Sometimes the most pressing questions are the most basic. Take the humble ballast block—this seemingly simple piece of concrete that holds down a flat roof mounting system can lead to a lot of questions about weights, dimensions, and materials. Since finding and placing the blocks is the installer’s responsibility, it’s a common and sometimes last-minute concern.

Unirac is your resource for answers. With ballast blocks, for instance, Unirac recently published Technical Bulletin TB-2020-010, “Procuring and using ballast blocks for Unirac Roofmount products.”, which covers sizing, half blocks, waterproofing, maintenance, and many other issues, and which you can download as a PDF for reference.

Blocks aren’t as straightforward as they look. Did you know that a block is technically called a “Concrete Masonry Unit” (CMU)? How about that Unirac recommends that the ones you source comply with ASTM ( concrete roof pavers designation C1491-14 or C90-16a for a “normal weight” density with a compressive strength of 3000 PSI?

Despite being standard sizes, they can have various weights depending on manufacturer. If you take nothing else from this article, the most important thing you can do as an installer is to make sure your block weight matches that in the “inspection” section of your Unirac report. If you’re using U-builder, you can change actual block weight in the “Describe the project” section to match what you have available. Otherwise, please contact us before proceeding—blocks are an integral system element and a careful part of the overall design.

Another frequent question has to do with sourcing. In general, you’ll want to find a supplier that understands the requirements outlined in the technical bulletin—this should be a good screen if you have any doubt about your vendor. Appropriate suppliers will tend to be specialized construction outlets that can ensure a quality block that’s fit for purpose—local hardware or home-improvement stores generally won’t carry products that will last a full 25 years and meet all your project requirements. Some names we’ve heard of customers having general success with include National Roofing Partners (national), St. Vrain Block Company (Colo.), A. Jandris & Sons (Mass.), RCP Block & Brick (Calif.), Cromwell Concrete Products Inc. (Conn.), and Oldcastle Infrastructure (national). If you have trouble finding one in your area, please contact us and we can assist.

Unirac aims to provide not only world-class products but also world-class support. Along with Technical Bulletins, you’ll find Installation Manuals, Design and Engineering Guides, Detail Drawings, and Certification Letters at

And of course, if you have questions that you don’t see answered here or in your installation manual, please reach out to us. We will get you the resources you need quickly.