A skylight on a St. Louis roof is a hole with a window in it, and the part that keeps water out is not the glass. It is the curb, the flashing kit around the curb, and the metal or membrane that laps over the top edge. Most roof bids say "replace skylight" or "flash skylight" and stop there. A homeowner comparing two bids needs to know which curb, which flashing kit and which counterflashing each roofer means, because those three parts decide whether the first hard rain after the job stays out of the ceiling.
Material and spec
The roof spec should name the skylight by brand and model number if the existing unit is staying, or name the replacement unit if it is changing. The number is printed on the label inside the frame or on the glazing edge. Write it down before the crew arrives. A curb or flashing kit is sized for one unit, and a kit bought for a different width leaves a gap at one side that no amount of sealant closes cleanly.
Ask whether the curb is a site-built wood curb that will be framed, or a factory curb that ships with the unit. A site-built curb gives the roofer the most control over the framing height and the slope of the top face, but it depends on the carpentry being right. A factory curb is consistent, but it sets the height above the deck, and that height has to clear the shingle courses that will run up to it. The spec should state which one is going on, and the curb height in inches.
The flashing kit is the metal that sits on the deck and on the curb. Most kits have a head flashing, two side flashings and a bottom apron. The spec should name the metal, the gauge and the finish, the same way it would for a valley or a drip edge. Galvanized or aluminum is common. If the spec only says "flashing kit", the crew will use whatever came in the box, and the box may not match the metal on the rest of the roof.
Counterflashing and the shingle course
Counterflashing is the piece that laps over the top of the flashing kit and sheds water onto the shingles above. On a skylight, the upper counterflashing should lie over the head flashing with the shingle courses above it running down and over. The lower edge of the shingles should lie over the apron. If the bid says "install flashing per manufacturer" without naming the counterflashing, ask for a sentence describing which piece sits on top of which.
Shingle course placement matters here more than most homeowners expect. The shingles directly under the skylight curb are cut so that each course overlaps the one below it in the right direction. A crew that installs the shingles first and the flashing second, instead of the reverse, often leaves the head of the curb sitting on top of the shingle instead of under it. Ask each roofer which goes on first, and ask for a photo of the head flashing in place before the shingles reach it.
Ice and water membrane usually runs around the skylight curb on the lower sides, where snow and meltwater sit in winter. In St. Louis, the freeze and thaw cycle pushes water up under shingles at any low point, and a curb is a low point when the membrane stops short of it. The spec should say how far the membrane runs up each side of the curb, measured in inches from the curb edge, not just that it goes "around the skylight".
Sealant, and what it is allowed to do
Sealant is the last layer, and it is the layer most often used to hide a missing piece. A bead of sealant at the top corner of a curb does not replace a counterflashing that was left out. Sealant belongs at the joints where two metal pieces meet and at the seam where a curb touches the flashing, and the spec should name the product. Keys installs Atlas and Malarkey shingles only, and any sealant written into the spec should be one that the roofer will name and stand behind in the written scope, not a general "roof sealant".
Sealant has a cold limit as well. Late in the season, a bead that is applied below its recommended temperature may not cure before the first freeze. A spec that names the sealant also gives the crew a reason to hold the caulk gun until the temperature allows. Ask what the crew does if the temperature drops below the sealant's limit on the day the skylight is flashed.
What it costs
A skylight flashing kit is a small line on a roof bid, and that is why it gets left off or priced as a lump sum. The kit, the curb and the labor to fit them are a separate item from the shingle install. Ask each roofer to price the skylight as its own line, with the curb, the kit, the counterflashing and the membrane listed under it. Two bids that differ by a few hundred dollars on the skylight line may differ because one includes a site-built curb and one includes a factory curb, and those are not the same work.
If the skylight is being replaced, ask whether the old curb stays. An old curb that has been leaking may be rotten at the framing, and the new flashing kit cannot fix wood that has already soaked. The bid should say what happens if the decking around the curb is found soft once the old shingles come off. Ask for the unit price for replacing a section of deck, in writing, before the work starts.
What to ask
- What is the exact skylight model and width, and does the curb match it?
- Is the curb site-built or factory, and what is the finished height above the deck in inches?
- What metal, gauge and finish is the flashing kit, and does it match the metal on the rest of the roof?
- Which piece sits on top of which, from the head flashing down to the apron?
- How far up each side of the curb does the ice and water membrane run?
- Which goes on first, the flashing or the shingles, and will the crew send a photo before the shingles cover the head flashing?
- What sealant is named in the spec, and what does the crew do if the temperature is below its limit?
- What is the unit price for replacing soft decking around the curb?
Put the answers in the roof spec, not in a phone call. If a roofer cannot name the curb, the flashing kit and the counterflashing in one sentence each, the skylight line on that bid is not yet a complete scope. The office at Keys can be reached at (314) 220-2333 if you want to walk through the spec before you choose a roofer.