If you haven't typically been an Air-con user, I think you'll probably enjoy the mod. It does bring in quite a bit of light. Also, it can be non-tinted light, which I like (since the side windows are tinted). And of course it's totally reversible (and if your Air-con is broken, you'd already be having to put a new one up there, so why not try without it if you are so inclined).Rokrover wrote:Quick question about the correct screws to go in the fiberglass roof as my googling about wasn't very helpful. I’m thinking more and more about replacing the A/C with a manual vent hatch. The 14” x 14” hatch I looked at doesn’t come with screws or caulking tape so I need to source these supplies, should I proceed. There are so many screw choices and I’m not sure metal or wood threads work best in fiberglass. Don’t want to promote stress cracks and, as they say, measure twice - drill once!
There are a few schools of thought on screws into fiberglass, and most RV's use the most basic way, and for the most part it seems to work. The reason I'm kind of "into it," is that the Chinook is made a lot like a boat, and boating is my background. The most basic way being to just drive "pointy screws" into the fiberglass skin. I think a coarse thread would be a better choice than a fine one. This always makes me slightly shudder, but 99% of RV's seem to do just fine that way, so.
I would use stainless screws. If you spend time near the ocean, you might order grade 316 which you can get through McMaster Carr. If not, then the typical 304 will probably be fine. 316 is less prone to corrosion.
But let's back up and I'll describe a few of the prep steps I took. I can't remember if yours is still up on the roof, but first of all I made sure there was no AC power source (inverter, shore power, generator), then I removed the inside shroud. I have photos of each step if you need them, but I remember it being pretty straightforward. I took off the little clippy filter panels, then accessed the shroud screws inside. There are two basic wire connections: the"telephone line" for thermostat, and AC power (which is in a metal junction box on the passenger side. I pulled the AC wire back toward its junction box behind the microwave so my skylight garnish would fit (I used the interior garnish from a regular RV roof vent). It could still be pulled back for a new Air-con.
Once the interior shroud is off, you can unfasten the unit from the roof. There are no holes for fasteners, but rather it's a clamp type system. I think there were three long bolts. Two forward and one aft. Then you unscrew the little "air box" square.
I can't recall if there was any DC power to the air-con unit, but don't be fooled by a bunch of DC wires that (may) run through the opening on the driver's side. On my rig this was a set of wires that came in through the roof cavity from the wall over the couch, then split off forward and aft (for the original fan and the two ceiling lights in the vicinity. I did have to add some wire to those because as-built they ran through the hole and I of course didn't want wires going through my skylight/vent. I crimped on extensions so I could run them around the perimeter instead.
Then it's just heave-ho. I was by myself so I got a couple of 12' 2 x 6 boards and made two holes in each one near one end. Then I lashed them to the roof perimeter rail side by side. I also parked near a hill so the slope of the boards to the ground was more gradual. I pulled the unit out onto a towel, then slid it over to the boards, and then down them (I had it tethered just in case). Buh-bye! It went down in a slow and controlled fashion though, thanks to friction/tethers.
Once the air-con is gone, you will be able to see exactly how thick the roof is, and how it's made. Mine has a fiberglass top skin, a nidacore core (looks like vertical plastic tubes), then an inner fiberglass skin.
The original vent screws basically grab by just going through the top skin. And that's basically workable. I don't like the idea of water potentially getting into the core (can be bad news even when the core is not wood), so when I re-do something like this (or do it anew), I overdrill the holes (larger diameter), fill with thickened epoxy, and then re-drill into that new "column." But if you don't do that it will probably be fine. One thing is that boats commonly last 50-100 years, so there is incentive not to "kill" them prematurely with deck rot. A Class C is never going to last that long, so it's more of an optional step, IMO. Also, the bedding compound usually provides some grip. That said, there is a fair amount of stress on the thing, especially if you add vent hoods.
IIRC the vents take #8 screws. Probably 1" would be a good length, but you'll be able to check that when you have the edge exposed. 3/4" might be fine too. One thing to watch for is that when you tighten them against the bedding, because the flange is plastic, you may get "screw dip, hump, screw dip, hump," etc. That's not great because now the holes are "dips." But that somewhat depends on how cold it is when you do it (how resistant the bedding is to compression). I'm going to be re-placing/re-bedding the forward fan, and I may use some bar stock on top of the plastic flange - will have to see.
I'm a big fan of butyl tape, but ONLY if it's the good stuff. The ones that work like putty suck, IMO. I've only found one source for the real good butyl tape nowadays, and that's "Bed-It" through Compass Marine. He has worked with the mfgr. to produce a really good butyl. I've tested half a dozen others (even from classy places) and ... BZZZZZT.
Another choice would be 3M4000, which is a UV resistant polyether. That'll work great, but is much more of a pain when it comes time to remove-redo vs. butyl, which comes right off with mineral spirits. IMO, all the bedding should go under the flange, except for a small worm of butyl that you wrap around the base of the screw head. I don't know why RV tradition is to put a bunch of sealant on top of the item
I mean, do you put neosporin on top of a band aid? On boats every last bit of whatever bedding/sealant one uses is under the flange (except for a small worm of butyl placed on the screw shank just under the head, but you don't even do that with more "flowy" things like polyether. There's also no need to have the bedding extend out to the sides once past the flange. I just trim it clean. Now it can't funnel water in or trap dirt. I would purchase and use the Bed-It butyl. Leftovers keep for decades, and you can use it on other re-bedding jobs.Back to the screws: If you want to get fancy you can overdrill, then fill with thickened epoxy. Then either drill and tap that for machine screws, or you can put pointy screws in when the epoxy is not quite completely hard (then it sort of "threads" vs just making the fiberglass powdery). Pointy screws into a fiberglass skin is not great, but like I said seems to work so it's probably fine. There are a lot of screws, and bedding is somewhat adhesive. Plus no-one's going to be cursing you in fifty years
BG
PS: Probably goes without saying, but you want the roof surface clean and decontaminated. Which solvent to use somewhat depends on what bedding you are using, but mineral spirits works well for butyl. I also wipe down the screws as they often have machine oils on them. You may also want to see how flat your roof is in that area. Mine has a slight depression from the weight of the Air-con, but I have heard of others having one a bit more pronounced. The vent wants a flat surface.
The next step was that I backed up to the outcrop and ran the planks from there into the back door and slid the unit in for taking to storage.
) where there is a skinny half-size looking junction box. The only purpose of this box is to cover a joint in the Air-con wire. I imagine they ran the in-roof wire at one point in the construction process, then joined it with the wall wire later, and AC wire junctions require a box. So what I did was grab the wire between that skinny box and where it goes into the roof cavity, and I pulled it so that the wire in the Air-con hole was juuust inside the edge of the hole (where it can be grabbed again if Air-con wanted to be put back). Without doing that it would have interfered with my skylight garnish (or a vent).