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Roof fan install on curved fiberglass top?
#764856 August 17th 2020 3:15 pm
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Anybody have any tips on installing a roof fan with the standard 14x14 cutout on a curved fiberglass hi-top? The top fin or high part of the roof pretty much goes down the length of the van (check out the attached pix for what I mean) and is a little over 14" wide but that means the trim piece of the fan bracket (the part you screw down) will need some special love on the sides to be sure it's affixed properly and doesn't leak. I'm thinking either just multi layers of butyl tape and pour on the sikaflex or trying to fashion some sort of wood bracket (and butyl / sikaflex). Would love some input if you have any ideas...

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Re: Roof fan install on curved fiberglass top?
James02DRV #764869 August 17th 2020 9:56 pm
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I dont know what all is included in bonding fiberglass, but the install that would last the longest would be sanding down that whole part of the roof, and building up the surface with fiberglass like it's made of. Then body working and painting over it.

Personally, I wouldn't want to cut into my roof without knowing it wont leak for at least a decade. If you opt to put some kind of support under there, you'd want it to be metal, not wood. And you'd have to run through bolts and seal them, or epoxy the metal on. Which sounds awful. That would take me back to option A.


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Re: Roof fan install on curved fiberglass top?
James02DRV #764872 August 17th 2020 10:33 pm
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There's a lot of different ways to skin this cat.

Making a flat spot via wood/foam, covered with fiberglass, would likely be best, but working with fiberglass and resin, on the center a Van roof, without lots of previous experience with fiberglass, could be a curse fest to end all previous curse fests.

While quality butyl tape is awesome stuff, it is not really intended to be stacked, to fill large gaps, and sikaflex is good stuff too, but again, huge gaps are problematic.

I'd probably masking tape off an 18x18 inch " square on roof, sand through the gel coat to green fiberglass below within this square. I'd peel the paper backing off some 1 " thick insulation foamboard, and use epoxy to adhere a 17.5x 17.5 inch square of foam to the roof, using weights to hold down the edges over the bend in the roof.

regular 'fiberglass resin', which is actually polyester resin, will dissolve most foams, whereas epoxy will not

Once the epoxy cures, come back with sandpaper on a long sanding block, and sand it flat. Then cover with fiberglass saturated with epoxy, then cut the 14x14" hole, then seal the interior of this cut out with more epoxy, thickened to almost bondo consistency with fine wood sanding dust, like that collected from a belt sander.

Once fiberglass sanded flat and smooth then butyl tape and install the vent and then use sikalfex or Dicor on the edges outside the butyl tape.

Epoxy resin and hardener must be mixed in a ratio precisely, by volume or weight, and then thoroughly mixed in the mixing cup. If one skips precision on this, they will regret it.

The foam can be replaced with wood, but it is harder to sand/shape and still needs the edges well sealed.

Cured Epoxy needs to be painted to protect from UV light. Polyester resin should be too, but is more resistant to UV.

Depending on how your roof is built and where you cut the hole, there could be some roof reinforcement required. so the flattening and fiberglassing can spread the load around the hole.

Re: Roof fan install on curved fiberglass top?
James02DRV #764900 August 19th 2020 5:11 pm
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Originally Posted by wrcsixeight
I'd probably masking tape off an 18x18 inch " square on roof, sand through the gel coat to green fiberglass below within this square. I'd peel the paper backing off some 1 " thick insulation foamboard, and use epoxy to adhere a 17.5x 17.5 inch square of foam to the roof, using weights to hold down the edges over the bend in the roof.


This sounds like a great idea.. I think I'm gonna give that a go. I'll try to take some pix and post em here as I go through it. Thanks the input DrCaldre and thanks for the details wrcsixeight!

Re: Roof fan install on curved fiberglass top?
James02DRV #765518 September 13th 2020 7:03 am
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Have had any luck on the install?


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Re: Roof fan install on curved fiberglass top?
James02DRV #765530 September 14th 2020 1:19 pm
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Not yet Jim... I've got the stuff to do it but had my last few weekends dealing with a fuel pump replacement. I test fitted the fan base on the roof and there's really only about a 1/2 " overhang on each side so I just need a small amount of filler but I'm going to play around with some fiberglass / epoxy / insulation off the van before I try it where it counts!


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