Commercial Roofing Construction Detail: Flashing A Drain with Liquid Flashing (CTL-SF-05)


Welcome Back Installers! 

I’m excited to bring you another chapter in the Joe Knows series. If you are new to my blog, welcome aboard and get ready to learn! I am sharing my extensive knowledge about low-slope roofing methods and practices to help you and your team get the job done quickly and efficiently.

Let’s Put Our Hard Hats On and Get to Work!  

One of the intricacies of low-slope roof installation is flashing a penetration. If a leak is found on a flat roof, it is likely due to a flashing detail. 

A drain, like other types of roof penetrations, relies on the precision of the flashing installation in protecting a roof from water damage. Since its function is to collect water, it is an extremely important flashing detail. 

For successful installation, please follow the provided instructions and watch my quick video demonstration. As I walk you through the steps to Flash a Drain with Liquid Flashing (Construction Detail, CTL-SF-05),  I want you to practice precision and focus with each step. As I’ve said before, and will say again, the devil’s in the details.

1. First, use our FlintBoard Hinged Target Sump to achieve a positive slope from the field down the drain. 

2. Position your Flintlastic® SA PlyBase, and roll it out over the drain. Allow the bolts to penetrate through the membrane before cutting out the drain access. 

3. Repeat Step 2 with the Flintlastic SA Cap Sheet, positioning both the base and cap sheets to avoid side laps or endlaps running across the drain. 

4. With the membrane in place, cut it flush with the edge of the drain bowl.

You’re Now Ready to Flash In

5. Start by preparing the polyester fabric reinforcement, determining the fabric’s length and assuming a 2” overlap within the drain. 

6. Once you have that length cut, cut fingers into the top and bottom of the polyester. 

7. Next, prepare the target patch. Take the 6” strips, each measuring a minimum of 12” in length, and cut out the fabric to accommodate the bowl. Overlap the pieces as necessary to square the edge. 

Please see our SmartFlash®ONE Brochure for additional details.

8. Tape off the edges for a clean aesthetic (not necessary for waterproofing).

You’re Now Ready for Resin

9. SmartFlash ONE is thick, and easily applied with a brush or roller without any dripping. Apply the resin around the drain and 4” into the drain bowl at approximately 30 wet mils thick. 

10. Then, lightly press in your finger flashings. 

11.  Now, fully cover the fabric with resin, making sure to put resin in the 2” overlap. You should not see any fabric.

12. Next, press in your target patches. Again, fully coat the surface and overlapped areas with resin at approximately 30 wet mils thick. 

2-4 Hours Later…

13. For full 20-year warranty coverage, allow 2-4 hours to dry, and then apply a final coat, approximately 30 wet mils thick. 

14. For aesthetic purposes (not required for warranty) broadcast CertainTeed color-matched granules into the wet resin.

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

Nice job, you now have the knowledge needed for flashing a drain with liquid flashing. 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.

Meanwhile, keep an eye out for my next blog and video post detailing the installation method for base flashing on a parapet wall (CT-06). Oh, and if 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)

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

Happy Building, 


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