Ways in which solar panel arrays can affix to a commercial building are as many and varied as the buildings they serve. In this article, we’re focusing on one popular method of attachment – the Ballasted roof mounted system.
Many commercial solar prospects in and around the Maryland/Washington DC area have buildings with flat or low-sloped roofs. These are generally defined as a roof with a 3:12 pitch or less. For these applications, ballasted systems can offer a way of attaching solar arrays without any (or few) roof penetrations. Many building-owners prefer this method of attachment as it negates the risk of leaking, particularly on flat roofs that may collect water. With a ballasted system, the arrays are held down using the weight of the existing solar panel arrays, racking and – most importantly – concrete blocks. These blocks are placed methodically throughout the system design to secure the arrays in place, resisting wind loads or other types of disturbance.
The addition of the ballast blocks to the solar system arrays adds additional weight to the roof. As with any commercial solar project, this weight load is analyzed and approved by a licensed structural engineer as required by the permitting jurisdiction – with some differences in each locale. Ballast racking manufacturers also specify the amount of blocks required throughout a design. The number of blocks per panel typically varies throughout design due to array proximity to building edges, parapets, or other inconsistencies that can affect wind and snow loads.
Although ballasted systems have many advantages, as with any design, they have their disadvantages and are not compatible with every type of commercial building. Let’s look at a generalized comparison scenario between a roof penetrated system and a Ballasted System:
Roof Penetrated System
|Labor Costs and Criteria||HIGH: Penetrations require flashing and sealing techniques – sometimes requiring coordination with a roofer.||LOW. Labor skills required for installation are fairly straight-forward and require no roofer coordination.|
|Panel Count||INCREASED: Systems attached to roof substructure require less roof space for mounting.||DECREASED: Ballast-blocks require roof space and can limit the available space for solar panels|
|Roof Loading||DECREASED. No concrete/ballast block||INCREASED: Ballast blocks add weight to the roof|
|Design Criteria||FLEXIBLE: Attached racking systems can usually negotiate hatches, HVAC equipment||LESS FLEXIBLE: Because of the increased space required per panel – designs may be more rigid. However most ballast racking does have reasonable flexibility.|
|Roof Pitch||5 – 45 degrees||Generally less than 10 degrees|
Multi-roofed commercial applications may have a variety of racking types for both sloped and pitched roofs. There are other options for flat roofs also, including attached and hybrid solutions that use a combination of both penetrated and non-penetrated techniques.