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Key takeaways
- Lap sealant is a flowable product applied over and around fasteners and edges. Butyl tape is a preformed strip that sits between a mount's foot and the roof.
- Roof surface matters most. One major maker states its lap sealants suit EPDM, TPO and PVC membranes, and says nothing about fiberglass or aluminum on the pages.
- Different tapes and sealants carry different use limits. One butyl tape from that maker is labeled for professional use only.
- Sealant keeps water out. The mount's fasteners hold the panel to the roof, as the mount maker's instructions say.
- If the roof surface or a product's compatibility is unclear, ask the RV maker and the sealant maker before you seal anything.
Why the choice matters for solar mounts
A solar mount adds holes and edges to a roof, and every one of them is a place for water to enter. Water behind a roof membrane or into a wall can do a lot of damage before it shows. That is why sealing is not an afterthought on a solar install. It is part of the job, and the choice of product matters.
It also matters because sealing materials differ in how they stick, how they cure and what they tolerate. A product that works well on one roof surface may not bond, or may react, on another.
This article compares the two common choices by what they do and what to check. For choosing between adhesive and mechanical mounting, see our guide to installing rigid panels and the mounting kit guide.
How butyl tape and lap sealant work
Butyl tape
A strip of tacky, rubber-like tape that you press between a surface and the thing mounted to it, so that the joint seals as the hardware is tightened. It goes on clean and dry. Dicor's butyl seal tape page says surfaces must be clean and free of moisture and contamination, tells you not to stretch the tape while applying it and advises against using it where acidic or alkaline chemicals are present.
Lap sealant
A flowable sealant that you apply along seams, over screw heads and around the edge of a foot, where it spreads, cures and forms a seal. Dicor's FAQ notes that its sealants contain a small amount of petroleum distillates that can cause bubbling and rippling as they cure, and that full cure takes much longer than becoming waterproof. Bubbling is a characteristic to expect, not a reason to panic, but follow the maker's cure notes.
The two are often used together: tape under a foot and lap sealant over the fastener heads and the foot's edge. Whether your mount maker wants that is in the instructions, so read them before you mix products.
Match the product to the roof surface
The roof's material decides what you can use. Start by finding out what you have, from the RV maker's manual or the maker itself.
| Roof surface | What the maker pages say | What to do |
|---|---|---|
| EPDM, TPO or PVC membrane | Dicor's FAQ says its 501 and 551 sealants are compatible with EPDM, TPO and PVC membranes | Confirm the exact product name on the label, and follow its application notes. |
| Fiberglass | The Dicor pages do not mention fiberglass | Ask the RV maker and the sealant maker which products they support. Do not assume. |
| Aluminum or other metal | The pages do not address it for the sealants. One butyl tape page mentions adhesion to galvanized metal | Check the product page and the RV maker. Do not assume it carries over. |
| Unknown | No information | Find out first. The RV maker, the dealer or the owner's manual can tell you the roof material. |
Cleaning matters too. Dicor's FAQ says not to use cleaners or conditioners containing petroleum distillates, harsh abrasives or citric-based cleaners on the roof. Use what the roof maker and the sealant maker recommend.
What really holds the mount
Neither tape nor sealant is the thing that keeps panels on the roof. The mount's fasteners do, installed as the mount maker instructs. Go Power's installation notes for its kits put it plainly: all four mounting feet must be securely fastened to the RV roof, or the panels could lift and separate while the RV is moving.
Sealant has a different job: keep water out of the holes. Treat the two as separate tasks and do both properly. If your mount maker specifies a particular sealant or a sealing method, follow it exactly..
Cable entry points are another leak risk. Our guides to choosing a roof cable entry gland and installing on a curved roof cover those details.
Where sealing goes wrong
Most leaks around solar mounts trace back to a handful of avoidable mistakes.
- Sealing over dirt or moisture. Both makers' pages stress clean surfaces, and tape in particular needs a clean, dry surface to bond.
- Stretching butyl tape. Dicor's page tells you not to stretch it while applying it.
- Using a product the maker does not list for your roof. A sealant that suits one membrane may not be stated for another surface.
- Cleaning with the wrong product. Some roof cleaners contain ingredients the sealant maker tells you to avoid.
- Skipping the cure notes. Cure and recoat timing is on the label for a reason, and rushing it can undo the work.
- Forgetting the cable entry. The mount feet get attention while the hole the wires pass through is left until last.
None of these needs special skill to avoid. They need patience and the maker's documents in front of you while you work.
A sensible process
Work through it in this order.
- Identify the roof material from the RV manual or the maker.
- Read the mount maker's instructions for sealing and the sealant maker's data for that roof surface.
- Clean and dry the area with a cleaner the roof and sealant makers allow.
- Test-fit the mount. Plan fastener locations and cable routes before you open a tube.
- Apply tape or sealant as the makers instruct, noting temperature and weather limits on the label.
- Fasten the mount to the maker's instructions, then seal fastener heads and edges.
- Leave the sealant to cure as the label says, and inspect it again after the first few trips.
Add a seal check to your routine. Our spring RV solar check includes a look at the roof and the mounts, and roof seals deserve an inspection after any heavy weather. Water stains inside the coach near a mount are a reason to stop and investigate, not wait.
Frequently asked questions
Is butyl tape or lap sealant better for RV solar mounts?
Neither is better in general. The roof surface and the mount maker's instructions decide, and they are often used together: tape under the foot and lap sealant over the fasteners and edges.
Can I use any lap sealant on a fiberglass roof?
Do not assume so. The Dicor pages state compatibility for EPDM, TPO and PVC membranes and do not mention fiberglass, so ask the RV maker and the sealant maker.
Does sealant hold the panel mount to the roof?
No. Fasteners installed to the mount maker's instructions do that. Sealant keeps water out of the holes.
Why does lap sealant bubble while it cures?
Dicor's FAQ says its sealants contain a small amount of petroleum distillates that can cause bubbling and rippling during cure. Follow the maker's cure guidance.
How often should I check roof seals around solar mounts?
Check them regularly and after severe weather, and look for cracks, gaps and water stains inside. Your sealant and RV makers give specifics.
Sources and verification
- Dicor Products FAQ: its 501 and 551 sealants are compatible with EPDM, TPO and PVC membranes, petroleum distillates can cause bubbling and rippling during cure, avoid cleaners containing petroleum distillates, harsh abrasives or citric bases, cure timing differs from waterproofing. The page does not mention fiberglass or aluminum for the sealants.
- Dicor butyl seal tape product page: clean, dry surfaces, do not stretch the tape, avoid acids, bases and organic solvents, labeled for industrial and professional use only, adhesion to galvanized metal stated. Fiberglass, aluminum, EPDM and TPO are not mentioned on that page.
- Go Power! support, cautions and safety for solar kits: all mounting feet must be securely fastened to the roof or panels could lift and separate in transit.
Statements about product compatibility come from these makers and apply to their products only. No temperature, cure-time or quantity figures are quoted, and no compatibility is claimed for roof surfaces the pages do not cover. Confirm your roof material with the RV maker and your sealant with its maker. This article makes no structural or roof-load claim.