How the two differ in kind
Snow does not sit evenly on a roof, and the design accounts for that with two separate checks.
The BALANCED load is the uniform one: the ground snow load for the location, modified for the roof's exposure, its thermal condition, its slope and the importance of the building. It is the number most people mean by snow load, and it is checked across the whole roof.
The DRIFT surcharge is local and much larger. Wind blows snow off an upwind surface and deposits it in the lee of an obstruction, where it piles up to a depth that can be several times the balanced depth. The load is concentrated over a limited width, so a member perfectly adequate under the uniform load can be substantially overloaded within that band.
Where drifts form is predictable, which is what makes the check tractable. Against a higher adjacent wall, where snow blowing off the upper roof or the wall face accumulates below. Against a parapet, which traps snow blowing across the roof. Behind rooftop plant, screens and enclosures, which act as obstructions in exactly the same way. In the valley of a multi-span or sawtooth roof. And at a step between two roof levels, which combines the first and the last.
The depth of the drift depends on how much snow was available to be blown into it, which is why the calculation takes the LENGTH of the upwind roof as an input: a long upper roof supplies a large drift and a short one supplies a small one.
The consequence people miss is that drifts can be created after a building is finished. Extending a roof, adding a plant screen, building a taller adjacent structure, or a neighbour building higher next door all introduce or enlarge a drift on a roof that was designed without one — and nothing on the roof itself has changed.
The factors that actually differ
Show
| Balanced snow load | Drift surcharge | |
|---|---|---|
| Distribution | Uniform across the roof. | Concentrated in a band at an obstruction, tapering away from it. |
| Magnitude | The design figure for the location, modified for exposure, thermal condition, slope and importance. | Potentially several times the balanced depth within the drift width. |
| Where it governs | The general roof structure — most members, most of the time. | Members near a wall, a parapet, a screen, a valley or a roof step. Almost always the governing case there. |
| What drives the magnitude | Ground snow load and the roof's own characteristics. | The LENGTH of the upwind roof supplying the snow, plus the height of the obstruction. |
| Created by later work | Unchanged by an extension or a new screen. | Yes — a roof extension, a plant screen, or a taller neighbour can create a drift where none was designed for. |
| Slope | Reduces it, since snow slides or is less readily retained on a steep roof. | Sliding snow can ADD to the drift on a lower roof below, which is a separate surcharge again. |
| Thermal condition | A warm roof loses snow faster, which the factors account for — and an unheated one holds it. | Same inputs, and a heated roof below a cold upper roof is a common combination at a step. |
| Rain-on-snow | An additional surcharge in some codes on low-slope roofs, where rain is absorbed rather than drained. | Compounds it where drifted snow is already deep. |
| Ponding interaction | Deflection under load allows water to collect, which adds load and increases deflection. | Worse, because the drift deflects the structure locally and creates the low point. |
| What the check is for | The roof generally. | The specific members and connections within the drift zone — frequently a stronger member in a band rather than everywhere. |
Which one, and when
Choose balanced snow load when…
- Assessing the roof structure generally, which is where every snow check starts.
- A simple single-level roof with no adjacent higher structure, parapet or rooftop plant.
- Establishing the design snow load for the location before any local effects are considered.
- Comparing a roof's capacity against the code figure for its region.
Choose drift surcharge when…
- There is a parapet, a higher adjacent wall, a roof step, a valley, or rooftop plant — that is, most commercial roofs.
- A roof is being extended, or a screen, enclosure or new plant is being added.
- A taller building is going up next door, which changes an existing roof's loading.
- Assessing an existing roof for a change of use or additional equipment.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why are drifts so much larger than the balanced load?
- Because they collect snow from an area much larger than the area they sit on. Wind moving across a roof picks up snow and carries it until an obstruction slows the air enough to drop it — so the snow that fell on a long stretch of upwind roof ends up concentrated in a band a few metres wide in the lee of a parapet or a wall. The load per square metre in that band is therefore a multiple of what fell there. That is why the calculation takes the upwind roof length as an input: it is effectively asking how much snow was available to be moved. It is also why the drift tapers — deepest at the obstruction and diminishing away from it — and why the structural check is about the members within that band rather than about the roof as a whole.
- Where exactly do drifts form?
- In the lee of anything that interrupts the wind, and the locations are consistent enough to check by walking a roof plan. Against a higher adjacent wall, including the wall of a taller part of the same building. Against a parapet, on the inside face. At a step between two roof levels, on the lower roof. In the valley of a multi-span, sawtooth or barrel roof, where snow blows off both slopes into the low point. And behind rooftop equipment, screens, enclosures and anything else standing proud of the surface — which is the one most often forgotten, because plant is added after the structure was designed. Windward drifts also form on the upwind side of an obstruction, smaller than the leeward one but not always negligible, and codes deal with both.
- Can adding rooftop plant overload a roof?
- Yes, in two separate ways, and the second is the one that surprises people. The obvious one is the unit's own weight, which is checked as a point or distributed load on the structure and is what everybody looks at. The second is that the unit or its screen becomes an OBSTRUCTION, and a drift forms in its lee where none existed before — a load the original design never included, applied over a band of roof that was sized for the balanced case. A large screened plant enclosure can create a substantial drift. The same applies to a new roof extension creating a step, or a neighbour building higher against a shared boundary. It is why a structural check for added rooftop equipment considers drifting rather than only the equipment's mass.
- How is the balanced load derived?
- From the ground snow load for the location, modified by factors for the roof's own condition — and each factor has a defensible basis rather than being a fudge. Exposure accounts for whether wind can remove snow from the roof: a sheltered roof surrounded by taller buildings or trees keeps more than an exposed one. The thermal factor accounts for heat loss through the roof melting snow from below, so a heated building's roof carries less than an unheated structure's, and a well-insulated roof carries more than a poorly insulated one — which is a real and slightly counter-intuitive consequence of improving a roof's insulation. Slope reduces the load on steeper roofs, more so for slippery surfaces where snow slides. And an importance factor reflects the consequences of failure for the building's use.
- What is sliding snow and does it matter?
- Snow shedding off a steep or slippery upper roof onto a lower one, and it is a separate surcharge from drifting that frequently applies at the same location. A metal upper roof is particularly effective at releasing its snow, and the whole of it can arrive on a lower roof in one event — so the lower roof is checked for the sliding load as well as for any drift, and codes give methods for both. It is also a safety matter beyond the structure: snow released from an upper roof falls onto whatever is below, which is why snow guards are used over entrances, walkways and parked cars. Where an upper roof discharges onto a lower one and a drift also forms there, the two effects are considered together rather than in isolation.
- How does this interact with ponding?
- Badly, and drifts make ponding more likely rather than less. A drifted load deflects the structure locally, and that deflection creates a low point exactly where the load is greatest — so meltwater or rain collects there. That water is additional load, which increases the deflection, which collects more water: a progressive mechanism that has caused roof collapses, particularly on low-slope roofs during rain-on-snow events where drainage is blocked by ice. The defences are the ordinary ones done properly: adequate fall to the outlets designed with deflection allowed for, overflows set so that a blocked primary outlet discharges visibly instead of loading the roof, and keeping outlets clear. On a roof with known drift locations, the drainage design deserves particular attention at exactly those places.
- Should snow be cleared from a roof?
- Sometimes, and it is a decision with its own hazards rather than an obvious precaution. Clearing reduces the load, which matters where an unusual accumulation has occurred or where a roof is known to be marginal. It also puts people on a snow-covered roof, which is a serious fall risk; risks damaging the roof covering with tools; and can make things worse if done unevenly, since removing snow from one area and leaving it on another creates an unbalanced load the roof was not designed for. Where clearing is genuinely needed, it is done to a plan: symmetrically, leaving a layer to protect the covering, with fall protection, and by people who know where the drift zones are. For most buildings, the answer is a roof designed for the load rather than a plan to shovel it.
- Does better insulation increase the snow load?
- Yes, and it is a genuine consequence of improving a roof rather than a technicality. A poorly insulated heated building loses heat through its roof, which melts snow from beneath and reduces what accumulates — so the roof carries less than the ground snow load would suggest, and snow codes recognise that with a thermal factor. Insulate the roof properly and that heat loss stops, so the snow stays, and the roof carries closer to the full load. For a new building this is simply part of the design. For a retrofit it is worth a thought: upgrading the insulation on an older roof whose structure was designed when it was leaking heat can move it toward a load case it was not checked for. It is rarely decisive on its own, and it is worth mentioning where the structure is marginal.