Structure
Designing a Roof for Snow
Snow does not load a roof where it falls but where wind and slide pile it, so frame the drifts before the field.
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Reading the Roof as a Catchment
Snow arrives more or less evenly across a roof and never stays that way. Wind scours the exposed field and drops what it lifts into the first sheltered pocket downwind: the low side of a step, the throat of a valley, the strip inside a parapet, the alley between two rooftop units. A roof that carries the uniform figure with room to spare can still come down at one of those pockets, because the load there is a multiple of the average, concentrated over a band a few feet wide, sitting on exactly the members nobody gave any reserve to.
Failures in snow country are rarely a story about a freak winter. They are a story about geometry — a wing added ten years after the original, a mechanical screen bolted to a clean plane, a dormer that turned a shedding slope into a catcher. The uniform case gets calculated because plan review asks for it. The collection cases get missed because nobody drew them.
Treat the roof as a map of catchments before you treat it as a set of spans. Print the plan and mark every change in roof height, including height changes against a neighbouring building on the next lot; every parapet, screen wall and dormer cheek; every valley and re-entrant corner; every obstruction that will stand taller than the settled snow; and the prevailing winter wind. What is left unmarked is the open field, and the open field is the easy part. Everything you marked earns its own load case, its own supporting members, and its own line on the sheet the truss fabricator receives.
The Open Field
Standards hand you the uniform load nearly complete. ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, begins from a site ground snow load and adjusts it for how exposed the roof sits, how warm the roof surface runs, the consequence class of the building, and the slope. The National Building Code of Canada and EN 1991-1-3, Eurocode 1: Actions on Structures, Part 1-3, General Actions, Snow Loads, follow the same shape under different coefficient names. The arithmetic is short; the judgement behind each factor is not.
Exposure is a site call, made standing on the ground and looking around. A roof screened by mature conifers or shadowed by a taller neighbour keeps everything that falls on it and gains whatever blows off the taller surface. A roof on an open ridge line loses a share to the wind, but only the field loses it, and what leaves goes somewhere. Thermal condition splits along whether the assembly is heated, ventilated, or an unheated shed or carport, with the unheated case carrying the higher factor because nothing below melts the underside. Slope reduction is available only where the surface is genuinely slippery and genuinely unobstructed.
Ground snow load itself is jurisdictional. Mapped values exist, but mountainous and coastal terrain is commonly designated case-study territory, and many municipalities publish a local value that exceeds the map. The authority having jurisdiction governs, and the number should arrive in writing before framing is ordered rather than during plan review. Low-slope roofs also carry a code minimum that can exceed the calculated value in mild regions, which catches anyone assuming a warm climate means no snow design at all.
Every drift, slide and unbalanced case further down this page is a surcharge riding on top of the balanced number, so the exposure, thermal and slope factors are worth applying properly here rather than carried forward from memory into nine more load cases.
Flat-roof design snow load
0.1523 psi
With the figures above, the flat-roof design snow load comes to 0.15 psi. This is presented for United States. Building in another market? Change the selector above and the units and terminology follow.
Add the equipment this sizes
This result is a specification — 0.1523 psi — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
The Step: Lower Roofs Under a Taller Wall
A lower roof against a taller wall produces the load that puts more buildings on the ground than any other snow case. Wind blowing across the upper roof carries snow over the edge and drops it in the dead air behind the wall, building a leeward drift on the lower roof. Wind from the opposite quarter scrapes the lower roof and piles material against the same wall from the other side, giving a windward drift computed as a fraction of the leeward case. Both get checked, whichever is larger governs, and neither can be assumed away because the site supposedly gets no wind from that quarter.
The surcharge is triangular, deepest at the wall and tapering to nothing out in the field, and it sits on top of the balanced load rather than replacing it. Its height grows with the upwind fetch — the length of roof or ground feeding it — and with the ground snow load, so a long upper roof feeding a short lower roof is the worst arrangement available. Where the height difference is too small to hold the full pile, the drift truncates and spreads wider and flatter instead of disappearing, and that case gets misread in the unsafe direction more than any other.
On site the failures are dull and repeatable. Fetch measured off the wrong roof, or measured to the parapet instead of the far edge. Trusses ordered from a plan drawn before the addition was designed. The first joist at the wall doubled while the second, third and fourth carry a surcharge nearly as large and stay single. Sheathing and its fastening left at the field schedule under a pile several feet deep. And the wall itself, now taking an eccentric vertical load along a line the stud pack below was never detailed for.
The Lee Side of the Ridge
Wind across a gable strips the windward slope and builds the leeward one, so the design case loads one plane heavily and leaves the other nearly bare. Rafter-framed roofs with a structural ridge feel that asymmetry directly: the beam picks up a heavier reaction on one side, and the posts and the load path beneath them have to accept it. Very shallow and very steep roofs fall outside the unbalanced requirement, while the broad middle band of ordinary residential and light commercial pitches sits squarely inside it.
Truss roofs handle the unbalanced case routinely inside the fabricator's software, provided the fabricator was told the right ground snow load and the right exposure. ANSI/TPI 1, National Design Standard for Metal Plate Connected Wood Truss Construction, sets the responsibility split, and that split is the trap: the truss designer engineers the truss for the loads given, while the building designer owns the loads themselves and the supports under them. Scissor, cathedral and dropped-chord profiles change how the unbalanced case resolves and deserve a specific note rather than a generic snow figure.
Ridge members deserve their own arithmetic rather than a section carried over from a similar house. Tributary width does not change under unbalanced snow, but the load riding on that width does, and a beam sized on the balanced figure can be short by a wide margin at the same span. Follow it down: the post at each end, the bearing under the post, the header it lands on if there is an opening below, and the footing. A ridge beam bearing on a stud pack that dies at a window head is a common and quiet defect.
Unbalanced snow is the point where the ridge stops being a geometry problem and becomes a load problem, so the beam's tributary load should be re-run with the heavier slope's figure before any section is committed to the framing package.
Ridge beam load per linear meter
7.92 kN/m
Assumes simply-supported rafters bearing directly on a structural ridge beam with no rafter ties — verify actual load path with a structural engineer for irregular framing.
With the figures above, the ridge beam load per linear meter comes to 7.92 kN/m. The method behind this is well established, though site conditions and material batches will move it somewhat. This is presented for United States. Building in another market? Change the selector above and the units and terminology follow.
Add the equipment this sizes
This result is a specification — 7.92 kN/m — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
The Valley Throat and the Inside Corner
A valley collects from two planes at once and drains nothing while the temperature stays below freezing. Snow slides in from both sides, meets in the throat, and consolidates there into the densest material on the roof; settled and refrozen snow weighs several times what the same depth of fresh snow does, so the depth you see understates the load badly.
Valley rafters are already the longest and most heavily loaded members in a hip or intersecting-gable roof before any surcharge is added. Upsizing the valley itself is the obvious move and usually gets done. Checking what it bears on at the bottom — often a corner of plate, a header, or a point on a wall with no post under it — gets done far less often, and the bearing is the part that crushes.
Re-entrant corners on L-shaped and U-shaped plans behave like valleys in plan rather than in section: they catch drift from two fetches at once, and the two piles overlap in the corner. Crickets behind chimneys and curbs are drainage devices, not snow devices, and a cricket sized to shed water will still hold a drift on the uphill side of whatever it serves.
Behind Parapets, Screens and Dormer Cheeks
Anything standing above the snow surface builds a drift on the roof behind it. Wind crossing the field meets a parapet, slows, and drops its load in a triangular pile along the inside of the perimeter — the strip framed with the lightest members and the shortest spans, and the strip carrying cantilevered edges. Deep parapets on a large low-slope roof can govern the perimeter framing outright.
Parapet height relative to the snow matters more than parapet height on the elevation. A low parapet fills to the top and then stops collecting, which caps the drift. A tall screen wall, an elevator overrun, a signage frame or a mechanical enclosure keeps collecting for the whole storm, and open frames still count when the infill is dense enough to slow the air.
Dormer cheeks act as small parapets with fetches nobody measures, because a dormer reads as a detail rather than as a structure. Each cheek makes a drift in the roof plane beside it, sitting on jack rafters cut short and hung off a header. Where a run of dormers repeats, the drifts between them merge and the whole band across the roof face loads up.
The Lee of Every Curb, Chimney and Array
Rooftop units, chimneys, duct runs and solar racking each generate a pile on their lee side and in the alleys between rows. The surcharge is local, but the framing beneath it has usually been headed out to make the opening for the unit, so the worst load and the weakest framing arrive at the same place.
Solar arrays do two things to a snow roof and both matter. They stop a slippery metal or membrane roof from shedding, which removes the slope reduction and leaves the full load on the structure for the life of the array. They also shed themselves: modules dump onto the roof below in a concentrated band, and onto whatever sits beneath the eave if the array runs to the edge. Racking loads arrive as point loads through standoffs into rafters or purlins sized for a uniform figure.
Snow guards are an obstruction installed on purpose. Their function is to keep snow on the roof, which means the structure must hold what it would otherwise have dropped, with no slope reduction taken anywhere on that plane. Guards also transfer a down-slope force into the covering and the deck, and a guard rail screwed into sheathing rather than into framing will strip out and take the snow, the guards and the panel edge with it.
The Eave, and Where the Slide Lands
Whatever leaves an upper roof lands somewhere, usually on something nobody designed for it. Sliding snow off a metal upper slope arrives on the lower roof as a band along the base of that slope, and the standards treat it as a separate surcharge with its own extent. Lower roofs beside steep slippery upper roofs frequently carry sliding load and leeward drift in the same place and need both checked, not the larger of the two assumed to cover both.
At the eave, heat loss from the conditioned space melts snow up-slope and the meltwater refreezes over the cold overhang. The ice band that builds there loads the rafter tails and the cantilever of the deck, and water backing up behind it gets under the covering. Continuous insulation and ventilation carried to the eave over an airtight ceiling plane is the cure, and it is building science rather than framing — but the framing consequence is a concentrated load at the tail of every rafter, typically the smallest section on the roof.
Look at what sits under the discharge path before signing the roof off. Entry doors, gas meters, condensers, egress routes and parked vehicles beneath a slippery slope are a life-safety problem, not a nuisance. Three moves exist: hold the snow with guards and design the roof for it, catch it with a canopy designed for a drop rather than for rain, or relocate the thing underneath.
The Low Spot, Once It Melts
Meltwater collects where snow already has. Low-slope roofs in milder snow regions carry a rain-on-snow surcharge for exactly this reason: a snow pack holds water like a sponge, and the drains that water should reach are the first things to freeze. A blocked drain in February turns a drift into a reservoir, and water weighs the same whether it was planned for or not.
Ponding instability is the failure mode in which deflection feeds itself — the roof sags, the sag collects more water, the extra water deepens the sag. Shallow roofs, roughly those under a quarter inch per foot, require an explicit ponding check rather than a drainage assumption. Camber helps but substitutes for nothing: truss camber offsets dead-load deflection, and treating it as reserve capacity for snow and water misreads what it was calculated for.
Secondary drainage has to work with the primary blocked, and it should spill somewhere visible so a blocked primary announces itself. Winter clearing needs a plan too, because uneven removal creates an unbalanced case nobody designed for; stripping one slope and leaving the other can be worse than leaving the whole roof loaded.
The Drift You Have Not Built Yet
The drift that lands on an existing roof is often created by a building that has not been designed yet. A new taller wing beside an old low-roofed structure imposes a leeward drift the old framing never saw, and the permit covers the new work while the overload lands next door. Any addition, raised roof surface, new parapet or screen wall, array or dormer redraws the catchment map of everything around it.
Check the existing structure whenever geometry changes near it, including on the neighbouring property where the setback is small. Original truss drawings, if anyone kept them, give the design loads and let you judge the margin quickly; without them the roof needs measuring and rating from scratch. Retrofits at a drift zone are workable — sistered members, added posts, a beam under the drift band — but they are straightforward while the framing is open and disruptive once the finishes are in.
Leave the load basis behind in the building. Ground snow load, exposure and thermal factors, and a marked-up plan showing each drift zone belong in the operations file and, laminated, on a wall in the mechanical room. The next person to bolt something to that roof will either find that sheet or guess, and the guess is what the next collapse investigation writes up.
Before the Framing Package Goes Out
Five items that decide whether the drift cases get framed or discovered.
- Site ground snow load, confirmed by the authority having jurisdiction — Mapped value, local amendment or case-study determination — in writing, before material is ordered.
- Marked-up roof plan showing every height change, parapet, valley and obstruction — Include adjacent buildings within the upwind fetch, on this lot and the next.
- Upwind fetch length measured at each step and parapet — Scaled off the plan to the far edge of the feeding surface, not paced or eyeballed.
- Load sheet issued to the truss or joist supplier — Balanced, unbalanced, drift and sliding cases listed separately with the locations they apply to.
- Obstruction and discharge schedule — Curbs, arrays, chimneys and snow guards on top; doors, meters and parking underneath.
- Drainage check for the blocked-primary condition — Secondary path sized and spilling somewhere visible; ponding check on anything under a quarter inch per foot.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.
Drawn from
- ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures
- International Building Code, structural design loads provisions
- National Building Code of Canada, snow and rain load provisions
- EN 1991-1-3, Eurocode 1: Actions on Structures — Part 1-3: General Actions — Snow Loads
- ANSI/TPI 1, National Design Standard for Metal Plate Connected Wood Truss Construction
- ANSI/AWC NDS, National Design Specification for Wood Construction
Guidance, not a specification. Local codes, the engineer of record and the product manufacturer’s instructions govern where they differ from anything written here.