how to flash a roof top Air conditioner ducting and what to do with the duct?
Hey all,
Just purchased a home that i need to replace the roof on.
I am an engineer and an avid home construction weekend-warrior and am looking to take it on myself.
I am going to go with GAF products because so far they have the most complete documenation on their roofing system that i have found and feel most comforable with going with that since this is my first time doing a full house roof job (i have done previous small jobs on relatively large chicken coops 🙂 )
that said, i have one thing i am not sure on and can’t find much documenation on and that is how do i flash the HVAC system on top of my roof and, as you can see from the photos if i can get it uploaded…. the ducts are large and coated in something that clearly did not perform as anticipated.
My questions are:
1. how do i properly flash these ducts where they penitrate the roof and as well the metal stands the AC unit is ontop of
2. What should i do with these ducts to better insulate them? At this point, should i just replace them before implemeneting the new solution? how hard is that?
thanks folks
-Ben
Replies
In terms of flashing, study up on how you'd flash a chimney.
Dreadful!! Sorry to be so frank!
The duct work looks rusty and carelesly installed. The heavy use of sealant points to a questionable installation. If possible, start over from within the attic. The portion of duct that sits on the roof deck should have L-shaped flanges around it. These act as flashing and should extend a good 6-12" below the shingle. Since step and counter flashing do not appear to be an option, hand seal each shingle as it is laid on top of the flashing. The lower leg of the duct lies ON the shingle and the side legs lie underneath shingle.
While the details are not clear, it appears that part of the AC rests on a flat roof (with a membrane of some kind?). If so, you have little recourse...sigh. AC-related work will impact the roof membrane. It MAY be possible to re-do the AC work and lay patch-membrane material over the existing membrane; glued to it. It's hard to know without more detail. There are insulating materials that can be sprayed directly on the metal housing. It is also possible to build an insulated structure that houses the AC. This will call for a good design, since three trades (HVAC, carpentry, roofing) overlap.
BTW if you go new, raise the duct work off the flat roof membrane. Doing so virtually guarantees installing a new membrane. Be careful...any proposed fix could place you in a hornet's nest! :(
I hesitate to say more. You need the input of an HVAC expert, a roofer who knows his stuff, and someone who can oversee the work of three trades. In my many years as craftsman/remodeler I constantly run into problems where trades overlap. My best wishes...and...uh...do open a beer on honor of the Cubs! :)
Mel froscarpentry.com
Thanks Mel!
yeah i knew i was going to get a hybrid-answer. Where i'm from (maryland/east coast) i don't ever remember seeing these roof top AC units. Possibly because the pitches are so steap for snow shed it doesn't make sense. But when i came out here to california, i notice them EVERYWHERE. and i have always thought "i wonder how the roofer and HVAC techs work together to make that happen....."
To follow up on your info, can you point me to a URL that provides the new ducting that you're talking about? I don't know what it's called so i can't figure out a good search phrase. Same on the insulating materials that can be sprayed directly on these incase i can't/don't want to go the route of re ducting.
Thanks so much and any more info/pictures of proper install would be super benificial!
Tomorrow is friday (thankfully!!!!) and i'll clamber up on the roof and take better pictures. this pic was straight out of the inspectors packet. The unit does NOT (thankfully) go on any flat-membrane coated roof. I am however planning to reroof the enitre house and the flat membrane room included.
pictures tomorrow.
thanks
Customized metal work required
I am not an HVAC mechanic, though I coordinate my work with that trade. You need a HVAC company that will custom form its metal work to fit your situation. You won't be able to find what you need in a box store. This is not a job YOU can do UNLESS you have a metal folding device and are familiar with the processes involved. I also can not point you to suitable spray-on insulation material. Talk to an HVAC expert. Finally, how the unit rests upon the roof is something that needs to be analyzed on-site. Consider the load this unit places upon ceiling joists. Your project is not a week-end undertaking. I am afraid it's going to bleed greenback$ from your wallet...sigh.
Mel froscarpentry.com