Quick definition
A ballasted mount sits on top of a flat or low-slope roof and stays put through sheer weight, usually concrete blocks locked into a metal racking frame, rather than bolts driven through the roofing material.
It's the standard mounting choice for flat commercial roofs, where avoiding penetrations helps protect an expensive membrane roofing system from leaks.
At a glance: ballasted racking is most common on TPO, EPDM, and other membrane roofing found on commercial and flat-roof buildings, far less common on residential asphalt shingle roofs.
Why it matters
Roof penetrations are the biggest long-term risk on any flat membrane roof. Every bolt that goes through the membrane is a potential leak point, and on a large commercial roof with hundreds of racking anchors, that risk multiplies fast. A ballasted mount sidesteps the issue entirely by staying on top of the roof surface instead of going through it.
The tradeoff is weight. A ballasted system needs enough concrete or heavy material to resist wind uplift without any mechanical anchoring, which usually means significantly more dead load on the roof structure than a bolted-down system carrying the same panels. That's why a structural engineer typically has to confirm the roof can handle it before a ballasted system gets approved, backed by the kind of rooftop structural testing Sandia National Laboratories has conducted on solar PV specifically.
Wind zone matters a lot here too. A building in a high-wind region, per the wind load requirements in ASCE 7, needs more ballast weight to keep the array from shifting or lifting than the same array would need in a calmer climate, which can push the weight penalty even higher in coastal or storm-prone areas.
Where you'll see this
Deciding between the two mounting styles for a flat roof is exactly what the ballasted mount vs penetrating mount comparison tool is built for, weighing roof risk against added structural load. If ground mount is also on the table as an alternative to a heavy ballasted roof system, the ground mount vs roof mount space calculator compares the space and structural tradeoffs directly.
Once you've settled on ballasted racking, the flat roof solar mounting cost tool estimates hardware and installation cost for your specific roof size.
This term comes up most in commercial and flat-roof residential installs, in structural engineering reviews, and any time a roofing membrane's warranty terms restrict penetrations.
Related terms
Frequently asked
Neither is universally better. Ballasted mounts avoid roof penetrations, which matters most on membrane roofs where leaks are a real concern. Penetrating mounts anchor more securely and add less dead weight, which matters more on roofs with tighter structural limits.
Not typically. Ballasted mounts rely on gravity holding the weight in place, which works well on a flat or low-slope roof but doesn't hold securely once the roof pitch increases. Sloped roofs almost always use penetrating racking instead.
It varies by wind zone and system design, but ballasted systems typically add noticeably more dead load than penetrating racking, since the ballast itself is doing the job that anchor bolts do in a penetrating system. A structural review is standard practice before installing one.
Yes. Even without roof penetrations, most jurisdictions in the US and Canada require a permit and often a structural engineering review, since the added weight still has to be confirmed safe for the roof's load rating.