Quick answer
Ballasted Solar Mount A ballasted solar mount is a racking system for flat or low-slope roofs that holds solar panels in place with weight, usually concrete blocks in trays, instead of bolting through the roof.
The blocks resist wind trying to lift or slide the array. An engineer sets how many blocks go where, and the roof structure must be able to carry the extra weight.
Quick facts
The key facts about ballasted solar mount, with sources:
Key takeaways
- Ballast uses weight, not bolts, to keep panels on a flat roof.
- The main design force is wind uplift, worked out under ASCE 7.
- The roof structure must carry the panels, racking and blocks. A structural check comes first.
- Block counts come from the maker’s engineering tool and a stamped plan, never a guess.
- Most common on commercial flat roofs; seen on some homes with flat sections.
How a ballasted mount differs from other racking
Most home racking bolts into rafters through flashed holes. The general picture is on the solar mounting system page. A ballasted mount takes the other route. It sits on the roof surface and relies on mass.
The frame is a low tray or "chassis" under each row of panels. IronRidge’s BX chassis holds up to four full-size concrete blocks. Unirac’s RM10 EVO uses "ballast bays" that nest together and flex with the roof surface. The panels sit at a low tilt, 5 or 10 degrees, so wind passes over them.
Some designs add a few roof anchors where wind is strongest, a mix often called hybrid. The BX manual covers layouts with third-party roof attachments, with or without concrete blocks in the chassis.
How weight beats wind uplift
Wind flowing over a roof creates suction, called uplift. It is strongest at corners and edges. It also pushes sideways, which can slide an array. A ballasted system must resist both with weight and friction alone, or with a few anchors.
Engineers calculate these forces with ASCE/SEI 7, "Minimum Design Loads and Associated Criteria for Buildings and Other Structures". The current edition is ASCE 7-22. Inputs include the local design wind speed, building height, roof zone and the array’s shape.
Unirac says its RM10 EVO was designed and tested to meet the requirements of SEAOC PV1, PV2 and ASCE 7, which are wind and seismic design guides for rooftop arrays. IronRidge says the BX was certified to the International Building Code and ASCE/SEI 7, with wind tunnel testing.
The output is a ballast map: how many blocks go in each tray. Corner and edge trays usually need more. Unirac’s guide says deviations from the block plan must not be made.
Kinds of ballasted system
| Type | How it holds | Where it fits |
|---|---|---|
| Fully ballasted | Weight only; no holes | Roofs with spare structural capacity and moderate wind |
| Hybrid (ballast + anchors) | Blocks plus some roof attachments | Higher wind zones or weight-limited roofs; the BX fire rating covers layouts with roof attachments, with or without blocks 1 |
| South-facing, low tilt | Single row facing south at 5–10° | Most common; maximises output per panel 2 |
| East-west (dual tilt) | Panels back to back, facing east and west | Packs more panels on a roof; lower wind exposure |
Example: how much one tray can weigh
This uses only the numbers printed in the IronRidge BX manual. It shows scale, not a design. A stamped plan sets real counts.
| Item | From the manual | Result |
|---|---|---|
| Blocks per chassis | Up to 4 full-size (16 × 8 × 4 in) or 8 half-size 1 | — |
| Weight per full block | 28–34 lb, ±2 lb 1 | — |
| Max block weight on one chassis | 4 × 28 to 4 × 34 lb | 112 to 136 lb of concrete per chassis |
| System design load rating | 30 psf down, 20 psf up, 5 psf lateral 1 | Actual capacity set by the PE-stamped project design 1 |
Multiply that by dozens of chassis and the blocks alone can weigh several tons on a commercial roof. That is why the roof check comes first.
Unirac adds that block weights in its system range from 26 to 38 lb, and weight has a major effect on the design, so verify them before use 2.
Where ballasted mounts are used
Commercial buildings with large flat roofs are the main market: warehouses, schools, offices and stores. They often have membrane roofs where every hole is a leak risk. See commercial solar.
Homes use them less often. Row houses, condos and modern homes with flat roof sections are the typical case. For shared roofs, read solar on condo and shared roofs.
Pitched roofs are not candidates. The BX is approved only for roof pitches under 7 degrees 1.
Benefits and drawbacks
What it does well
- Few or no roof holes, which suits membrane roofs.
- Fast to place: trays sit on the roof and blocks drop in.
- Low tilt keeps the array out of sight from the street.
- Easier to lift off and replace when the roof needs work.
Where it falls short
- Adds a lot of weight to the roof structure.
- Low tilt yields a bit less per panel than an ideal angle in many places; see solar panel tilt.
- High-wind or seismic sites may still need anchors.
- Not for pitched roofs.
Limits: roof strength, slope and seismic zones
Roof strength. The building must carry the array, racking, blocks, snow and people walking on it. A structural engineer checks this. Older roofs may not have room for ballast at all.
Slope. Weight holds best on flat surfaces. IronRidge limits the BX to pitches under 7 degrees. Unirac also restricts minimum and maximum roof slopes in its manual.
Seismic areas. Unanchored arrays can shift in an earthquake. Unirac says roof pads are required for unattached systems in certain seismic areas.
Roof membrane. Blocks and trays rest on the membrane. Unirac says to check with the roofing maker whether protective pads are required and compatible.
What drives ballasted mount cost
We do not list prices. The drivers: the amount of ballast the wind map demands, whether anchors are needed, structural engineering and any roof reinforcement, block delivery and crane lifts, and roof pads. A strong roof in a moderate wind zone needs the least.
Lifespan is tied to the roof. If the roof will need replacing soon, do that first. See re-roof before solar and solar cost.
Warranty terms sit with each maker. IronRidge says warranty details are on its website; check the current terms for any system on your quote 1.
How a ballasted array is designed and installed
Every step is done by licensed professionals. This is what to expect, not a how-to.
- A structural engineer checks the roof’s spare load capacity.
- The designer runs the maker’s tool with wind speed, building height and roof zones under ASCE 7.
- A PE-stamped plan set with a ballast map is submitted for the building and electrical permits.
- The crew marks fire setbacks, places trays, lays panels and loads blocks exactly per the plan 1.
- The array is bonded and grounded per the NEC and the UL 2703 listing 2.
- Building and electrical inspectors sign off; the utility grants permission to operate.
Upkeep
Check after major storms that no blocks have cracked, shifted or blown out and that trays have not slid. Concrete blocks must resist freeze-thaw where needed; the BX manual requires blocks made to ASTM C1491 with 3,000 psi minimum strength 1.
Keep drains and scuppers clear. Debris can pond water around trays. A roofer and installer should coordinate any roof repair under the array.
Warning signs and when to call a pro
- Trays or panels shifted from their marked positions.
- Cracked, missing or crumbling blocks.
- Ponding water, sagging deck or new interior leaks.
- Loose wiring or panels after high wind.
- Any of these: call the installer and, for sagging or leaks, a structural engineer or roofer.
Safety rule
Never add, remove or move ballast blocks yourself. The count in each tray is engineered for wind and roof load, and changing it can let the array lift or overload the roof.
Standards and codes that apply
ASCE/SEI 7-22 is the current edition of the US load standard for wind, snow and seismic forces. Your building code adopts a specific edition; the engineer uses that one.
UL 2703 covers mounting systems, bonding and grounding. UL says it is also used to set the fire class of a rooftop PV system.
The BX manual lists a Class A system fire rating on low-slope roofs under 9.5 degrees with listed module types, and says Class A systems can go on Class A, B and C roofs without changing the roof rating.
SEAOC PV1 and PV2 are structural engineering guides for rooftop solar that Unirac cites for seismic and wind design.
Building and fire codes set roof access setbacks. The BX manual starts by marking fire setbacks on the roof.
Ballasted mount vs other mounting options
| Option | Holds by | Roof holes | Best for |
|---|---|---|---|
| Ballasted mount | Weight | None or few | Flat and low-slope roofs |
| Railed roof mount | Bolts into rafters | Yes, flashed | Pitched home roofs; see roof penetration |
| Ground mount | Posts or footings in soil | None | Open land |
| Solar carport | Steel structure on footings | None | Parking areas |
Misconceptions
- Myth No holes means no risk to the roof.
- Reality The roof still carries the weight and the membrane takes the wear; pads may be required 2.
- Myth Any flat roof can take ballast.
- Reality Only if the structure has spare capacity, confirmed by an engineer.
- Myth More blocks are always safer.
- Reality Extra weight can overload the roof; the stamped ballast map is the rule.
- Myth Ballasted systems work on any pitch.
- Reality They are limited to low slopes, under 7 degrees for the BX 1.
Ballasted solar in SC, GA and VA
Coastal South Carolina, Georgia and Virginia face hurricane winds, which raise the design wind speed an engineer must use under ASCE 7 and can push a design toward anchors or more ballast. Your local building department enforces the code edition it has adopted. Flat-roof systems are common on SC businesses and farm buildings.
Is a ballasted mount right for your roof?
If your roof is flat or nearly flat and has a membrane you do not want pierced, ballast is worth asking about. Ask for the structural letter, the ballast map and the wind speed used.
If the roof is pitched, a railed or rail-less mount is the norm. If the roof is old, plan the re-roof first. Check solar panel weight and roof loading before you sign.
Questions about ballasted solar mount
Do ballasted solar panels blow off roofs?
They are designed not to. An engineer sizes the ballast for local wind under ASCE 7, with more blocks at corners and edges. Problems come from wrong block counts, cracked blocks or skipped engineering. Check the array after big storms and call the installer if anything moved.
How much weight does a ballasted system add?
It depends on the wind map. The IronRidge BX manual allows up to four full blocks of 28–34 lb each per chassis, so up to about 136 lb of concrete on one tray. Across a whole roof that adds up, which is why a structural check comes first.
Can ballasted mounts go on a pitched roof?
No. They need flat or low-slope roofs. IronRidge approves its BX system only for roof pitches under 7 degrees. Pitched home roofs use racking bolted into the rafters instead.
Do ballasted systems need any roof penetrations?
Often none, but not always. In high-wind or seismic areas, or on weight-limited roofs, designs add some anchors. The IronRidge BX manual covers layouts with roof attachments, with or without blocks. Unirac requires roof pads for unattached systems in certain seismic areas.
What tilt do ballasted solar panels use?
Low tilts, usually 5 or 10 degrees. IronRidge sells 5° and 10° chassis, and Unirac’s RM10 EVO is a 10° south-facing system. Low tilt reduces wind load and lets rows sit closer together.
What standard covers wind load on rooftop solar?
ASCE/SEI 7, the US standard for minimum design loads on buildings. The current edition is ASCE 7-22. Engineers use it, with the edition your local code adopts, to calculate uplift and sliding forces on the array.
Are ballasted systems fire rated?
Yes, under UL 2703, which sets the fire class of the panel and racking together. The IronRidge BX manual lists a Class A system rating on low-slope roofs with listed module types. That rating holds only when installed per the manual.
What kind of concrete blocks are used for ballast?
Solid concrete blocks of set size and weight. The BX manual calls for 16 × 8 × 4 inch blocks made to ASTM C1491, freeze-thaw resistant where needed, at 3,000 psi minimum and 28–34 lb each. Use only what the plan specifies.
Can I re-roof under a ballasted array?
Yes, but the array has to come off first. Ballasted arrays are easier to lift and replace than bolted ones because there are few or no roof holes. Coordinate the roofer and the installer, and keep the original ballast map.
Sources
- IronRidge, BX System Installation Manual, Version 1.8 (2019; LA County-hosted copy), retrieved .
- Unirac, RM10 EVO Installation Guide (PUB2021DEC01; LA County-hosted copy), retrieved .
- ASCE, ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, retrieved .
- UL Solutions, PV Mounting Systems Certification (UL 2703), retrieved .
- Unirac, RM10 EVO product page (10-degree south-facing ballasted flat roof system), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Ballast, tilt, slope, load rating, block and fire-rating details copied from the named IronRidge BX v1.8 and Unirac RM10 EVO manual revisions; load standard edition from ASCE; UL 2703 scope from UL Solutions. No block counts or prices are given because they are project-specific.
Suggest a correction. We fix errors and say what changed.