Curved roof. How to cast mold for vent fan base?

Annaleigh · 01-29-2022, 07:15 PM

I have a mark III high top conversion van. The top is one of those that is low over the cab and has 2 glass windows in the front where the top raises up higher. The top slants slightly lower towards the rear of the van.

So I want to install a vent fan if in the high area near the front but the fiberglass top has a curvature to it from right to left. My vent has a metal frame and when I lay it on the roof there is a 1/4" gap at each corner from right to left. That is quite a gap to seal and I don't think the steel frame will bend without distorting. What I would like to do is make a template some how and was wondering if any of you all have some ideas how to do this?

The fan opening is 14"x14" and the metal flange is about 1" to 1 1/4".

There is a company I contacted that makes adapters but they said that they did not have one for my van.

I was thinking that if I had a template then i could perhaps buy a 1/2" thick 18"x18" HDPE cutting board, cut it to size, and pattern it after the template.

Thanks for any all advice!
Annaleigh

Twoxentrix · 01-29-2022, 07:56 PM

Seems to me you would just duplicate the same issue using a 1/2" thick HDPE cutting board since it really doesn't bend much, and even if it did conform to your roof the mounting plate still wont bend (as you described) - think you'd need something that would accommodate both surfaces...what if you used 1/2" (or 3/4") rubber floor matting, with some type of sealer applied to top & bottom ? At least it would conform to both sides of the mounting surface.
Something like these:
https://www.amazon.com/American-Floo...3511337&sr=8-6

https://www.northerntool.com/shop/to...2334_200912334

dhally · 01-29-2022, 08:30 PM

Normally the fan base is mounted on butyl putty. The putty and sealant should fill the gap. Maybe add shims to prevent bending the base.

1der · 01-30-2022, 05:46 AM

As dhally and two centric have suggested, what is needed is a material to create wedges and spacers that result in a flat surface to mount your vent to.

The best material I have found to make the wedges and spacers is a piece of PVC Trim 'lumber' from a big box store. It cuts and sands easily. It is waterproof. Create your wedges for the left/right taper of the roof and spacers for the fore/aft edges of the vent. Make the pieces the same or slightly larger than the mounting flange.

Use Lap Sealant or equivalent and possibly some butyl to create the seal between the wedges/spacers and the top. Then mount the vent onto the now flat surface.

Dearranged · 01-30-2022, 06:43 AM

I replaced an a/c with a vent fan without using the gasket material off the a/c.

Mask off the fan flange that is going to contact the gasket surface. Mark an outline around the intalled fan so you know where to install the new gasket material, then or before clean thoroughly.

Get yourself a tube of 100% silicone, the good expensive stuff is all I use now, it costs about $8 a tube. Apply inside of the outline of the fan enough so when you set the fan on it will form a seal. Just set the fan on top, don't fasten it down or wiggle it around or pick it up, when you set it down leave it for a couple days.

Remove the fan after a couple days and you will have a gasket that will outlast the van. This is where masking the fan flange comes in, you can use painters masking tape also. Drill for fasteners or however you plan to do it, on final install apply a little silicone to the fasteners to seal them.

If you feel you missed a couple areas that might leak you can still apply some silicone on the final install.

Annaleigh · 01-30-2022, 07:01 AM

Thanks for the replies and thoughts. Originally I was thinking about making a form and perhaps finding some kind of plastic like material that could be heated and poured in place to make the adapter...

As to the HDPE cutting board. My thoughts there were to cut the plastic to the correct size to fit under the vent. So I would have a square ring say 14"ID x 15.5" OD. If I had a casting of the area of van roof where the vent mounts, then I could glue on some sand paper to the casting and sand the ring to fit.

I have been thinking since then, What if I taped a sheet of poster board down on the roof and then glue the sand paper to the poster board. I could take the square ring and sand it to match the surface. Before I was thinking if I just taped the sand paper to the top of the van, it would probably damage the finish around the area. With the poster board though I should be able to keep the surrounding area clean and avoid scratching the gel coat.

On the older ford vans there is a recessed body line that is about 1/2 in. When they installed the side windows they used weather strip like material that flattened down when the windows were fastened in place but the weatherstrip also seals the 1/2" recess.
If the rubber mat would give 1/4" then that would work.

Years ago my father used to build those truck covers. Back then they were wood framed and covered with aluminum siding. I remember he used butyl putty on all the windows and trim pieces. That stuff stays pliable for years and does not shrink. With some shims that might work..

The PVC trim is a great idea as it would smaller and easier to work with while sanding. It is also UV resistant while the HDPE is not. I just liked the idea of making it all in one piece.
I am not sure that being UV resistant would be an issue as only the outer edges would be showing.

Thanks
Annaleigh

Annaleigh · 01-30-2022, 07:09 AM

Thanks Dearranged, I just saw your post after posting my reply. I didn't think about a silicone gasket..
Years ago I was involved in a solar project and we needed a seal around a hinged lid. We put wax paper down on the flat surface first and then applied a thick bead of silicone. Next we applied wax paper to the hinged lid and closed the lid. 3 days later we had a near perfect gasket!

Annaleigh · 01-30-2022, 08:16 AM

BTW. Here are some pics of an install I found last year on the internet. I am not sure where they came from as I saved them for my personal use. The install is on the same type of fiberglass roof as mine. The main roof appears to be about 3/4" thick and the support beams appear to be 1x3"s . They are using a plastic framed fan and the pic shows the frame fitting much better then the metal frame on my roof.
I also plan to add the support framing like they did.

boywonder · 01-30-2022, 08:19 AM

I like the thick HDPE idea.......trace the curvature of the roof on the HDPE (maybe put masking tape on the edge) to get the profile....same way you cope woodwork. Machining the HDPE may be tricky. Perhaps run it though a table saw with a dado blade at various heights to get the profile close (and stair-stepped), then sand the stair steps out to finish.

Dearranged · 01-30-2022, 09:33 AM

Quote:

Originally Posted by Annaleigh

Thanks Dearranged, I just saw your post after posting my reply. I didn't think about a silicone gasket..
Years ago I was involved in a solar project and we needed a seal around a hinged lid. We put wax paper down on the flat surface first and then applied a thick bead of silicone. Next we applied wax paper to the hinged lid and closed the lid. 3 days later we had a near perfect gasket!

Ah yeah, I had the waxed paper idea after I did mine and all the painters tape didn't come off. Waxed paper would be better

. Silicone is the wonder drug of the 20's.