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Old 01-08-2021, 05:23 PM   #1
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Black & Grey Tank Roof Vent Leak - Need to know

After removing several black and grey tank roof vents during roof repairs, I noticed a common rusting problem with the screws mounting the vent cap to the roof. This problem has shown up across all 18 of the black and grey tank vents I have removed so far. I am not sure what year Sunline started using this style of tank/plumbing vent.

I am passing along the findings so when club members change their tank vents due to damage or are up on the roof caulking other areas, they can seal the vent in a better way to prevent this leak in the future.

This rusting screw issue may be a larger industry-wide issue. I know Jayco and Coachmen have used this vent, and both companies sealed them as Sunline did.

The tank/plumbing vent I am referring to is a Custom Plastics two-piece plumbing vent. The vent has a lower base to hold and seal off the vent pipe's sides and an upper cap with three mounting legs covering the open vent pipe. Here is the original manufacture, I do believe. Custom Plastics | Specialty (Select Roof Vents) They are an excellent vent cap; I have no issues with the vent itself; the problems come with how the installer seals the vent to the roof.

Here are some pictures to show problems. These two tank vents came off of different campers with extreme opposites in roof maintenance over the camper's life, yet the rusted screw issue displays the same mode of rusting.

This vent cap is on my 2004 T310SR I changed in 2019, which I have maintained the roof since 2007.

In 2017 this vent cap was removed from my 2004 T1950 project camper, now in restoration. This camper did not have very much roof maintenance since it was new. I can tell the original caulk still existed in many areas with no touchups.

The screw rusting leak mode was in the same place between those two extremes and the other 14 vents I have had off over the years.

Here is the problem as I see it happening. These pictures were from my T310SR when I changed the vents in 2019 as they started to become brittle. I know the caulking was still in good condition, yet the rusting screw issue still prevails.

The problem starts at the three screws that attach the vent cap through the vent base and then onto the roof circled in red. This picture shows how the water was getting to the screws even with good caulk on the perimeter, which was well maintained.

Not every screw rusted the same; how much water collected in each location could be different. The top of this screw holding the cap and base in the red circle is not rusted at the head like the other two on the same vent. The other vents I removed over the years had similar patterns; some had all three screws heads rusted, others only one or two.

The red arrows and circles point to the problem areas, which have no added sealant to prevent water from getting to the screw threads from the inside of the vent cap. Water collects and puddles in the gap between the vent cap's three legs in the red arrow area as the vent cap covers the vent base.

The circles show the dirt witness trail the water followed to get to the screw threads.

The screws removed in this picture are next to their corresponding holes. Two of the three screws that go through the cap and base have the most advanced corrosion. Once the screws start rusting at the exposed threads at the cap and base junction, the rust keeps migrating up and down the entire screw over time. In some cases, the wood in the roof supporting the screws was rotted, not leaving screw clamping pressure to the cap.

What is missing is sealant on the backside of the vent cap legs at the screw locations, like this. Here is the fix if you are not removing the vent. Clean the area and caulk with Dicor lap sealant. You can do more if removing the vent. (See the next reply in this post.)

The simple lack of caulking behind the vent cap legs is the problem. There are no directions that come with these vents on how to seal them, and the issue may still be ongoing today on new campers. I never realized this issue until enough of them keep coming up with rusted screws; there is a repeating problem. Some of the screws in the pictures above have rust only at the tip of the screws; this I feel is from attic moisture rusting from the tip of the screw upward. The 1” long screw hangs below the 1/2" OSB reinforcing pad used on the Sunbline rubber roofs exposing the screw end to the attic environment. This vent is next to the shower steam dome, a general area that allows moisture to enter the attic.

Here is a vent cap off one of my good friends Jayco. You can see there is no caulk behind the vent cap legs.

If you are looking for replacement vent caps of this style, here is where I have found them to be cheaper. The cost on the web is all over the map. Free shipping also differs. I have also noticed the patent may have run out on this vent, as other brands are for sale with very slight tweaks in the molding process.

I have used RV Upgrades in the past and had a positive transaction.

I will add a second reply with the “how-to” remove and install a new vent for those who have not been through this before. These plastic vents do not last forever; sun damage makes them brittle.

Hope this helps


For more pics on the tank vents, see my Flickr page here

Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC

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Old 01-08-2021, 05:26 PM   #2
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Here is a picture of “how-to” remove and install your tank vents.

Most Sunline roofs are not a direct walk-on roof. When servicing items on the rooftop, precautions need to be in place to prevent damage to the roof system or you falling through it. Start with putting a canvas or other protective material over the rooftop to stop abrasion.

Using a sturdy ladder, reach from the side, place manageable sized plywood pieces onto the tarp to spread your weight out over a larger area. 3/8” or thicker plywood 24 – 30” wide by 48” long works well.

Feel the roof for the stiffness of the rafter locations that are under the rubber or metal membrane. Place the plywood over a minimum of two rafters to support the single sheet of plywood. Take care getting on and off the ladder. Crawl around on your hands and knees while working on the roof helps also.

Using a heat gun, gently warm the old sealant, use a metal putty knife with all blade end edges dulled/rounded up, and scrape up the old caulk.

Remove the six screws.

Warm the lower base of the vent to soften the butyl tape. Gently work the putty knife under the base flange. Take care not to dig the roof. Lift the old cap and base off the roof.

Gentle heat and scrape what you can to get the old sealant and butyl off the roof.

With nytril gloves on, use mineral spirits on a rag and wipe the area clean. Do not pour a cleaner on the roof; only apply with a rag. Rubber roofs are affected by these cleaners when left for a long time. You can scrub. Wipe the roof dry as soon as you have completed the mineral spirits cleaning. Follow with a high flash cleaner, Naphta, denatured alcohol, lacquer thinner, etc., on a rag and wipe clean and dry the entire vent area. The high fash cleaner will remove leftover residue from the mineral spirits and dry quickly.

Prepare the new tank vent base with high-quality butyl tape. Tape 1” wide x 1/8” thick works well. Here is the brand I use and where I buy it. GSSI MB-10A butyl tape Other steel building or roofing supply places sell other good brands as well. I caution to “not” buy butyl tape from Amazon. I have had too many unusable rolls shipped by their packing process.

Clean the base and top of the mounting flange with the high flash cleaner. Place the butyl tape on and fold/bend it to a circle making pleats as you go. Do not stretch it; it will thin. Leave the release paper on until you are ready to install.

Apply three strips to the places the vent cap will mount.

After the butyl tape is on, hand wipe 303 UV Protectant on a rag inside and outside of the vent base and cap, wipe on thin, and wipe until dry. This product
You can get it at the auto store or retailers. This protection will extend the life of all plastic and rubber roof items. I apply 3 to 4 times a year after cleaning the roof. Inside the vent will only get the initial treatment.

I install a plastic window screen over the vent pipe to keep insects and spiders from nesting. Cut the screen large enough to cover the pipe. The screen will collapse over the pipe. Test fit if needed before removing the release paper on the vent base.

Remove the release paper from the vent base, and warm the butyl with the heat gun. Align the screw holes in the vent base to the roof holes. An ice pick helps. Press base over the pipe holding the screen until the vent base starts going over the pipe.

Using stainless or coated steel, no. 8 x 1” long screws, install only the three base screws that do not hold the vent cap down. Warm the vent flange. You can use a drill driver to start the screws, but stop short and tighten by hand. The butyl will ooze, and that is OK.

Remove the release paper from the three cover butyl tabs.

Align the vent cap leg holes with the base holes and install the cap. An ice pick aligns the holes quickly.

Warm the cap legs butyl and install the 3, #8 x 1” lg. screws. Drill driver OK but stop short and tighten by hand. Butyl will ooze.

Using a plastic scraper with a rounded edge, trim the excess butyl off the roof and base. Take care not to dig the roof.

The vent cap should look like this with the butyl sealing the gap, where water can leak in.

Using Dicor 502LSW or 501LSW self-leveling lap sealant, Seal over the top of the butyl inside the cap legs as shown.

Continue applying the sealant over the vent base and screws.

Note: Sunline used 1 1/2" ABS sch 40 pipe vents on the black tanks, and on “some” grey tanks, they used 1 1/4” ABS sch 40 pipe.

The 1 1/2" measures approximately 1 7/8” across the hole.

The 1 1/4" measures approximately 1 5/8” across the hole.

There are different part numbers of tank vents for the various pipe sizes to make a good tight fit between the vent base to the pipe. I have not yet found the 1 1/4" tank vent for sale that Sunline used, but I found a workaround. At the local lumber yard/hardware store, buy an ABS plastic 1 1/2 to 1 1/4 reducing bushing. They cost about $0.60 or did the last time I bought some.

The bushing creates an excellent tight fit. You do not have solvent weld the fitting on, only press it on. There is no air pressure on this vent pipe, and the bushing can not go down or up to come off the pipe as the vent base traps it in location. The insect screen will still work with the fitting. You may have to trim the vent pipe to compensate for the added bushing to allow the base to sit flat. “Check” the pipe height “before” installing the vent base.

The 1 1/2" tank vents are advertised on some websites to work on both sizes, but I do not like the big gap between the pipe and the vent base using a 1 1/2 base on a 1 1/4 pipe. Get the adapter bushing. If the top cap cracks off, and it will in time, that gap exposed at the base will leak into the roof as there is no tight fit to the pipe.

I hope this helps.


For more pics on the tank vents, see my Flickr page here

Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC

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Old 01-12-2021, 05:47 PM   #3
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This is a good thing to note. When I've replaced my vent caps in the past, I've always used what the local dealer carries here, which is a little different style. This is definitely an issue for all factory installed ones.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR
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