Roofing Around a Curb

What is the proper way to seal a Rheem roof curb onto a flat roof.

  1. Run the roofing materials up the side of the curb, And set the AC unit on after the roofing is done.

  2. Don’t go up the sides at all.

  3. Other.

We are installing a packaged unit air conditioners onto a flat roof. The unit will be set onto a metal curb which is provided by the manufacturer.

if its a welded aluminum curb with a 4" or better flange,
then i dont see the need to turn the roofing up the side.
just screw the flange down good and strip it in .

check your deck. dont put no heavy ass a/c up there.
and not have a strong enough deck to support it.



You run your material up the sides of the curb and inside the curb.

Then you set your unit on the curb. The unit will act as the termination bar.

lefty`s right here

Hey… I used to work in the HVAC industry. I’m not a roofer, but Lefty is correct. For change outs we had to set the curb adapter on top of the original curb.

You always want to base flash your equipment curbs, not just strip in the flanges. In addition, depending on what the curb looks like, you may have to insulate the curb prior to installing your base flashing. Obviously, if the curb isn’t canted, you will also want to install a cant strip along the base of curb.

they make a curb now that is pre insulated on the
inside and doesnt need to be wrapped up and in with
cant strip,
which by the way is on its way out,
no need to make 2 cut pionts any more,
roof lays in tighter and better at a strait 90 dgree.

next yall will be tellin me ya want to put cant strip around a 4" lead boot and then flash roof up over the lead.

whats that? its ok to just flash the flange of a lead boot or grv but not an a/c curb?
hmm funny. both can leak just as bad.

thats what i thought
save me the explanation.

if your just gonna wrap the a/c curb,
then just build a cheap wooden one.

if you just have to use an aluminum one, and
cant seal just the flange, then by all means,
flash it like ya stole it.


The main difference is the size of the penetrations between a soil stack flashing and an HVAC curb. In addition, the HVAC curb is typically fastened to either a wood nailer or the roof deck, whereas the soil stack lead is set on top of the roofing membrane in roofing cement. Come on gweedo, you’re talking apples and oranges here, and if you don’t believe me take a peek at your NRCA manual.

if this is a large unit.
then yes it has to be fastened directly to the deck,
and you cannot just seal flange.
i was thinkin small residentual.


the guy mentioned a separate curb which the a/c would be set on,most single ply materials do fine w/out cant strip,w/hot tar you need it,but most mfgr.s call for no cant strip today,and we all know,they don`t back up the material unless you do it to the mfgr.s specs

firstly the curb needs to be fastened to the substrate(deck). this will require the removal of the old roof. then once securly fastened the insulation needs to be replaced next to the curb. then, a new field sheet needs to be installed to the top of the cant strip. a cant strip IS required.then a separate flashing sheet is installed on the side and over the top of the wood nailer.also be sure to use asphalt primer on the new curb before flashing it.class dismissed.


The curb is made of steel.

The flanges are about 2 inches.

The curb dimensions are 4 feet long, 3 feet wide and the height varies. On 3 sides it is 8â€Â

oh lord.
mr pines, your tryin to mess with some stuff that us
old pros have to work hard at to get rite.

i dont say this much here,
i beleive in helpin mortals and saving them money,
but you need a roofer. a good one.

were are you.
maybe someone here is close to ya.

if not ,ask your neighboors and freinds.
someone knows one.


ditto gweedo,i dont un derstand why the consumers come in ,ask us detailed questions,but dont even tell us their area of the country,apparently they don`t realise ther are different techniques dependent on the area,like r-values necessary and such :?

i hate those things. They are a pain.

I am in Tucson Arizona.
Yes, I am looking for a good roofer.
No, I have no intention of doing the project myself.

Actually a professional roofer has attempted to seal the curb in and failed miserably. He installed the base sheet and told the ac guy he was finished. The ac guy mounted the unit. Then I complained. The roofer came back and applied peal and stick mineral cap sheet to the base sheet but cut it about 1 inch short of the ac unit. Within an hour the stuff was falling off because he forgot to apply the material with force. Additionally he neglected to remove the strip that says in 2 languages “Remove this strip completelyâ€Â

reread my last post.

It is my professional opinion that AC curbs are only to b e mounted to the structure, not simply the roof deck.

This often means that you have to add supports to steel decks and framing to wood decks, and pads to insulating concrete down to the structural concrete.

aaron is 100% correct,you absolutely need to frame it to handle that weight!