden curbs (2×2″ to 2×6″), around which the roof shingles are caulked or thin-metal flashed to prevent any rain leakage around them. These domes can also be surface mounted directly onto the tar paper and wooden sheeting directly beneath the shingles.
In the latter case, the dome’s surrounding shingles themselves are directly laid over its flat outside flanges similar to the way the shingles are commonly laid over the flat metal of roof vents, furnace stacks, sewer vents and so forth. This installation minimizes the dome’s height.
Problem. Either way, or no matter how the domes are mounted, they can and will produce unwanted condensation and similar leaks even if they have an inner plastic shield within them. These domes are also vulnerable to being cracked from large hail, fallen tree branches, earthquakes, or careless roofers. When these events happen, the skylights will leak rain water as well in addition to forming moisture.
Solution. Build and install a shallow-box-like 5-1/2″-high hard transparent cover over the dome on the roof. This cover will act like a storm window does by taking the brunt of temperature change and the formation of most condensation. For this article, a 20″ square dome (which includes a 1″ flange on each side of the 18″-square dome itself), 5″ high, and surface mounted directly to the roof sheeting will be used as the example to be covered with the box-like storm cover. Similar covers can be built proportionally for other dome sizes.
Materials and tools (total cost for parts: $50-90, depending on the availability of the acrylic sheet).
- One 2x6x96″ pressure-treated board ($5)
- One 22×22″ sheet of heavy 0.22″ thick acrylic plexiglass ($25-60, depending on the cut sizes available at the outlet). If the piece must be cut from a larger sheet, leave the protective films unpeeled on both sides of the sheet until the sawing is done. Use a sharp fine-tooth saw blade. Otherwise, let the outlet cut it to size for you.
- Twelve #12×3″ Philips screws
- 20 to 30 – #8×1″ Philips drywall screws (wide head)
- One small tube clear silicone caulk ($4)
- Two tubes silicone window/door caulk similar in color to the roof shingles ($8)
- Four perpendicularly flat 2-1/2″ L-shaped brackets ($5)
- Eight #10×1″ wood screws (for attaching the L-brackets to the cover frame)
- Eight #10×1-1/2 to 2″ wood screws (for attaching the frame’s L-brackets to the roof through the shingles)
- One quart oil-base water-proof paint similar in color to the roof shingles ($4)
- Tools: electric circular saw, electric drill, bits, Philips screwdriver, caulking gun, putty knife, rat-tail file, paintbrush
1. Cut the 2×6″ board into four square-cornered pieces: 2 22″-long ones and 2 19″-long ones.
2. Assemble the 22″ square box-like frame by sealing/securing the four corners with the silicone window/door water-proofing caulk and 3″ screws.
3. On what will be the lower-end of the bottom side, use the file to make at least three half-moon groves for moisture drainage.
4. Paint both the inside and outside of the wooden frame. Let dry.
5. Place the acrylic plexiglass flush over the frame.
6. Mark on the plexiglass where the screw holes will go, about 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. Then, by using a narrow bit, drill holes 1/2″ deep into the wood frame dead-center through the plexiglass holes.
9. Remove the plexiglass from the frame, noting how it fits onto the frame.
10. Brush away any plastic or wood shavings from the frame and plexiglass.
11. With the silicone caulk, add a narrow bead lengthwise on each side of the frame’s drill holes all around it.
12. Carefully replace the plexiglass atop the frame, aligning it to the original drill holes accordingly.
13. Install the screws to medium tightness, so not to crack the plexiglass. The caulk will flatten and spread between the plexiglass and the frame, sealing it.
14. Add at least one flat L-shaped bracket to each side of the frame flush to its bottom side with the 1″ wood screws.
15. Paint the brackets.
Cover installation (the easy way).
1. While using safety precautions, carry the completed cover up to the roof, assuming the roof is a low-pitch and safe to walk on.
2. Center it over the dome skylight, square to square.
3. Attach the dome to the roof through the shingles with the 1-1/2 to 2″ wood screws, depending on the thickness of the shingles.
4. Seal the bottom side of the upper end and both sides of the cover with the silicone window/door caulk. Seal the brackets and screws the same way. Leave the bottom side of the lower end unsealed for drainage. It is a storm window.
5. If necessary, touch-up the frame and brackets with paint.
By installing the cover atop the shingles, the wood sheeting beneath the shingles is also protected from any moisture damage. This placement also elevates the cover over the dome for adequate air space.
This cover will assume the formation of almost all the condensation that would otherwise occur within the dome, none of which will make it through the dome into the house nor beneath the shingles. It will also protect the dome from hail, fallen tree branches, and other damages. To learn more about skylight types and their maintenance, see these sites.