Snow and Wind Load on Roof Joists: A Complete Guide

Snow wind load on a roof joist is one of the most important factors in roof design, and ignoring it can cause sagging, cracking, or even collapse. If you’ve ever wondered how much snow your roof can really hold, or how wind might push against the structure, this guide breaks it all down for you. 

Using formulas, examples, and building codes like ASCE 7-05, we’ll explain how to calculate snow load and wind load, how they affect roof joists, and what you need to check before finalizing your roof design. 

Whether you want to use a roof joist calculator, learn the formula in kn/m², or see an example calculation for pitched roofs, everything you need is right here.

Key Takeaways

• Snow load is the weight of snow and ice pressing on the roof.
• Wind load is the pressure or suction from moving air.
• Roof joists must be sized and spaced to carry combined loads safely.
• ASCE 7-05/16 provides standard formulas for calculating loads.
• Factors like slope, exposure, thermal condition, and building importance affect design.
• Calculators simplify estimating roof joist requirements.
• Ignoring load risks sagging, cracking, or roof collapse.

Why Snow and Wind Loads Matter in Roof Joist Design

When I first started looking into this, I thought a roof was just about shingles and beams. But the truth is, snow and wind loads are the hidden forces that make or break the structure. A roof joist isn’t just holding up roofing sheets—it’s carrying:

• Dead load (the permanent weight of the roof materials).
• Snow load (how much snow/ice piles up).
• Wind load (forces pushing or lifting from storms).
• Live load (temporary loads like workers or equipment).

The combined forces are what engineers call the design load. That’s the number that decides whether your joist spacing and size are safe—or at risk of failure.

Quick Definitions

• Snow Load (kn/m² or psf): The weight of snow and ice pressing down on the roof.
• Wind Load: The pressure from wind hitting, lifting, or sucking at the roof.
• Roof Joist: The horizontal framing member that supports the roof.
• Dead Load: The fixed, permanent weight of the roof structure itself.

Snow Load vs Roof Load

Snow load is only one part of the total roof load. Roof load combines:

• Dead load: Permanent roof weight (shingles, rafters, insulation), typically 10–20 psf.
• Live load: Temporary weight (maintenance workers), about 20 psf.
• Snow load: Varies by location.
• Wind load: Depends on gusts and exposure.

Table – Common Roof Loads

A total roof load is the sum of all these. Ignoring one is asking for trouble.

How Snow and Wind Affect Roof Joists

Roof joists carry loads to the walls and foundation. Too small, too far apart, or wrong material, and they bend or break.

Pain Point: Sagging ceilings are a common sign of overloaded joists. I once visited a house mid-winter with a sagging living room ceiling. Snow hadn’t melted yet—the joists were at their limit.

Solution: Use proper calculations and joist tables. Tools like Roof Joist Calculator or Ceiling Joist Calculator make it easy.

Common Joist Sizes vs Span (40 psf live + 10 psf dead)

Snow load quickly reduces these spans. High snowfall? Go bigger.

Factors Used in Snow Load Calculations

Snow load is not the same everywhere. A house in Alaska faces a totally different design requirement than one in Texas. That’s why engineers follow formulas set by codes like ASCE 7-05.

Here’s what you need to calculate roof snow load (Pf):

1. Ground Snow Load (Pg):

   • Comes from building codes or maps.
   • Depends on the location.
   • Example: 30 psf ground load.

2. Exposure Factor (Ce):

   • Accounts for wind exposure and terrain.
   • Open areas = more wind = less snow accumulation.

3. Thermal Factor (Ct):

   • Heated roof = less snow stays on.
   • Unheated roof = higher load.

4. Importance Factor (I):

   • Hospitals, fire stations = higher safety margins.

5. Slope Factor (Cs):

   • Flat roofs hold more snow.
   • Steeper roofs shed snow faster.

Formula for Flat Roof Snow Load (ASCE 7-05)

Pf=0.7×Ce×Ct×I×Pg

• Pf = Flat Roof Snow Load (psf)
• Ce = Exposure Factor
• Ct = Thermal Factor
• I = Importance Factor
• Pg = Ground Snow Load (psf)

Example: Snow Load Calculation

Let’s do a quick one.

• Pg = 30 psf
• Ce = 1.0 (partially sheltered)
• Ct = 1.0 (unheated roof)
• I = 1.1 (school building = important)

Pf=0.7×1.0×1.0×1.1×30

So the roof must be designed to safely hold at least 23.1 psf of snow.

Wind Load on Roofs

Snow isn’t the only thing pushing down. Wind can actually lift up parts of the roof. Engineers calculate wind load by looking at:

• Basic wind speed (V): Based on region (mph).
• Exposure category: Open terrain vs urban.
• Pressure coefficients (Cp): Depend on roof angle and shape.
• Height factor: Taller buildings catch more wind.

Wind Load Formula (Simplified ASCE 7-05)

q=0.00256×Kz×Kzt×Kd×V2×I

• q = Wind pressure (psf)
• Kz = Exposure factor by height
• Kzt = Topographic factor
• Kd = Directionality factor
• V = Wind speed (mph)
• I = Importance factor

How Snow and Wind Loads Affect Roof Joists

When I first looked at my own attic, I realized that the horizontal beams—roof joists—aren’t just holding up boards; they’re taking on the combined weight of snow, wind, and the roof itself. If these forces aren’t accounted for, the results can be ugly: sagging joists, cracked ceilings, or even a collapsed roof.

What Engineers Consider:

• Total Design Load: Dead load + Snow load + Wind load + Live load.
• Material Strength: Wood, steel, or engineered lumber affects load capacity.
• Span and Spacing: Longer spans or wider spacing need stronger or larger joists.
• Roof Pitch: Steeper roofs shed snow faster, reducing load.

Fun fact: Even a few inches of snow can weigh hundreds of pounds across your roof. That’s why calculating the exact load in kn/m² or psf is critical.

Quick Table: Snow Load vs Roof Pitch

Example: If your flat roof’s snow load is 30 psf, and your roof pitch is 35°, multiply 30 × 0.7 = 21 psf.

How Wind Loads Interact with Joists

Wind doesn’t just push down—it can lift the roof. Joists must resist both compression from snow and uplift from wind. Areas like roof edges and corners experience the highest pressure, which is why edge joists are often stronger or reinforced.

• Positive pressure = pushing down
• Negative pressure = pulling up

💡 Tip: If you’re curious about exact numbers, tools like the Roof Joist Calculator or Ceiling Joist Calculator let you input span, spacing, roof type, snow, and wind load to get safe joist sizes.

Step-by-Step Snow Load Calculations for Roof Joists

Calculating snow load might sound scary at first, but if you break it down step by step, it’s manageable—even for a DIY-minded person. Here’s how I usually approach it:

1️⃣ Find the Ground Snow Load (Pg)

• What it is: The baseline snow weight for your area. Usually provided in psf (pounds per square foot) or kN/m² by local building codes.
• Example: Your area has Pg = 25 psf.

Tip: You can check your region’s snow load map or ASCE 7 standards.

2️⃣ Apply Roof Slope Factor (Cs)

• Formula: Pf=0.7×Cs×Ct×I×Pg

Where:

• Pf = roof snow load

• Cs = roof slope factor (from table above)

• Ct = thermal factor (1.0 for unheated roofs)

• I = importance factor (1.0 for normal buildings)

• Example: Roof pitch = 30° → Cs = 0.8

• Unheated roof → Ct = 1.0

• Normal building → I = 1.0 

Pf=0.7×0.8×1.0×1.0×25=14psf

3️⃣ Consider Exposure Factor (Ce)

• Ce accounts for the surrounding terrain. Sheltered roofs = lower Ce. Open, exposed roofs = higher Ce. Typical range: 0.9 – 1.2.

• Multiply Pf by Ce.

• Example: Exposed roof → Ce = 1.1 

Pfadjusted​=14×1.1=15.4psf

4️⃣ Convert to kN/m² (if needed)

• 1 psf ≈ 0.04788 kN/m² 

5️⃣ Verify Joist Capacity

Now you know the roof snow load. Check your joist:

• Material type (wood, steel, engineered lumber)
• Span and spacing
• Design load > snow + dead load + live load

If the calculated load exceeds the joist capacity, increase joist size, reduce spacing, or adjust roof pitch.

Quick Reference Table: Snow Load Factors

💡 Pro Tip: Keep an Excel sheet with Pg, Cs, Ct, I, Ce for your area. That way, you can quickly adjust for different roof pitches or locations.

Wind Load Calculations for Roof Joists

Wind load can actually be trickier than snow because the pressure varies across your roof. Here’s a simple way to handle it step by step.

1️⃣ Determine Basic Wind Speed (V)

• What it is: Local code or ASCE 7 gives wind speed in mph or m/s.
• Example: Your area → V = 90 mph

2️⃣ Calculate Velocity Pressure (qz)

• Formula (simplified ASCE 7-05): 

Where:

• Kz = exposure coefficient (depends on terrain & height)

• Kd = directionality factor (usually 0.85)

• I = importance factor (1.0 for a normal building)

• Example: Exposed roof → Kz = 1.0 

qz​=0.00256×1×0.85×902×1=17.6psf

Tip: Kz tables are in ASCE 7-05; choose based on surrounding terrain.

3️⃣ Apply Roof Pressure Coefficients (GCp)

• Roof shape changes wind pressure. Edges get higher forces than the middle.
• Example: Simple gable → GCp = 0.9 (average) 

4️⃣ Convert to kN/m² (optional)

• 1 psf ≈ 0.04788 kN/m² 

Integrating Snow & Wind Loads

For roof joist design, both snow load and wind load matter. Here’s a simple guideline:

1. Snow-dominant areas: Use snow load + dead load as the main design load.
2. Wind-dominant areas: Check uplift pressures; use wind load + dead load.
3. Critical buildings: Always consider both simultaneously with safety factors.

Combined Design Load Table (Example)

Note: Dead load usually includes roof material weight and minor live loads.

✅ Practical Tips

• If your joists sag or feel weak under these loads, increase spacing or beam size.
• Always check local building codes; snow/wind factors vary by region.
• Tools like Roof Joist Calculator or Ceiling Joist Calculator help quickly verify your design.
• Consider pitched roofs: steeper roofs reduce snow accumulation but may increase wind uplift.

FAQs – Quick Answers

• How to calculate snow load for a roof? Use ASCE formula: (P_f = 0.7 \times Ce \times Ct \times I \times Pg).
• How much snow load can a roof hold? Depends on joist size, spacing, and slope. Typically 20–70 psf.
• Minimum snow load? Usually 20 psf, even in light snow regions.
• Roof pitch for snow? 30° or more.
• Difference between snow load and roof load? Roof load includes snow, dead, live, and wind.
• Overhang snow load factor? Edges collect more snow; factor = 1.2–1.3.

Safety Tips

1. Check local building codes before construction.
2. Don’t skimp on joist size. Replacing hidden joists is tough.
3. Consider overhangs—snow drifts increase edge load.
4. Use calculators or tables for preliminary design.
5. Hire a structural engineer if loads are extreme or irregular.

Conclusion

Snow and wind load are not just numbers—they determine whether your roof stands or sags. Proper calculation, factoring in ground snow, exposure, thermal properties, importance, and wind, ensures joists are strong enough. Tools like calculators make design easier, but understanding the basics prevents costly mistakes.

Remember: Roof pitch, joist size, and spacing matter just as much as the snow and wind numbers. A smart design today keeps your roof intact tomorrow.