RV Skylight Reseal and Replacement: When to Seal It, When to Swap It

Most skylight calls start the same way: a brown ring on the ceiling over the shower, and an owner who swears the roof looked fine last month. From a distance, it did. Get up there and the sealant is cracked open, the dome is yellow and chalky, and there is a hairline crack running from a screw hole. That skylight did not fail last month. It failed slowly, in the Florida sun, over a couple of years.

Good news: this is one of the most DIY-friendly roof jobs there is — if you make the right call first. Reseal a sound dome, replace a dead one.

First call: reseal or replace?

Reseal it if the dome is still healthy: clear or evenly tinted, flexible when pressed gently, no cracks, no crazing — that spiderweb of fine lines — no chalky brittleness. If the only problem is sealant that has cracked, shrunk, pulled away from the flange, or left screw heads exposed, that is a reseal job.

Replace it if you see any of this:

• Cracks, especially radiating from screw holes or corners
• Yellow, hazy, or chalky plastic that feels brittle to the touch
• A dome that flexes and crackles, or has taken a branch or hail hit
• Water stains below after a past reseal — the dome is letting go

Sealant over a cracked dome is a bandage, not a repair. The crack keeps moving with heat and road vibration, and water wicks under the flange until the roof deck goes soft.

Measure before you order anything

Do not order by the hole you think you have. Measure three things: the rough opening in the roof, the outside flange dimensions of the old dome, and its height. The most common opening I run into is 14 by 22 inches, but the flange that covers it is several inches larger all around, and neo-angle shower skylights are a completely different shape. Brands like ICON and Specialty Recreation size by the opening, but flange and height vary — and if you are keeping your inside garnish, height has to match it. Measure the old part, photograph it, then order.

Material matters too. Polycarbonate takes impacts far better than basic acrylic, which goes brittle faster in UV. In Florida, that difference is not academic.

Resealing a sound dome

Clean first. Wash the area, then pick out any sealant that is loose, cracked through, or separating. Sound, well-bonded sealant can stay — do not scrape a roof membrane raw to prove a point. Use a plastic scraper, never metal, especially on EPDM or TPO: one gouge and your reseal just became a roof repair. Wipe the joint with denatured alcohol and let it dry.

Then run a bead of Dicor 501LSW-1 Self-Leveling Lap Sealant over the flange edge and every screw head. Self-leveling means exactly that: on a flat roof surface it flows out and settles into the low spots on its own. Give it an hour or two, look for gaps or pinholes, and touch those up. Never use household silicone up here — it peels, nothing sticks to it later, and it turns the next reseal into a scraping nightmare.

Replacing the dome, start to finish

Scrape the old sealant off until the screw heads show, back every screw out, then work a putty knife gently under the flange to break the old bedding loose — holding the roof skin down as you lift so you do not tear it. A little warmth from a heat gun on low helps. Patience beats force here.

Clean the mounting surface down to sound roof. Old butyl tape and sealant residue have to go; new sealant over an oily, dirty surface is a leak with a waiting period. Wipe it down and let it dry.

Check the deck while it is open. Soft, dark, or crumbly wood means water has been in there a while — fix that now, not after the new dome is on.

Lay fresh butyl tape on the underside of the new flange, continuous, no gaps at the corners. That tape is the real seal — everything on top is the second line of defense. Set the dome, start every screw, then snug them down evenly. Do not overtighten. Cranking a screw until the flange bows is how brand-new domes crack at the screw holes, sometimes before the job is even done.

Cap the perimeter and every screw head with the Dicor. Plan on up to two tubes for a full-size skylight — running short halfway around is not where you want to be. EternaBond RoofSeal tape has its place on seams and emergency patches, but for a fresh skylight install, butyl under the flange plus lap sealant on top is the combination that lasts.

Test it, then check it on a schedule

Once the sealant has skinned over, hose-test it while someone watches inside with a dry paper towel around the garnish. A drip found now is a five-minute touch-up. A drip found next month is a ceiling panel.

After that, put the skylight on the same 90-day look you give every other roof seal: cracks, exposed screws, sealant pulling away. Ten minutes on the roof a few times a year is the difference between a tube of sealant and a roof deck rebuild.

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