The best way to put a AC condenser on a flat roof?


#1

I have an AC condenser sitting on a plastic pad on my roof. I know that’s not a great way to do it, and It’s not hooked up yet, so I’d like to change it before the AC is connected. I’m concerned about moving the unit for reroofing in the near future. I want to know the best way for the equipment to be mounted, from a roofers point-of-view, for ease of reroofing. I’ve seen other common way to do it around here, with treated 4x4s. I’ve also seen the metal stands and wood framed curbs. I don’t mind spending money now to avoid trouble later.

I’d like to have the AC condenser mounted so that it does not have to be moved or lifted to reroof. <<<

I’m in Albuquerque NM so we don’t get a lot or rain or snow. The current roof is I think what’s commonly called tar&gravel or BUR. It consists of a wood base, with what appears to be rolled roofing and some tar-like stuff, and gravel on top. I might consider some sort of membrane when I reroof, but I don’t know much about that.

Any advice from roofers would be appreciated.


#2

Google roof curb rails. Buy ones big enough to cover the unit. Put them on the roof where you want the unit to sit. Cut the roof out to the deck. Use 2x6 blocking to raise them up. Flash them in with modified bitumen roofing (torch, or cement applied). Secure the top of the membrane with screws and a termination bar. Put the cap that comes with the rails over the top to act as a counter flashing. Set and secure the unit.
The reason for the blocking is so that if you lay over your roof when you get a new one, the walls of the rails will still be tall enough to have good and acceptable flashing height.


#3

[quote=“durosh*t”]Google roof curb rails. Buy ones big enough to cover the unit. Put them on the roof where you want the unit to sit. Cut the roof out to the deck. Use 2x6 blocking to raise them up. Flash them in with modified bitumen roofing (torch, or cement applied). Secure the top of the membrane with screws and a termination bar. Put the cap that comes with the rails over the top to act as a counter flashing. Set and secure the unit.
The reason for the blocking is so that if you lay over your roof when you get a new one, the walls of the rails will still be tall enough to have good and acceptable flashing height.[/quote]

Ah, I see now, like a RTU curb but for when you don’t need a penetration.

Using this method, the equipment won’t need to be disturbed when reroofing? There would be about 9 inches clearance under the unit, which is 34x34 inches.


#4

All you will have to do when re-roofing is tear the flashings off of the curb rails. Screw a modified base sheet to the curb rail and torch or cement in a cap sheet. This way ur rail wont be covered in asphalt when you tear the flashings off.


#5

Is there another way to mount the condenser, without a curb or curb rails, so that lifting for reroofing would be minimized? It would be about 9 inches off the roof surface and the unit is about 34 inches square. Some kind of metal frame or 4x4 posts?


#6

Ok, I’m thinking I’ll just build the curb myself. It will be 34"x34", and about 8" high (2x6 or 2x8 lumber) , with a 3/4 plywood top deck. I’ll get my sheet metal guy to make the cap, 18ga or 20ga, 36"x36" with 2" counterflashing. Then just spud the gravel and cement the thing down to the roof. There’s no penetration, so I just need to seal it so no water gets underneath and it doesn’t move around (the condenser is only like 150lbs). Then I can get a roofer to flash it in, unless I can figue out how to do that myself too. Should be better than some of the hack jobs I see around here, just sitting on 4x4s or plastic pads.

I might do the lineset supports (3 or 4 of them) the same way, only they’re much smaller, like 6x12 or so.


#7

You could mount a steel platform with legs penetrating the roof. Where the legs penetrate they will be subject to movement and cracks. Install the legs into a chem curb which will not crack or split. You should strongly consider a Spray Polyurethane Foam application over your flat asphalt roof when you go to re-roof. That way you keep your good asphalt base with a good fire rating and encapsulate it with SPF which will insulate and protect your asphalt roof. And the cost is a fraction of what it would cost to tear off sun beat asphalt and put asphalt back on which will start leaking in five to six years. SPF lasts forever if its done right and the coating on top only needs to be re-sprayed every ten to twelve years for pennies compared to traditional roofing. SPF roofing will allow you to use your A/C much less in the Summer and Winter. Call me if you have any more questions.


#8

In order to properly do the roof, the unit will have to be removed completely from their support (moved by hand or crane, depending on the size). Most of the time roofers just jack them up to try to shove some flashing underneath, or they don’t flash it properly at all (use a termination bar just underneath the existing metal flashing).

The fewer penetrations in the membrane, the better. Personally, I would probably just stick it on some pavers and styrofoam. Your 34x34x8" curb idea with a metal cap is good, but I would properly tie it into the roof as the vibration from the AC unit would eventually break down your sealing attempt.


#9

The steel platform sounds interesting, and I’ve seen the chem curbs. The steel frame might be overkill in this case though. My condenser is just a 2.5 ton and only weighs about 250lbs. Around here they usually set them on 4x4 sleepers just sitting on the roof.

I’ll definitely consider the SPF when I reroof. I wouldn’t want it to go up over the top of the parapets through. I’ve seen some roofs done like that and it looks like crap as you can see it from the street.


#10

[quote=“shazapple”]In order to properly do the roof, the unit will have to be removed completely from their support (moved by hand or crane, depending on the size). Most of the time roofers just jack them up to try to shove some flashing underneath, or they don’t flash it properly at all (use a termination bar just underneath the existing metal flashing).

The fewer penetrations in the membrane, the better. Personally, I would probably just stick it on some pavers and styrofoam. Your 34x34x8" curb idea with a metal cap is good, but I would properly tie it into the roof as the vibration from the AC unit would eventually break down your sealing attempt.[/quote]

For the wood-framed curb, I was thinking of just setting it on the exisiting roof, using neoprene cement to seal under it. Then I’d lag bolt it into 2 trusses (24" centers). On the sides where it meets tyhe roof I’d use a cant strip and flash it up under the counterflashing. Since there’s no penetration (other than the lag bolts which are surrounded with neoprene) I just need to keep water out of the inside of it. I might foam the inside to help with sound isolation and fill the void since it’s just an empty box.


#11

Mount on 4" by 4" leave enough slack in the soft lines were it can be raised by a careful contractor. With the small amounts of rain received you will probably be replacing the wood again when you re roof make sure the wood is pressure treated as well


#12

Due to the copper refrig lines I want to mount the AC condenser so it doesn’t have to be moved when the roof is redone. It’s a heat pump so I also want to get it up a little higher than a 4x4 provides for when it snows (not often here, but it does). I think that means using either a steel frame platform, or a steel/wood equipment curb (platform). In either case I think it should probably be bolted into the trusses.


#13

I’ve been flip-flopping a little on how I should mount it. One question I had is, how do they mount condensers on flat roofs in residential new construction these days?


#14

I’ve gotten a lot of different opinions on this. The input from roofers is different from the HVAC guys, so it’s been interesting to try to factor it all in. Here’s my current tentative plan.

The equipment curb will be wood-framed, using 2x6 framed lumber with a 1" top. Dimensions of the frame and with top deck will be 34W x 34D x 6.5H. A sheet metal cap 36 x 36 (1 inch bigger than the top of the platform) with a 2 or 3-inch overhang will protect the top and act as a counterflashing. I’ll use a cant strip where the frame meets the roof. I’ll just spud the gravel where I’m putting it and bolt it right down over the current roofing material. I’ll lag bolt it to the rafters (24 inch centers), using neoprene or polyurethane flashing/roofing cement to seal under it. Since there’s no penetration other than the lag bolts that are surrounded with cement, I just need to keep water out of the inside of it. I might foam the inside to help with sound isolation and fill the void since it’s just an empty box. Then I’ll probably get a roofer to flash it in, unless I think I’m due for a re-roof soon, in which case I can just cover it with something temporary (maybe an elastomeric coating).

Enhancements:
I might get the metal cap made so that the counterflashings are removable and not part of the cap. This would make redoing the curb flashings easier than if the roofers have to put them up under the cap lip. Got any ideas on that? How would I seal between the cap and the removable counterflashing?


#15

It would look something like this when done, only smaller of course.


#16

It might look like this before it’s installed. :wink:[attachment=0]P1090140a.JPG[/attachment]


#17

As shown, the best way is to build a curb. In real life, most of the small units are mounted on two 4x4’s on top of a piece of walkway pad directly over the mod roof. Make sure you situate the 4x4’s so they don’t block the waterway to the drain. Try to get pressure treated wood to extend the life of the 4x4’s.


#18

Here’s the equipment curb with the pipe supports. The pipe supports will have the same kind of metal covers as the equipment curb.
[attachment=0]P1090176a.JPG[/attachment]


#19

Your post is really very helpful for me i have got some new ideas for my new home.


#20

What would be the best sealant to use under this type of curb?
-Neoprene type such as Black Jack 1010
-Ruberized asphalt type such as Henry 906
-Polyurethane type such as Loctite PL

With the attached cant strip, these are going to have a sealing area all around the perimeter of approximately 3" to 6" wide. Are there any curing issues with any of these flashing cement types, considering a sealing area this wide?