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Roof Top Ac Unit


Nam

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When I got the 1990 Itasca 321RB, it had the old roof top AC. That winter it leaked, so I removed it and installed a powered roof vent with fan instead.

Years gone by, I feel like having the AC again. Here are my concerns: the Coleman Mach 1 PS (power saver) 11k BTU fits the bill as my Honda eu2000i can run it reliably (total cost ~800). Issue is the roof sagged a little bit after bearing the old AC on top of it for over 20 years. I could raise the lips of the roof around the 14"x14" opening so water won't get in but water will be standing around the AC when it rains. Any idea how to mitigate this?

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When redoing my roof on the 87 Itasca I considered putting an AC unit back in. The thing was I also have a sagging roof. One solution I thought of was to pick up a scrap piece of EPDM roofing and take a 3/4" piece of exterior plywood big enough to be just a bit larger than the foot print of the AC (maybe a foot all around) and glue the sheet too it. I would use a round over bit on a router so the top edge isn't square. Using the same liquid roof EPDM coating I did my roof with as a seal and adhesive for the underside of the now covered board, fasten it in place as both a means to elevate the AC and reenforce the sag in the roof. The EPDM will have no problem with the water and the AC unit will only be elevated up by a little over 3/4" when all done.

In the end I just replaced the vents with fan units but I still think the idea had merit. The gasket up against the new reenforced roof section should have no problem sealing out water and there will also be far less of it around the gasket as the AC is mounted up above the roof run off. This idea remains untested but perhaps worth some thought or as the basis for something similar.

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Hi Don, I really like the idea. If I tackle this project, I will seal the roof the same way you did as well. Let's me give it some more thought. Thanks for sharing

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Ok then, here is the rest of the design.

To help secure and reenforce the roof and the plywood panel, I was going to take a couple pieces of inch thick oak for inside. Before covering the plywood, cut the matching 14x14 hole and then clamp the plywood in place. Drill 4 holes both front and back edges of the panel through the panel right through the roof and ceiling. There is a piece of wood spanning across the entire length of my Itasca in the front and some wires that come across the back so figure out what is there first but if you investigate a little it is not hard to figure out. Looking through the fridge vent on top you can see the wire bundle and gauge where it crosses. The back of the plywood panel should be far to the rear of the wiring if yours is similar but check.

On the inside of the RV, cut the oak boards so that they are sized to fit across the span then mark where the center of the AC hole is in relation. Remove the clamps and panel and clamp the two pieces of oak centering them to the AC hole and mark the drill hole locations. Drill counter sunk holes into the oak so that you can use 1/4-20 bolts with washers. In the plywood panel, install t-nuts into the holes. Find bolts long enough to thread into the t-nuts but not to long as so to poke out above the plywood. Clamp everything back in place and dry fit before covering the panel with the EPDM and you should be good to go. After final installation, the bolt heads can be covered by a piece of trim. Paint or stained, your preference.

Yes, I've given this idea way too much thought. Still probably going to do it at some point and install an AC unit but just not this year.

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Lets see

Plan A do what you originally planed and just raise the gasket.

Plan B Same as plan A but you make the shim thing look like the letter H and run the legs to the side walls. They will support the AC. Some one did this with a welded aluminum.

Plan C Do it Dons way.

Plan D Look inside can you find a counter or back of a dinette seat to use a stair rail post to raise the roof back to original height. I did this with some turned posts and when stained they looked good.

Plan E Go the Sunrader route and put couple of curved beams inside, bolted to the wall. This has been done on this site with bent wood and bent aluminum beams. You do get a couple of possible head bangers this way

Lots of ways

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If you were to go with Plan A look into cellular PVC sheets. No painting required and is not affected by water. You'll need to seal it against the roof but unlike my plywood/oak beam combo doesn't need the EPDM covering. Unfortunately it isn't structural and will not help with supporting the sag. The plus is that it works with normal woodworking tools. So cut it to length with a saw, route a radius if you want with a router bit and so forth. Home Depot carries it locally 1/2" thick in 24" x 8' sheets. It will raise the AC and not be bothered by water. PVC, they make pipes with the stuff.

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To help secure and reenforce the roof and the plywood panel, I was going to take a couple pieces of inch thick oak for inside.

My Minicruiser had fairly rotten 1" X 4" cross supports on the roof. Originals were wood but what species I don't know. Doug Fir maybe? I wanted something as structurally strong as possible and also rot resistant. Doug Fir or Tamarack is damn near impossible to get in my area anymore. Pressure treated wood (Southern Yellow Pine) was not something I wanted because #1 it's not dry, and #2 the chemicals react badly when they touch aluminum and my side-frames are aluminum. I wound up getting White Oak. Red rots too easy, but White Oak is perfect for the job. Just wasn't easy to find in my area with a full 1" thickness (5/4" boards). I finally got some 1" X 14" boards at a small mill and ripped them down to suit my needs.

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My Minicruiser had fairly rotten 1" X 4" cross supports on the roof. Originals were wood but what species I don't know. Doug Fir maybe? I wanted something as structurally strong as possible and also rot resistant. Doug Fir or Tamarack is damn near impossible to get in my area anymore. Pressure treated wood (Southern Yellow Pine) was not something I wanted because #1 it's not dry, and #2 the chemicals react badly when they touch aluminum and my side-frames are aluminum. I wound up getting White Oak. Red rots too easy, but White Oak is perfect for the job. Just wasn't easy to find in my area with a full 1" thickness (5/4" boards). I finally got some 1" X 14" boards at a small mill and ripped them down to suit my needs.

Just to clarify that in this scenario the oak would be interior and not exposed to the outside. Best you could do is finish it like trim. That said, if there is concerns with rot on wood mounted to the ceiling on the inside, I would think there are other bigger problems. Sort of sounds like treating my baseboard trim like my roof shingles, I'm just thinking of a way to craft a bolt in support sandwich. The plywood would need to be wrapped in water tight material as it it the only part exposed to the outside. The biggest issue I was trying to speak to was the originators question regarding mitigating standing water around the AC seal. The EDPM sealed panel attached to the roof wouldn't care a bit about the water. EDPM, they make ponds with the stuff. The raised nature of the plywood platform would raise the AC gasket out of the water at minimum.

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I had a small leak on mine it ran across the interior roof panel and dripped out around the cabinets. The roof on mine was one piece fiberglass so I removed the Ac unit that in it's self should not have been bad but some where along the line some one "fixed" it with silicone I did not think I would ever get the thing off the roof thank God for vibrating blade tools.The gasket was flat as a pan cake I could not believe how thick the new one was it is nearly an inch thick and along with it comes a new piece that fits under the back end of the AC to level it up. Not only did it fix the leak it reduced the noise level by a huge percentage. I bought it from a family run RV shop (not Camping World) and the owner told me that they really should be replaced every 5 years and I now see why. It was not a get rich scheme on their part it was only $15

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I once saw a winnie warrior where the owners had made a bracket put of what looked like angle iron (aluminum?) that layed on the exterior on top and supported the AC and appeared to pull the roof up. Not sure if it comprised of a box form for the AC roof opening or if it only supported the front & rear of the AC unloading the roof.

My roof sags a bit - no leaks at all.

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  • 2 weeks later...

Definitely but it needs two hoses and I can't think of a space for the portable AC unit.

Using an upright a/c is another option.

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If you want to make a spacer to raise the height of the AC get some HDPE marine board. It comes in a variety of thickness and is made to use outdoors in salt water environments. While it is not lightweight however you will be cutting out the middle of it and leaving just a ring so that will take away most of the weight of the sheet. It is fairly easy to machine you can drill it and you can cut it with saws and routers.

https://www.onlinemetals.com/merchant.cfm?id=1573&step=2&top_cat=181

Wood will be prone to cracking, rotting etc but HDPE is not.

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Good idea corbin. Thanks for sharing. Question is how much the roof can withstand the AC on it. I'll snap some picture of the roof sag tonight.

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Good idea corbin. Thanks for sharing. Question is how much the roof can withstand the AC on it. I'll snap some picture of the roof sag tonight.

Since you have a framed RV you have more options for adding extra reinforcement to the roof by spanning over to the framing adjacent to the roof opening. A couple of pieces of stout aluminum I beam or L angle up top might do the trick. Share the load by distributing the load.

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