Leak-Proof Roof Hatch and Fan Openings - Fiberglass RV


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Old 09-13-2013, 02:27 PM   #1
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Leak-Proof Roof Hatch and Fan Openings

Our Surfside project trailer needed a lot of work. We had to gut it and replace the floor, make numerous fiberglass repairs, and more. Now we're just starting to get to the fun stuff, actually starting to install new stuff, and the first of those things is the roof prep for a new MaxxFan.

Here's the thing: Most roof openings eventually leak. They leak because rain falls on the roof, runs into flush-mounted vent or hatch, eventually finds a crack in the sealant around the vent, and makes its way inside.

My solution was to install a raised ring around my fan mount so the water would have to run up-hill to make it to my fan opening. This thread shows how I did that.
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Old 09-13-2013, 02:46 PM   #2
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My first step was to make a cardboard template for the mounting ring. I started by making a cardboard template that matched the edges of the mounting ring.

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Then measured the size opening the mounting ring needed, drew out a pair of rectangles the right size on a sheet of cardboard , and used my cardboard profile draw in the lines that matched the profile of the mounting ring. Then I cut the cardboard template out using a utility knife and scissors.

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Old 09-13-2013, 03:05 PM   #3
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The next steps were to test-fit the template to the mounting flange. You need a 1/8" (3mm) free play on the inside edges of the ring. Mine was too tight, so I trimmed the inside and tried again.

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After trimming the inside edges back a but, I had the cardboard template I needed.

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The next step was to lay my ring out onto the plywood sheet I planned to cut it from.

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My plywood was too narrow to fit the whole ring in one cutting, so I decided to do two halves.

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Which resulted in some very complex layout lines and cutting instructions, but it all worked out in the end. After cutting out two sets of half-rings and fiberglassing them together, I had my two mounting rings, one for the inside of the trailer, one for the outside.
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Old 09-13-2013, 04:21 PM   #4
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The next step was to cut out my 1/4" plywood rings. Whether you make your rings from one piece of plywood or two, the process is much the same. You drill holes in the corners, then use a saber saw to cut the inside lines out and round off the outside edges to match the layout lines.

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I don't have pictures of the completed rings or of the fiberglass shell preparations (sorry). I think you can visualize how the rings looked anyway. I didn't take pictures of the prep process because we were doing several things at once, and I think the pictures I would have taken would be very confusing.

So here's what need to do to prep your trailer.

First, you need to peel back the insulation and head-liner material that covers your ceiling. If you don't have a vent hole yet, you'll need to cut the hole.

Once you have a hole with the ceiling material peeled back, the next step is to grind or sand the gelcoat off around the top of the opening. Fiberglassing materials, even epoxy, don't stick to gelcoat materials very well, so exposing a roughly 2" (50mm) band of raw fiberglass around the opening allows you to attach the vent ring in a way that prevents leaks.

Which is kind of the point of all of this.

A good tool to use for doing this is a grinder with a 120 grit flap disk, available for $25 or less from Harbor Freight when they're on sale. A grinder removes the gelcoat almost effortlessly. (Which is why I don't suggest using a flap disk with coarser abrasive sheets; they can grind through gelcoat and through your shell much too easily.

I don't have a good picture showing how far I ground the fiberglass back from outside the trailer, but I do from inside and underneath. This picture shows what the opening looks like after my bottom-side ring was attached. You can see the light coming through the fiberglass from above where the gelcoat has been removed, and I think it shows the margins you need around the mounting ring pretty well.

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A couple things worth noting here. One is there's a notch cut in the forward part of my inside ring where the fan wiring can get through. The other two are, first, you can see where we patched the roof toward the back of our fan opening, because we wanted to move the vent forward several inches, and the fiberglassed-in roof supports. I'll talk about the latter two things another time, but do make sure you have a gap in your bottom ring where the wires can pull through.

Anyway, you've seen the after picture for the bottom ring. Getting the bottom ring into place isn't too hard to do. The easy way is to spritz the top side of the ring and underside of the trailer shell with water, then apply urethane glue to the upper side of your bottom ring, line it up with the opening, and clamp it into place. (Spritzing the surfaces with water might sound counter-productive, but urethane glue needs the moisture to set and cure.) Make sure to use lots of clamps to assure tight bonding between shell and wood frame, and trim off any excess urethane that oozes out during the curing process after the glue has fully set with a sharp utility knife.

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Mounting the outside ring is much the same, but, instead of urethane glue, you'll want to use a ring of thick, resin-saturated fiberglass mat or a thick layer of Bondo Glass filler to glue the ring into place. The glass mat or bondo helps make sure you have full and complete contact between the ring and the fiberglass of the outer shell.

The advantage of using the resin-saturated mat is it gives you about 15 minutes working time to position and clamp your ring into place. The disadvantage of resin-impregnated mat is it's drippy and smelly, and working with it overhead while you clamp it into place is messy. Wear a hat or shower cap while you work with it.

The advantage of Bondo Glass is it does an even better job of filling any voids between ring and shell and stays in-place on the ring as you move it from workbench to trailer roof, but Bondo sets in about five minutes, so you have to butter it onto the ring, get the ring up through the hole and lined up, then firmly clamp it into place all in the space of five minutes. That's not much time to get everything right.

Once the upper ring was in place, I sanded the inside edges of the opening smooth, then built a little 45-degree ramp of Bondo Filler all the way around the outside edge of the top ring. Nothing special about doing this; it doesn't have to be pretty, just fill the sides in and create a ramp. I built the edge up using two mixings of Bondo, and sanded any really rough spots smooth before the next step. (No pictures, sorry.)

Once the Bondo ramp had cured (about 15 minutes), I fiberglassed the top side of the ring to the fiberglass shell of the trailer shell using four 2-1/2" (63mm) strips of lightweight fiberglass mat.

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Here's a picture showing the angle of the Bondo ramp I built with the fiberglass over it.

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The final step before painting -- which will be a while, because we have other work to do around our trailer -- is to use Bondo filler and a power sander to smooth out the outer shell and ring ramp so it looks pretty.

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Old 09-13-2013, 04:28 PM   #5
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Sounds good, but without pics its confusing. Sketches of what you did?
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Old 09-13-2013, 04:36 PM   #6
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And that's as far as this mod has gotten. I'll update this thread when I get to the painting and fan installation steps, but that'll be a while. We have more than a few other things to do before we get to painting and installing our MaxxFan, so , to be fair, I'll give you a preview of the steps involved.

1) You have to paint the outside of your fiberglass, or the ultraviolet rays of the sun will degrade your repair and it'll leak. Not good! So:
..1a) Mask off the area around your fan installation.
..1b) Apply a light coat of automotive spray primer.
..1c) Let it dry completely, then sand lightly with 220 grit sandpaper.
..1d) Spray a second layer of primer.
..1e) Sand *very* lightly again.
..1f) Apply three or four light coats of matching spray paint. (You can have the paint guy at a paint store mix up a can of matching spray paint for this step. Just bring your trailer to his store so he can get the color right . . . and have him or her mix up a bottle of brush-on scatch cover while you're at it!)
2) Mount your fan's installation flange on top of your fiberglassed-in mounting ring, and drill pilot holes for the wood screws that hold it into place. Alternately, you can drill oversize holes and use machine screws and T-nuts to hold your fan down. (I'm a fan of using machine screws and T-nuts.)
3) Apply strips of butyl tape to the underside of the fan mounting flange, screw it into place, and install your fan according to the manufacturer's instructions.
4) Go camping!
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Old 09-13-2013, 04:38 PM   #7
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Dave, sketches of which step(s)?
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Old 09-13-2013, 04:50 PM   #8
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One thing I'd like to add is that, while all that grinding and fiberglassing might sound a little scary if you've never done fiberglass work before, this is really the ideal place and way to learn fibrglass basics.

First, it's a small project. Second, it's on the roof where, frankly, no one will see it. So any mistakes you make will be far from everyone else's view. You can just smile and say you did it yourself.

You also don't need much in the way of tools or supplies. You might need a grinder and saber-saw, but you probably have all the other tools you need. For supplies, you'll need some disposable latex gloves to do the fiberglassing, some scraper/smoothers to spread and smooth out the Bondo, popsickle sticks and Dixie cups for mixing the resin and Bondo, sandpaper, a small package of lightweight mat, a small package of heavy mat, a fraction of a quart of fiberglass resin and it's matching catalyst, and a quart can of Bondo.

The secret to success here is to take your time. Think things through as you lay out your cut lines on the plywood and fiberglass, test fit the plywood pieces to make sure they fit properly and you know where and in what orientation they go, organize your thoughts and act-out and rehearse the process of gluing or fiberglassing or Bondoing before you mix your resin or Bondo, because you don't have much time to think things through or correct mistakes you've made once the resin or Bondo has been mixed. Thinking things through is the key to getting it right and getting the result you want and need.
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Old 09-13-2013, 08:32 PM   #9
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THANKS Peter for documenting so thoroughly the steps. The pictures made it easy to follow along
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Old 02-28-2014, 03:28 PM   #10
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Fantastic Fan offers spacers for mounting fans on fiberglass. You can ask for one or two and they (used to?) send them free. Just give them a fantastic fan model name..... I've gotten about 6 of them in the past to use with Sunraders.

But I do know there are new owners, may not still be free, but very easy to use.
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Old 03-02-2014, 10:43 PM   #11
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That curb makes sense. The manufacturers should integrate the curbs into their molds. Have one for vent, escape hatch, and AC. Retrofitting like you are doing takes talent!
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Old 03-03-2014, 08:53 AM   #12
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Molded fiberglass trailers, motorhomes and slide-in campers have been around for more than 40 years and I have never seen a roof vent/fan/ac unit leak on one that wasn't caused by a lack of preventive maintenance, usually after about 10 years of use. Adding a flange as shown won't do a thing to reduce those leaks.
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Old 03-05-2014, 11:54 AM   #13
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Bob, the roof vent on our eight-year-old Scamp started leaking when it was four years old.
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Old 03-05-2014, 12:26 PM   #14
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And what was it sealed with????? I have seen Scamps with some kind of caulking gun stuff used.

I have pulled vents, windows and outside hatches off of 30+ year old FGRV's and found the original caulk intact.

The RV putty tape, properly installed, should last at least 10 years.....

As of late the builders don't seem to like putty tape because it takes longer and requires more skills than using a caulking gun.

We bought a brand new Coleman sticky last year and, not only did it leak when new, it developed two more leaks within a year. All were at calking gun sealed joints.
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