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.
The site's ground snow load, from your local building code's ground snow load map.
Accounts for wind exposure and terrain, which affects how much snow accumulates on the roof versus blows off.
Accounts for heat loss through the roof that can melt accumulated snow.
Based on the building's risk category, per ASCE 7.
Flat-roof design snow load
21.93 psf
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Ground-to-Roof Snow Load Calculator: 21.93 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 21.93 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The page returns pf from the equation alone, and ASCE 7 also sets a minimum roof snow load pm that governs instead on low-slope roofs: where the ground snow load is 20 psf (about 0.96 kPa) or less, pm is Is times pg, and above that it is 20 psf times Is. No floor is applied here, so the page's default pg of 1.5 kPa with an exposure factor of 0.9 returns 0.945 kPa, which is 19.7 psf and already below that 20 psf minimum.
- This is the balanced load spread evenly over the whole roof. Drift against a parapet, a roof step or a rooftop unit is a separate ASCE 7 calculation that puts a triangular surcharge over a limited width and can reach several times pf, so a rafter or beam sized on this figure can still be badly under-designed where the snow actually piles up. Sliding snow off an upper roof, and the rain-on-snow surcharge of about 5 psf (0.24 kPa) that applies to low-slope roofs where the ground snow load is 20 psf or less, are added on top of pf rather than contained in it.
- pf is a pressure on the horizontal projection of the roof, not on the sloped surface. Applying it to the true roof area counts the pitch twice: at 6:12 the sloped surface is 11.8 per cent larger than the plan area, so the snow total comes out that much high. Dead loads run the other way and do act on the sloped area, which is why the two are never taken over the same area.
- Ce, Ct and Is are multiplied exactly as typed and are never checked against the tables they come from. Entering Ce as 0.7 rather than 1.0 removes 30 per cent of the load on the strength of a wind exposure that has to hold for the life of the building, and conifers grow, a taller neighbour goes up, a screen wall gets added. The field also accepts Ce up to 1.3, beyond the 1.2 top of the ASCE 7 exposure table that its own help text describes, without objecting.
- The form used here, with a separate importance factor, is the pre-2022 one. ASCE 7-22 rebuilt the snow chapter around reliability-targeted ground snow loads mapped for each risk category and dropped Is from the flat-roof equation, so a pg read off those newer maps and then multiplied by Is of 1.2 applies the risk category twice and overstates pf by 20 per cent. Which edition your jurisdiction has adopted decides which of the two pg values belongs in the box.
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.
The combined dead and live load carried by the roof, per unit of horizontal roof area.
The full width of the building, measured eave to eave across both roof slopes.
Ridge beam load per linear meter
543 lbf/ft
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.
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Ridge Beam Tributary Load Calculator: 543 lbf/ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 543 lbf/ft — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Produces a load, not a beam. The section comes from applying that load over the clear span between supports and checking it three ways — bending, shear at the bearings, and a deflection limit tight enough that the ridge does not sag visibly over a cathedral ceiling — and doubling the span roughly quadruples the bending demand at the same load per unit length.
- Stops at the underside of the beam. All the load on it lands on the posts at its ends, and those posts have to run continuously down through the floors to a footing sized for that point load. A ridge beam bearing on a wall top plate with ordinary studs and no post beneath it is the common failure, and it shows as a dip in the floor below long before the roof complains.
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.
Guards trade a discharge problem for a retention problem, so settle how many go along the eave while the array layout is still moving — the count is a structural commitment, not an accessory line.
The horizontal width of the roof eave where snow guards are installed.
The center-to-center spacing between snow guards within a single row.
How many staggered rows of snow guards run parallel to the eave.
Total snow guards needed
42 snow guards
- Guards per row
- 21
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Metal Roof Snow Guard Spacing Calculator: 42 snow guards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- On a standing seam roof you cannot put a guard where the arithmetic wants one. Clamp-on guards land on the seams, so real spacing snaps to a whole number of panel widths — with 406 mm (16 in) panels, a 0.6 m spacing becomes either 406 mm or 813 mm, and the count moves with it. Lay the row out on the seam positions, then count.
- Holding snow back does not make it go away, it hands the load to the roof. Every kilo retained at the eave feeds through the panel into its clips and then into the framing as a steady downslope force the structure never saw while the snow was free to slide. Retention systems that work have pulled clips and torn eave flashing off buildings that were never checked for the load they now carry.
- Counts a single eave line, and that is rarely where the danger is. Snow discharges into valleys, onto lower roof planes, off dormers and over entrance doors, and those are the concentrated loads and the injuries — a row across the main eave does nothing for the slope that dumps over the front door.
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 band of ice loading those rafter tails is the same band pushing water back under the covering, and its extent is measured from the eave edge to a line inside the heated wall — a dimension worth fixing on the drawing rather than at the truck.
A strip along the eaves against ice dams, or a lining down every valley.
The combined length of all eaves requiring ice barrier protection.
The horizontal distance the roof overhangs beyond the exterior wall line.
How far the membrane must extend past the interior face of the exterior wall line, per IRC R905.1.2.
The roof's slope, expressed as rise per 12 units of run — used to convert horizontal coverage distance to along-slope distance.
Ice dam membrane area needed
246 sq ft
- Membrane width (along slope)
- 3.73 ft
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Roof Eave Ice Dam Protection Membrane Footprint (and Valley Lining): 246 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The two horizontal distances above are added at "the wall line", and a wall has thickness. R905.1.2 measures its 24in from the INSIDE face of the exterior wall, while an overhang is normally dimensioned from the OUTSIDE face of the framing or sheathing — so the 150 to 300 mm between those two faces belongs in the membrane width and is in neither box. On a thick wall, or one with exterior insulation, the membrane stops short of the point the code actually names.
- Gives a flat area, and the membrane is bought in rolls of fixed width — commonly 900 mm — that overlap at every side and end lap. The default inputs need a 1.13 m width, which is wider than one roll, so it goes on as two courses with a lap between them, and the sheet area to order runs above this figure by the lap on every seam plus the cut-off at each end of a run.
- Eaves only. The same membrane is normally run up every valley, behind sidewalls and dormer cheeks, around chimneys and skylights, and across any section shallow enough to need it — none of that is in this number, and on a cut-up roof it is often more area than the eaves themselves.
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.
Ponding starts as an ordinary sag nobody objected to, so put the span against the limit ratio it was designed to and see how much movement is permitted before water begins contributing its own.
The member's clear span between supports.
The code deflection limit that applies to this member, expressed as a fraction of the span.
Allowable deflection
0.65 in
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Roof/Floor Deflection Limit Calculator: 0.65 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.65 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The number returned is the allowance, not the deflection. How far the member actually moves depends on the load, the modulus of elasticity and the second moment of area, none of which this page asks for, so a 6 m span at L/360 gives 16.7 mm of permitted sag with no indication of whether the joist under it moves 8 mm or 40 mm.
- Codes normally impose two limits on the same member at once, one on live load alone and a looser one on dead plus live, and this page returns a single ratio at a time, so an L/360 allowance is the right yardstick only for the live-load half of that pair. The three options offered are the common generic set; brittle finishes, supported masonry and glazing attract tighter fractions in many codes. Some codes also pair the ratio with an absolute cap in millimetres that a long span reaches before the fraction does, and this page only ever divides.
- Span is used exactly as entered, which is the wrong input for a cantilever. Several codes define L for a cantilevered member as twice the projection, so a 1.2 m (4 ft) balcony joist entered as 1.2 m (4 ft) returns roughly half the allowance intended; how L is defined at a cantilever, and whether it is measured clear or centre to centre of bearings, is jurisdictional.
- The allowance has no time in it. Timber and concrete keep deflecting under sustained load, and codes deal with creep by multiplying the long-term component of the calculated deflection before it is compared with a limit like this one, commonly by a factor of the order of 1.5 for dry timber and 2.0 for green material, with the exact figure set by the governing code. A member that clears the limit on the day it is installed can exceed it a year later under the same dead load.
- Clearing a deflection limit is a serviceability result, not a structural pass: bending stress, shear near the supports, bearing crushing on the wall plate and buckling of an unrestrained compression edge are separate checks a member can fail while sitting comfortably inside its L/240 allowance. On a low-slope roof there is also ponding, where water collects in the deflected shape and the added weight deepens it further, which is a stiffness and drainage question no span fraction answers.
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.
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