Gambrel Roof Snow Load Calculator
A gambrel roof does not carry snow the way a gable roof does. Enter your ground snow load and both pitch angles below to see roughly how the load splits between the steep lower slope and the shallow upper slope — and where the knuckle needs a closer look.
Where the load lands
Lower slope Upper slope
- Lower slope factor (Cs, planning estimate)
- 0.25 at 60.0°
- Upper slope design load (planning estimate)
- 28.0 psf Cs 1.00 at 25.0° · this is the slope to design for
- Lower slope design load (planning estimate)
- 7.0 psf Sheds most of it once past roughly 60°
Drift check required: ASCE 7 requires a separate drift/surcharge check at the knuckle, where the lower slope meets the upper slope. This calculator flags that the check is needed — it does not compute the drift surcharge itself. Take that calculation to a stamped design.
Why Gambrel Snow Loading Runs Backwards From a Gable
On a gable roof, one slope runs the whole way from wall plate to ridge, so a snow load calculation treats the roof as a single surface and the load spreads evenly along it. A gambrel roof has no single surface — it has two, meeting at the knuckle — and those two surfaces do not behave the same way in snow at all.
The lower slope is steep by design, often 55° to 70°, because that is what buys the tall, square attic gambrels are built for in the first place. Snow does not accumulate well on a surface that steep; most of it slides off under its own weight well before it reaches any meaningful depth. The upper slope is the opposite case: shallow, usually 15° to 30°, sitting squarely in the range where snow holds and builds up exactly like it would on a low-pitched gable. Nearly the entire roof's snow load ends up resting on that one, smaller upper section.
The practical effect: the upper rafters and the knuckle connection are usually the members a snow load ends up sizing, while the lower rafters are more often sized by span and wind. That is the reverse of what most people assume walking in, and it is worth checking explicitly rather than guessing. See the gambrel framing walkthrough for how the upper rafters, purlin and collar tie carry that load down through the knuckle.
How the Simplified Slope Factor Works
ASCE 7 reduces a flat-roof snow load for pitch using a roof slope factor, Cs, read off Figure 7.4-1. The real figure is a curve, not a straight line, and it shifts depending on the roof's thermal factor and how "slippery" the surface is considered. This calculator does not reproduce that table. Instead it uses one deliberately simple straight-line stand-in for the general shape of the warm-roof curve, so you can see the effect of pitch on load instantly, without pretending to be a design tool.
Simplified Cs — planning approximation only
Cs = 1.0 for slope ≤ 30°
Cs = 1 − (slope − 30) / 40 for 30° < slope < 70°
Cs = 0 for slope ≥ 70°
This is a straight line drawn through the general shape of ASCE 7's warm-roof (Ct = 1.0) curve — full load up to roughly 30°, falling to nearly nothing by roughly 70°. It is not the tabulated Cs value. For a stamped design, read Cs from ASCE 7 Figure 7.4-1 or your local code's own table.
- Pg
- Ground snow load for your site, in psf, from your local code or hazard map.
- Ex.
- Simplified stand-in for Ce × Ct × Is. Defaults to 0.7; override it if you already know your site's actual exposure, thermal and importance factors.
- Cs
- The simplified slope factor above, calculated separately for the lower and upper pitch.
Planning design load per slope
Design load = Pg × Ex × Cs
Applied once with the upper slope's Cs, and once with the lower slope's Cs — the two numbers are usually far apart.
- Upper slope Cs. 20° is under 30°, so Cs =
1.0. - Upper slope design load. 50 × 0.7 × 1.0 =
35 psf. - Lower slope Cs. 65° falls in the middle band: 1 − (65−30)/40 = 1 − 0.875 =
0.125. - Lower slope design load. 50 × 0.7 × 0.125 ≈
4.4 psf— small enough that most of the practical design attention goes to the upper slope and the knuckle.
Simplified Slope Factor by Angle
This table applies the same straight-line approximation across the full range of angles a gambrel calculator deals with, so you can see where your own lower and upper pitch land without touching the calculator above. Remember it is a planning approximation, not a substitute for the code table.
| Slope | Simplified Cs | Typical on a gambrel |
|---|---|---|
| 10° | 1.00 | Shallow end of a upper slope |
| 20° | 1.00 | Common upper slope |
| 30° | 1.00 | Steep end of a upper slope |
| 40° | 0.75 | Uncommon on either slope |
| 50° | 0.50 | Uncommon on either slope |
| 60° | 0.25 | Common lower slope |
| 70° | 0.00 | Steep lower slope |
| 80° | 0.00 | Very steep lower slope |
Read across your own numbers: most gambrel upper slopes land in the 15–30° band, where this approximation stays at 1.0, while most lower slopes land at 55–70°, where it has already fallen most or all of the way to zero. That gap is the entire reason this page exists as a separate calculator from a plain single-slope snow load tool.
Checking the Knuckle for Drift
A slope factor tells you what settles on each roof surface in a balanced snowfall. It does not tell you what happens when wind or sliding snow moves load around and piles it up against the break between the two slopes. ASCE 7 treats that separately, as a drift and surcharge condition, and requires it be checked at any slope change — which on a gambrel means the knuckle, running the full length of the building. This calculator flags that the check is needed. It does not calculate the drift shape, because that depends on roof length, upwind fetch and site wind data that belong in a real design, not a planning estimate.
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Identify the slope break
The knuckle is the drift-check location on a gambrel, the same way a change in roof height is on any other structure. Mark it on your section drawing before anything else.
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Pull the drift and surcharge provisions
ASCE 7's snow drift provisions define a surcharge load added on top of the balanced case at slope breaks. Confirm which edition your jurisdiction has adopted before using its tables.
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Size the connection for the added load
The purlin and collar tie at the knuckle already carry the upper slope's balanced load. A drift surcharge adds to that, right at the joint with the least redundancy in the whole roof.
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Take the numbers to a stamped calculation
Ground snow load, exposure, thermal factor and drift geometry are all site-specific. A local engineer turns this flagged check into an actual number and a connection detail sized for it.
Using This Number in a Real Design
Treat everything above as a way to understand the shape of the problem before you talk to a designer or submit for a permit — not as the number that goes on a drawing.
This is a planning tool
It shows why the upper slope matters more than the lower one and roughly how big the gap gets. It is not a stamped calculation and does not replace one.
Get a stamped calculation
Ground snow load, exposure category, thermal factor and drift geometry all need to come from your local code and, in most jurisdictions, a licensed engineer's signature.
Truss design and framing
Once you have real numbers, they drive member sizing in the gambrel truss design guide and the connection details in the framing walkthrough.
Wind is the other load case a gambrel handles unusually — see the gambrel wind load reference for how the steep lower slope and the knuckle behave in wind, and back to the main gambrel roof calculator for rafter lengths, angles and area once your loads are settled.
Snow Load FAQ
Common questions about how gambrel roofs carry snow, and about what this calculator does and does not do.
Why does the upper slope carry more snow than the lower slope on a gambrel roof?
Because the two slopes behave completely differently in snow. The lower slope on a gambrel is steep — typically 55° to 70° — and snow simply cannot hold onto anything that steep for long; most of it slides or sheds off before it can build up. The upper slope is shallow, usually 15° to 30°, which is exactly the range where snow sits and stays. So almost the entire roof snow load ends up concentrated on the upper section, which is the opposite of a single-pitch gable roof, where the one slope carries the load evenly along its whole length.
What is a roof slope factor (Cs) and why does this calculator simplify it?
Cs is the multiplier ASCE 7 applies to a flat-roof snow load to account for snow sliding off a sloped surface — the steeper the roof, the lower Cs gets. The real relationship is read off ASCE 7 Figure 7.4-1, and it depends on the thermal factor and how slippery the roof surface is, not just the angle. This calculator uses a single straight-line stand-in for the general shape of that curve so you can see the effect instantly. It is a planning approximation, not the tabulated code value, and the page says so at every step.
Does ASCE 7 require anything extra at the knuckle?
Yes. Wherever a roof changes slope, ASCE 7 requires a check for drift and surcharge loading — snow that blows or slides from the steep lower section and piles up against the break instead of passing over it. On a gambrel, that break is the knuckle, running the full length of the building. This calculator flags that the check is required; it does not compute the drift shape or magnitude, because that calculation depends on roof length and site-specific wind and snow data that belongs in a stamped design, not a planning tool.
Is this calculator a substitute for a structural engineer's snow load calculation?
No, and it is not trying to be. It is built to show the shape of the problem — why the upper slope matters more than the lower one, and roughly how big the numbers get — so you walk into a conversation with a designer or building department already understanding the geometry. Every ground snow load, exposure condition and drift case still needs to come from your local code and a stamped calculation before anything gets built.
Should I use the ground snow load or the flat-roof snow load in the first field?
Enter your ground snow load, Pg — the number your local building code or an ASCE 7 hazard tool reports for your site, in pounds per square foot. The calculator applies the exposure/importance factor and the slope factor for you to get the planning estimate for each slope. If you already know your project's flat-roof snow load, set the exposure/importance field to 1.0 so the calculator does not apply that reduction twice.