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Garage & Flat Roof Material Calculator + Load Check

Calculate roofing materials (EPDM, Felt, Shingles) for garages. Includes a structural check for rafter spans under snow and storage loads.

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Category Roofing
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About

Garage roofs are often built with shallower pitches than main houses, making them susceptible to snow accumulation and water pooling. This tool assists in estimating the materials required for Flat (EPDM/Felt) and Low-Pitch (Shingle/Sheet) roofs. Unlike basic calculators, this tool integrates a structural "Sanity Check" based on standard lumber spans.

The load calculation includes a toggle for "Overhead Storage," which is critical for garages. Storing tires or boxes in the rafters adds a Live Load that, when combined with a heavy snow load, can exceed the capacity of standard 2x6 or 2x8 timber rafters. This tool flags potential structural deficits before you buy materials.

garage flat roof epdm rafter span snow load

Formulas

The Total Load Ltot is the sum of Dead Load (materials), Live Load (maintenance/storage), and Environmental Load (Snow).

Ltot = Ldead + Lsnow + Lstorage

Material Area Amat includes a 10% waste margin factor k (1.1):

Amat = (W × L) × k

Reference Data

Rafter Size (Inches)Max Span (No Snow)Max Span (Heavy Snow)Max Span (+Storage)
2x42.9 m2.3 m1.8 m
2x64.4 m3.6 m3.0 m
2x85.8 m4.7 m3.9 m
2x107.3 m6.0 m5.1 m

Frequently Asked Questions

Felt (bitumen) is the traditional low-cost option but typically lasts only 10-15 years and becomes brittle in cold. EPDM is a synthetic rubber sheet that is installed in one piece (no joins), is flexible, and can last 40-50 years, making it the superior choice for modern flat garage roofs.
Only if the rafters were sized for it. Standard roof rafters are designed to hold the roof up, not to hold heavy boxes. Adding storage adds approx 10-20 lbs/sqft (pounds per square foot) of load. This tool helps estimate if your lumber size is sufficient.
A 'flat' roof should never be perfectly flat. It requires a minimum fall of 1:80 (approx 1.25 degrees) to allow water to drain. Ideally, aim for 1:40 to prevent pooling water, which can degrade materials and encourage moss.