Commercial Roofing Construction Detail: Flashing A Drain (CT-12)


Hello Fellow Low Slopers, 

I’m back with another CertainTeed-approved installation detail for low-slope roofing. This is definitely a skill you’ll want to keep in your back pocket to preserve a strong reputation for successful, leak-free roof installations.

Let’s Get To Work 

In this session, I will be guiding you and your team through the process of Flashing A Drain (Construction Detail CT-12) with our Flintlastic® SA self-adhered membranes.

One of the most common questions I’m faced with when conducting my training course is “but why do we need to do it this way?” Well, as manufacturers we put a lot of research and field testing into products and their application, and together these findings prove out the techniques that are the most effective for preventing leaks. 

In this session, we are working with a metal surface. Unlike our products in the Flintlastic SA line, metal does not have the top blue surface that strengthens the bond to self-adhered membranes. To combat this, we need to use FlintPrime® primer. Priming the metal will secure the bond, preventing any potential leaks. 

For successful installation, please follow the steps below, and then watch my quick demonstration video to see it in action. 

1. First, you will want to make sure you have a positive slope from the field down into the drain with any drain detail. Our FlintBoard® Hinged Target Sump offers a quick, material-efficient way to achieve a positive slope.

2. Remember when working with self-adhered membranes to prime all metal surfaces. Apply FlintPrime Aerosol to the top surface of the drain, and let that flash off for 30 minutes or until it’s tacky to the touch. Then tape off exposed bolts to avoid getting primer in the thread.

30 Minutes Later…

3. Position your base sheet and roll out over the drain. Allow the bolts to penetrate through the membrane before cutting out the drain access. Your base sheet should be positioned to avoid side laps running across the drain of either the base or the cap sheet. 

4. The membrane should extend slightly over the drain edge. As with any self-adhered membrane, apply a weighted roller to remove any entrapped air.

5. Once your base sheet is in place, install a field-cut flashing collar of Flintlastic SA PlyBase or Flintlastic SA MidPly – sized to ultimately extend 4” beyond the lead on all sides. 

6. Position the collar and press the bolts to the membrane before cutting out the drain access.

7. Position your lead centered on the flashing collar. Be sure to round the edges of both the flashing collar and the lead. 

8. Use a mallet to define the bolt locations before cutting holes. Use the same technique to cut the drain access.

9. Now, set your lead in a 1/8” bed of FlintBond SBS-Modified Adhesive – Caulk or Trowel Grade. Set either to the back surface of the lead or onto the collar, and apply pressure with a hand roller. 

10. Apply FlintPrime Aerosol to the top surface of the lead, and again, let that flash off until it’s tacky to the touch – approximately 30 minutes.

After 30 Minutes…

11. Now you can install your Flintlastic SA Cap Sheet. Make sure you’re completely covering the drain bowl. You don’t want any sidelaps or endlaps in this detail. Just like the base sheet, the fastest way is to roll out your membrane, allow the bolts to come through, and cut out your opening. 

12. Once you’ve fully waterproofed the drain bowl, install the clamping ring.

Good work! You are on a roll with learning key skills for low-slope roofing installation. If you have any questions, you can submit via this form or leave a comment below.

For additional information on CertainTeed products and installation, please visit, or go directly to our Flintlastic SA Installation Manual

In the meantime, keep an eye out for my next blog and video post detailing the installation method for flashing a drain with liquid flashing (CTL-SF-05). Oh, and in case you missed it, check out my previous installation articles:

 How to Flash A Pipe with Liquid Applied Flashing (CT-13)

Commercial Roofing Construction Detail: Flashing A Pipe With Liquid Applied Flashing (CTL-SF-06)

Happy Building, 


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