Roof Load Calculator

Calculate your roof's total load including dead weight, live loads, snow, and wind to check against structural capacity

Calculate combined dead + live + environmental loads in PSF

Quick presets

Trusses/Rafters Needed

1

24" spacing • 0.0 ft rafter length

PRO

Professional Calculator

Load calculations, truss spacing, and code compliance

ft
ft

Estimated Materials

53 bundles

Roof Area

1,581 sq ft

Squares

15.8

Detailed Breakdown

Roof Area1,581 sq ft
With Waste1,739 sq ft
Roofing Squares15.8
Bundles53
How to Use This Calculator
The Roof Load Calculator helps you determine whether your roof structure can safely support its current or planned loads, following building code principles used by structural engineers.

Total Load tab: Select your roofing material, sheathing type, and underlayment to compute the dead load. Then enter your snow load (from our Snow Load Calculator or local code), the required code live load (typically 20 psf for residential), and any wind uplift. The calculator sums all downward loads to give you the total combined load in pounds per square foot (PSF) and total weight in pounds across your entire roof area.

Capacity Check tab: Enter the total load from the first tab and your roof framing's rated capacity. For engineered trusses, this value is printed on the truss shop drawing. For stick-framed rafters, it depends on lumber size, species, grade, span, and spacing — the calculator provides typical ranges. It computes the load ratio (demand / capacity) and flags any overstress condition. A ratio under 0.80 is good, 0.80-1.00 is marginal, and above 1.00 means the structure is theoretically overstressed and requires engineering review.

Reroof Weight Check tab: Planning a new roof? This tab tells you if your structure can handle the new material, whether overlaid on the existing roof or after tear-off. Enter your existing material, number of layers, new material, and installation method. The calculator computes the resulting dead load, verifies at least 20 psf of live load reserve remains, and gives a clear pass/fail verdict. This is especially important when switching from lightweight shingles to heavy tile or slate.

The Formula
The roof load calculator uses these formulas:

Total Dead Load (DL) = Roofing Material + Sheathing + Underlayment + Framing (est. 5 psf for wood trusses/rafters) Example: Architectural shingles (3.5) + 7/16" OSB (1.5) + Synthetic (0.2) + Framing (5.0) = 10.2 psf

Total Load (gravity) = Dead Load + MAX(Live Load, Snow Load) Per ASCE 7 load combinations: 1.0D + 1.0L or 1.0D + 1.0S (whichever governs) Example: 10.2 + MAX(20, 0) = 30.2 psf

Load Ratio = Total Load ÷ Rated Capacity Example: 30.2 ÷ 45 = 0.67 → 67% utilized (PASS)

Reroof Check (overlay): New Dead Load = Existing Material × Layers + New Material + Sheathing + Underlayment + Framing Available Live Load = Rated Capacity - New Dead Load Pass if Available Live Load >= Required Live Load (20 psf)

Reroof Check (tear-off): New Dead Load = New Material + Sheathing + Underlayment + Framing Available Live Load = Rated Capacity - New Dead Load

Common Material Weights (2026): - 3-tab shingles: 2.0-2.5 psf - Architectural shingles: 3.0-4.5 psf - Metal standing seam: 1.0-1.5 psf - Clay tile: 8-12 psf - Concrete tile: 9-12 psf - Slate: 8-15 psf
Example Calculation
Example: Converting Shingles to Concrete Tile in Arizona

Maria wants to replace her 20-year-old asphalt shingle roof with concrete tile on her 2,500 sq ft home in Phoenix. Her trusses are rated for 45 psf total load.

Step 1: Current Roof Load
• Existing architectural shingles: 3.5 psf
• 7/16" OSB sheathing: 1.5 psf
• 30 lb felt underlayment: 0.6 psf
• Framing (trusses): 5.0 psf
• Current dead load: 10.6 psf
• Live load requirement: 20 psf
• Current total: 10.6 + 20 = 30.6 psf (68% of 45 psf capacity — OK)

Step 2: New Concrete Tile Load (after tear-off)
• Concrete tile: 11.0 psf
• 5/8" plywood upgrade: 2.0 psf
• 30 lb felt + battens: 1.0 psf
• Framing: 5.0 psf
• New dead load: 19.0 psf
• Available for live load: 45 - 19.0 = 26.0 psf (meets 20 psf minimum — PASS)
• Load ratio: (19.0 + 20) ÷ 45 = 0.87 → 87% utilized (MARGINAL)

Step 3: Engineering Decision
• The roof technically passes at 87% utilization, but with only 13% margin
• Total roof dead weight: 19.0 × 2,500 = 47,500 lbs (23.75 tons)
• A structural engineer may recommend sistering rafters or adding collar ties for long-term safety
• Estimated reinforcement cost: $4,000-$8,000 for the 2,500 sq ft roof

Frequently Asked Questions

What is the difference between dead load and live load on a roof?
Dead load is the permanent weight of the roof itself — shingles, sheathing, underlayment, framing, insulation, and any fixed equipment like HVAC units or solar panels. Dead load is constant and does not change over time (unless you add material). Live load is temporary weight including workers performing maintenance, equipment, rain, and snow accumulation. Building codes require roofs to support a minimum live load of 20 psf for sloped residential roofs. Your roof structure must safely carry the combined dead load plus the worst-case live load simultaneously.
How much does a layer of shingles weigh?
A single layer of standard 3-tab asphalt shingles weighs approximately 2.0-2.5 psf (pounds per square foot), or about 200-250 lbs per roofing square (100 sq ft). Architectural (dimensional) shingles are heavier at 3.0-4.5 psf (300-450 lbs per square). Premium designer shingles can weigh 4.5-5.0 psf. For comparison: concrete tile weighs 9-12 psf, clay tile 8-12 psf, natural slate 8-15 psf, metal standing seam 1.0-1.5 psf, and wood shakes 3.0-4.0 psf. When evaluating whether your roof can handle new material, always add sheathing (1.5-2.3 psf) and underlayment (0.2-0.6 psf) to the roofing material weight.
What building code requirements apply to roof loads?
The International Building Code (IBC) and International Residential Code (IRC) require minimum roof live loads of 20 psf for sloped roofs (steeper than 4/12) and 20 psf for flat roofs, reducible based on tributary area and slope. Snow loads per ASCE 7 govern in regions where the ground snow load exceeds about 15 psf. Wind loads per ASCE 7 must also be calculated — the roof must resist both downward gravity loads and upward wind uplift. For reroofing, most codes prohibit more than two layers of asphalt shingles and require tear-off when switching to a material that weighs more than 10% more than the original.
When do I need to add structural support to my roof?
You need additional structural support when: (1) switching to a material significantly heavier than the original, such as from asphalt shingles (3 psf) to concrete tile (11 psf) — this nearly triples the dead load; (2) adding rooftop equipment like HVAC units, solar panels, or a green roof; (3) your region's snow load has been revised upward in the latest building code edition; (4) the existing structure shows signs of overstress like sagging, cracking, or deflection; or (5) you plan to add a roof deck or occupied space on a flat roof. Always consult a licensed structural engineer before adding load — reinforcement typically costs $3,000-$12,000 depending on the extent of work.
Can my roof handle a second layer of shingles?
Structurally, most residential roofs designed to current codes can handle the additional 2.5-3.5 psf of a second shingle layer — this is within the safety margin. However, the real concerns are: (1) code compliance — many jurisdictions limit you to two total layers and require that the existing roof be in good condition; (2) trapped moisture — a second layer traps heat and moisture, accelerating sheathing deterioration; (3) warranty — most shingle manufacturers void or limit warranties when installed over an existing layer; and (4) hidden damage — overlaying hides rotten sheathing, damaged flashing, and other issues. Tear-off is almost always recommended by roofing professionals for long-term performance.

Related Calculators

Snow Load Calculator

Calculate roof snow load from ground snow load per ASCE 7

Wind Load Calculator

Calculate wind uplift and pressure on your roof

Roof Truss Calculator

Estimate truss count, dimensions, and cost

Rafter Span Calculator

Find maximum rafter span for your lumber size

Roof Weight Calculator

Calculate total weight of your existing roof

Solar Panel Calculator

Estimate solar panel count and roof load impact

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