Built up roofing (BUR) is a long-standing method for creating durable, weatherproof layers on flat and low-slope roofs. Understanding the weight per square foot helps designers and builders ensure structural adequacy, evaluate loading on supports, and plan for insulation, drainage, and ballast. This article breaks down typical weights, influencing factors, and practical methods to estimate BUR load for American buildings.
What Affects Built Up Roofing Weight Per Square Foot
The weight of BUR depends on several variables, including the number of plies, the type of asphalt or adhesive used, reinforcement material, and whether ballast is present. Additional components such as granule surfacing, insulation, and flashing accessories contribute to the overall dead load. In practice, two common BUR configurations exist: unballasted (or lightly ballasted) systems and heavily ballasted systems with gravel or stone. The structural support must be designed to carry the resulting load with a safety margin for seasonal variations and maintenance activities.
Typical Weight Ranges By BUR Configuration
Weight per square foot (psf) is a practical unit for designing roof structures. The following ranges reflect common installations in the United States. Remember that 1 square equals 100 square feet, so multiplying by 100 converts psf to pounds per “square.”
- Three-ply BUR without ballast: roughly 3.0 to 6.0 psf
- Three-ply BUR with light ballast (e.g., single-layer gravel or mineral stabilizers): about 6.0 to 9.0 psf
- Three-ply BUR with heavy ballast (granular ballast or substantial gravel): approximately 10.0 to 20.0 psf
- BUR with insulation between plies (common in energy- retrofits): adds 0.5 to 2.5 psf per inch of insulation, depending on material
Note: These ranges are approximate. Exact weights depend on material grades, application methods, and local practice. For structural calculations, use manufacturer data sheets and project-specific drawings.
How To Calculate BUR Load For A Roof Deck
Accurate BUR load calculations require summing all dead loads and any applicable live loads or temporary loads during construction. A straightforward approach is:
- Determine base ply count and ballast type from the design or spec.
- Assign a psf value to each component (ply, membrane, ballast, insulation, and any additional layers).
- Sum all psf values to obtain total BUR dead load.
- Compare totals against the roof deck’s allowable dead load and the structure’s Design Load Capacity, accounting for safety factors and local building codes.
Example calculation: A three-ply BUR with medium ballast and 1.5 inches of insulation might have a base between 5.0 and 9.0 psf for the BUR layers, plus insulation at about 0.6 to 0.9 psf per inch, totaling roughly 7.8 to 11.7 psf. Multiply by 100 to get pounds per square: 780 to 1,170 pounds per square, then adjust for any additional features.
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Structural Considerations For BUR Weight
Structural engineers assess the roof system by considering total dead load, live load, wind uplift, and seismic forces. Heavier ballast BURs demand stronger supports and, in some cases, additional reinforcement such as upgraded joists or deck substitutions. Buildings with limited loading capacity, like older structures or retrofit projects, may opt for lighter, unballasted BUR designs or alternative roofing systems with reduced dead load. Proper drainage and insulation thickness also influence the total load and long-term performance.
- Deck compatibility: Ensure the substrate can support BUR weight throughout its service life.
- Thermal considerations: Insulation adds weight but improves energy efficiency and moisture management.
- Maintenance access: Ballast surfaces require periodic inspection and, if gravel is used, potential re-surfacing to maintain ballast uniformity.
Material Components And Their Impact On Weight
BUR typically uses multiple layers of reinforcing felt, asphalt, and aggregate or granules. The key weight drivers are:
- Number of plies: More plies increase capillary resistance as well as weight.
- Asphalt type and thickness: Heavier grades and thicker applications add psf load.
- Reinforcement: Fiberglass or organic felts have different densities and weight contributions.
- Ballast: Gravel or stone ballast can dramatically raise psf, often representing the largest portion of total load.
- Insulation: Between 0.5 to 2.5 psf per inch of insulation is typical, influencing overall weight.
Tables For Quick Reference
| BUR Configuration | Typical Weight (psf) | Notes |
|---|---|---|
| Three-ply BUR (unballasted) | 3.0–6.0 | Base weight; ballast not included |
| Three-ply BUR (light ballast) | 6.0–9.0 | Includes light ballast or stabilizers |
| Three-ply BUR (heavy ballast) | 10.0–20.0 | Ballast is gravel or similar material |
| BUR with insulation | +0.5 to +2.5 psf per inch of insulation | Adds to base ply and ballast weights |
Practical Tips For Builders And Property Owners
To manage BUR weight effectively, consider these practical steps:
- Work with a structural engineer to confirm allowable dead loads and safe design margins for the specific building.
- Choose ballast strategies aligned with climate, maintenance expectations, and roof life cycle goals.
- Optimize insulation thickness to balance thermal performance with weight considerations.
- Prepare accurate as-built drawings detailing all BUR components for future renovations or dry-out scenarios.
- In retrofit projects, evaluate lighter roofing options if structural upgrades are impractical.
Frequently Asked Questions
- What is the typical weight of a BUR roof per square foot? Typical BUR weights range from about 3 psf for unballasted three-ply systems to 20 psf or more for heavily ballasted configurations, with insulation and other layers adding to the total.
- Why does ballast increase weight so much? Ballast provides wind resistance and stability, but it adds substantial dead load compared to the bare membrane.
- How can I reduce BUR weight without compromising performance? Consider lighter ballast options, reduce insulation thickness where feasible, or select a low-weight BUR variation recommended by the manufacturer and structural engineer.
Understanding built up roofing weight per square foot is essential for safe, compliant, and durable roof design. By evaluating ply count, ballast, and insulation together, designers can ensure structures meet code requirements while delivering long-term performance and resilience.