The number that governs your racking is almost never the regional wind speed. It is which roof zone or which array row the equipment sits in, and whether anybody actually applied the factor that goes with it. Two arrays on the same building, using the same rail and the same clamps, can need ballast that differs by a factor of two — and the difference is entirely in the zone map, not in the wind map.
This guide follows the sequence we use when a customer sends a roof plan or a ground layout and asks how much ballast or how deep the piles need to be. It is written for installers, EPCs and structural reviewers who have to sign the calculation off, and it covers where these calculations go wrong in practice.
Start with the regional value, then distrust it
Every calculation begins with a regional wind speed or a ground snow load from the local code. That is the right start and it is not the answer. The regional value is an open-country figure at a reference height, and your array is neither at that height nor in open country in the way the code assumes. In the US the design loads come from ASCE 7.
Three adjustments come next, and skipping any one of them is the most common defect in a mounting calculation: height and exposure, which raise the wind pressure; the local zone or edge factor, which can double it; and the importance factor for the building’s occupancy, which some jurisdictions apply and some do not.
Roof zones: where most of the ballast actually goes
On a flat roof the wind does not load the surface evenly. The code divides the roof into zones — corner, edge or perimeter, and field — and the corner zone sees substantially higher uplift than the middle. A ballast figure worked out once for the whole roof is therefore either over-built across the middle or, much worse, under-built at the corners.
Two site details change the zone pressures materially, and both come from the building rather than the equipment:
- Parapet height — a parapet shields the perimeter. A low parapet may provide no shielding at all in the code’s terms; a tall one can move the whole perimeter into a lower pressure case.
- Building height — pressure rises with height, and a ten-storey building and a single-storey warehouse in the same wind region are not the same problem.
This is why the ballast schedule we issue is per zone rather than per roof, and why the roof plan and building height are the first two things we ask for. Without them any ballast number is a guess with a safety factor bolted on.
Ground arrays: the edge rows are the design case
On ground-mount, the pattern repeats. The interior rows of a large array shelter each other; the perimeter rows, and especially the corner rows, do not. The wind load on an edge row is meaningfully higher than on a row in the middle of the block, and a design that uses one foundation size across the whole array is sized by whichever case governs — which means the foundations are wrong everywhere else.
Where the array is small, a large proportion of rows are edge rows, which is why small arrays are surprisingly demanding per unit area. Where the array is large, the foundation schedule can legitimately differ between perimeter and interior, and that is worth doing because it is a real material saving rather than a paperwork exercise.
Snow is not a single number either
Ground snow load is a starting figure and the code then applies roof or array factors to it. Three effects matter on solar work and they are routinely missed:
- Sliding — snow can slide off a pitched roof or off a tilted array onto what is below it. On a roof that means onto walkways or a lower roof; on an array it means onto the row in front.
- Drift — snow drifts against parapets, against equipment, and against the upwind edge of an array. A drift surcharge on top of the balanced load is the case that bends purlins.
- Unbalanced load — snow removed from one side of a ridge or partially cleared from an array produces an asymmetric case that the structure has to tolerate.
On trackers there is an additional question: the stow position in snow, and whether the control strategy puts rows where the snow load is worse rather than better. That has to be in the design, not left to the default.
Uplift usually governs, not downward load
People instinctively design racking for weight. In practice the governing case on most solar mounting is uplift: wind getting under a tilted panel and lifting it. That is why ballast exists on flat roofs, why the edge and corner zones matter so much, and why the pull-out capacity of a pile or ground screw is usually the number that sizes a ground foundation rather than its bearing capacity.
The design checks both directions, and it checks them in combination with the dead load of the array, which helps resist uplift. Getting the dead load right therefore matters in both directions — and under-estimating module weight is a quiet way to end up with an uplift case that does not close.
The ballast arithmetic on flat roofs
On a ballasted flat roof the resisting weight has to exceed the uplift with the required safety factor, and the practical question is how much ballast per tray in each zone. That number is then checked against the structural reserve of the roof, which is the constraint people forget: the roof may be able to take the ballast in principle and not in the specific bay where the array is going.
Two things shorten this cycle. The first is having the roof’s original design loads or a structural survey before the layout is frozen. The second is accepting that the ballast schedule will vary by zone and laying the array out accordingly, rather than designing a uniform layout and then discovering the corners need twice the weight.
What the geotechnical number decides
On ground-mount, the soil report decides the foundation type and depth. It is not a formality:
- Driven piles — fast and economical in suitable soil, and their capacity depends on the soil profile along the shaft rather than at the surface
- Ground screws — quick to install and removable, which matters on leased land; capacity is again soil-dependent
- Concrete — used where the soil will not take a pile, or where the site specification requires it; slower and more expensive
- Ballast blocks — where the surface must not be penetrated, at the cost of much larger blocks and a surface able to take the load
The report also gives the soil resistivity and corrosivity, which set the galvanising specification and therefore the design life. That is a durability decision made from the same test data, and it is often left out of the foundation enquiry entirely.
What the calculation must contain
Reference table
Swipe horizontally to view all columns →
| Item | What it depends on | Failure mode if skipped |
|---|---|---|
| Regional wind speed and snow load | Site location and the code edition in force | Everything downstream is wrong from the first line |
| Height, exposure and terrain category | Building height, surrounding terrain, distance to coast | Pressure under-estimated; uplift case does not close |
| Roof zone map, or array edge and corner factors | Roof geometry and parapet, or array layout | Corners under-ballasted — the failure that actually happens |
| Dead load, including ballast | Module and racking weight, tray layout | Uplift case does not close; or roof overload |
| Snow sliding, drift and unbalanced cases | Roof geometry, adjacent obstructions, array tilt | Bent purlins after the first real winter |
| Foundation capacity | Geotechnical report, foundation type | Pull-out under uplift rather than settlement under weight |
| Corrosion class | Distance from coast, soil resistivity | Coating under-specified; design life not met |
| Fixing torque values | Frame material, clamp type, coating | Cracked module frames, and an installation defect rather than a warranty claim |
Before you place the order
Send the roof plan or ground layout, the building height and location, the roof covering if it is a roof job, and any geotechnical data you have. We confirm the proposed design-support scope, which may include a zone map, ballast or foundation schedule and structural calculation, together with any local engineer-review requirements and the delivery schedule.
Choosing the structure itself is covered in the mounting structure selection guide, and the roof and ground options are listed under roof mounting systems and ground-mount racking. Where tracking is being considered, the single-axis tracker page covers the wind stow question. The checks that belong before energisation are in the commissioning checklist, and the rest of the equipment in the balance-of-system guide. For anything site-specific, use the contact page.
BUYER CHECKLIST
Buyer considerations for this scope
Which load standard does the calculation have to follow?
Wind speed, exposure category, building height, roof zone, terrain and snow load all come from the governing code for that address, typically ASCE 7 in the US. A calculation copied from a previous site will be wrong somewhere, because corners and edges of a roof carry several times the uplift of the field.
Is the calculation project-specific, and who signs it?
A structural calculation is issued per project against the actual roof covering, purlin spacing and module dimensions. Where a stamped calculation is required by the authority, that requirement is stated on the enquiry so it is priced and scheduled rather than discovered later.
What has to be on the drawing before this can be priced?
Roof covering and pitch, purlin spacing and gauge, attachment type, module dimensions and weight, wind speed, snow load, building height and surrounding terrain. With those inputs the ballast or fixing pattern can be calculated per roof zone instead of guessed.
PRACTICAL ANSWERS
Questions buyers ask about this
Why does the corner of the roof need so much more ballast than the middle?
Wind does not load a flat roof evenly. The code divides the roof into corner, perimeter and field zones, and the corner zone sees substantially higher uplift. A single ballast figure for the whole roof is therefore either wasteful in the middle or unsafe at the corners, which is why the schedule is issued per zone rather than per roof.
Can we use a single foundation size across the whole ground array?
You can, but it has to be sized for the worst case, which is the perimeter and corner rows that are not sheltered by their neighbours. On a large array it is usually worth varying the foundation between perimeter and interior, because that is a genuine material saving rather than paperwork.
Does a higher parapet reduce the ballast we need?
It can. A parapet shields the roof perimeter, and a tall enough one can move the perimeter into a lower pressure case. A low parapet may provide no shielding at all in the code terms. The height has to go into the calculation rather than being assumed to help.
What do we actually need to send to get a calculation?
For a roof: the roof plan, the building height and location, and the roof covering. For ground: the layout, the site location, and any geotechnical data you have. With those we can issue the zone map, the ballast or foundation schedule and the structural calculation.
EXPLORE FURTHER