Air Conditioner Curb Installation- Flat Roof – Watch Video

 Air Conditioner Curb Installation 

Installing a A/C curb on a flat roof

The Bow Tie Royale 6 Cinema Theater contracted Commercial Roof USA to install an air conditioning unit on an EPDM flat roof at 542 Westport Ave., Norwalk, CT.  Heating and Cooling Company located in Norwalk, CT.  Mike Tavolacci marked precisely where the curb would be positioned on the roof. We installed the air conditioner curb with vibration dampers minimizing noise.

A/C Curb Installation – Process:

The a/c curb installation process required us to cut through the EPDM rubber at the markings. Then, we removed a 3″ Insulation ISO board. Under the insulation was a 5/8″ gypsum board. After removing the EPDM rubber, insulation, and gypsum board, we had to cut through the metal sub-roof, which is the opening of the intake duct to the A/C unit.

Images of A/C Curb Installation Steps:

  1. Make sure all the measurements are correct and the location is properly marked.
  2. Prep surface area by removing all debris and water.

Placing the Curb into Position

After making the opening for the duct work, we put the curb into position. Because the roof is slightly slanted, one side of the curb had to be lifted to be level. After leveling the curb, we anchored it down with 4″ screws to the metal sub-roof.

After anchoring the curb, we vulcanized the entire curb with EPDM cover tape, and carefully added the flashing, as well, to tie it into the existing rubber roof.

Air Conditioner Curb Installation - Preparing where the roof will be cut for the A/C curb
The spot is marked where the A/C curb will be installed

Removing the rubber and insulation

Cut the rubber and insulation away to expose the metal sub-roof.

A/C Curb Installation - Removing Insulation
Cutting the EPDM Rubber and insulation away

4. Below is the metal sub-roof after the EPDM and insulation were removed. Use a metal reciprocating saw to cut this section out.


Extreme caution must be taken during steps 3 and 4 because there could be electrical wires on the underside of the metal roof. In this case, a drop ceiling hanger wire was attached to the metal so we were fortunate enough to remove it without disturbing the drop ceiling.

Metal roof exposed under the rubber membrane and insulation
The metal deck must be cut

The metal roof had to be cut with a reciprocating saw.

5. Placing curb into position and ensuring it is level:


Since the roof was slightly slanted, we had to lift the curb by 5/8″ on one side to get it level. It is important to have it absolutely level; if it is not level, the bearings will wear off faster on the A/C unit.

Placing the curb in position to be flashed
The metal curb is placed into position

After removing the roofing material and cutting open the intake access area, we could place the curb in place.

6. Leveling the curb.


The roof is slightly slanted and it was necessary to level the curb before we started to install the flashing.  We used 5/8″ wood strips under the one end of the curb. After getting it level we secured the curb to the roof with 4″ screws that held it into place.

Air conditioner curb leveling
The curb must be leveled

This air conditioner curb had to be leveled before we could flash it.

7. Vulcanizing and flashing the air conditioner curb.


Once the curb was leveled we covered the entire outside of the curb with EPDM cover tape. To flash the curb, it is necessary to clean the existing roofing surface very well. We used lacquer thinners and rags to remove all the oxidation from the surface where we needed to adhere to the new EPDM cover tape.

After the cleaning, we brushed primer to the surface and waited for it to dry. After a touch dry test, we installed the cover tape and flashed the entire unit.

Cover tape is used for the flashing

It is essential to clean the surface from old oxidation before installing the flashing.

8. Installing the flashing.

In the case of an EPDM roof, it is necessary to use the compatible material to do the flashing. In this case, we used EPDM cover tape. It is very important to use a roller to roll the cover tape so it can properly adhere to the EPDM rubber.

Cover tape being rolled out.

It is important to roll out every inch of the cover tape to get a secure bond to the EPDM rubber roof.

9. Cover the curb with a tarp.


It is difficult to coordinate two different trade crews to complete an A/C unit installment on the same day, but if the A/C contractor marks out where the curb will be installed and leaves the curb on the roof, we can then notify them when the curbing has been completely installed and is ready for the A/C installation.

Covering a A/C curb with a tarp awaiting for unit installation
After installing the curb, we covered the curb with a Tarp

After installing the curb we cover it with a tarp. The A/C contractor can schedule a time to install the unit.

10. The proper material for the right job


Whenever there is a curb to be installed or flashing that needs to be done, it is important to use compatible materials. On an EPDM roof, only EPDM materials must be used. The same principle applies to a Modified Bitumen Torch Down Roof; only Modified Bitumen materials should be used. Sometimes, we find that this principle is not practiced by other roofing contractors. In the images below we are flashing a curb with Modified Bitumen Membrane Torch Down material.

Flashin curb with Modified Bitumen Membrane
Flashing a A/C curb with a torch

Flashing the curb with a Modified Bitumen Membrane

The image below is also a Modified Bitumen Membrane curb installation. It is essential that a special tar is used to try and seal any flashing, but it should be used very sparingly. Installing a curb is very important and should only be done by professionals.

More flashing repair on a A/C Curb

When the curb is installed on a Modified Bitumen roof, it’s important to use the same material to do the flashing.

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