Built-Up Roofing (BUR) is a time-tested, multi-ply roofing system that combines reinforcing felts, asphalt or coal tar, and optional ballast or insulation. Understanding the weight of a BUR system is essential for accurate structural design, budgeting, and long-term performance. This article explains the components that contribute to weight, how much a BUR system typically weighs, and how to estimate weight for planning and code compliance in American buildings.
What Is Built Up Roofing (BUR)?
Built Up Roofing, commonly referred to as BUR, is a composite roofing assembly built up from multiple layers. Each ply is typically an asphalt- or coal tar-saturated felt that is hot-applied or cold-applied to the roof deck. A BUR system may include:
- A concrete or wood roof deck as the substrate
- Several plies of reinforcing felt saturated with asphalt or coal tar
- A cap sheet or reflective top layer
- Optional ballast such as gravel or stone over the top surface
- Additional components like insulation boards, cover boards, or flashing details
Because BUR is assembled with multiple plies and can add ballast for durability, its total weight varies with the number of plies, the type of asphalt or coal tar used, whether ballast is included, and whether extra insulation or cover boards are installed.
How Much Does Built Up Roofing Weigh?
The weight of BUR depends on configuration. The following ranges reflect typical installations in the United States, but exact weights must come from manufacturer data sheets and structural calculations for a given building.
- BUR without ballast: 3 to 6 pounds per square foot (psf) for a 3–4-ply system with standard asphalt saturants. Heavier felts or additional plies can push this higher, but ballast is not included.
- BUR with gravel or stone ballast: 12 to 20 psf. Ballast adds substantial weight and is a primary driver of total system mass.
- BUR with insulation and cover boards (no ballast): 5 to 9 psf, depending on insulation type and thickness, plus any fasteners or edge details.
- Total typical BUR weight (with ballast and insulation when present): 16 to 30 psf, though some heavy ballast configurations can exceed 30 psf on larger projects or with thick insulation.
| Configuration | Estimated Weight (psf) |
|---|---|
| No ballast, 3–4 plies | 3–6 |
| Ballast on top (gravel/stone) | 12–20 |
| Insulation added (per inch, typical polyiso) | 0.4–1.0 |
| Total BUR with ballast and insulation | 16–30+ |
These ranges illustrate how ballast and insulation often dominate the total weight. Actual numbers depend on the specific products used, including the weight per ply, the asphalt or coal tar formulation, and ballast density and depth.
Key Factors That Influence BUR Weight
- Number of plies: More plies increase base weight. A common BUR design uses 3–4 plies; adding plies or using heavier felts raises the dead load.
- Asphalt or coal tar formulation: Different saturants have different densities. Coal tar-based products can add weight relative to lighter asphalt-based systems.
- Ballast type and thickness: Gravel or stone ballast significantly increases weight, while some reflective caps or single-ply top sheets may reduce weight.
- Insulation and cover boards: Adding rigid insulation (mm to inches) and cover boards contributes additional psf, varying with material thickness and density.
- Deck type and surface conditions: A plywood, wood, or concrete deck affects how the BUR is applied and can influence the total weight by adding or absorbing materials.
- Additional components: Flashings, penetrations, curbs, and edge detailing add minor weights but can be material-dependent.
Important takeaway: Ballast weights often dominate the overall BUR mass, but insulation and plies also contribute meaningfully, especially in retrofit projects or cultural stone ballast scenarios.
Estimating BUR Weight For Your Roof: A Step‑By‑Step Guide
- Identify the roof configuration: number of BUR plies, presence of cap sheet, ballast type/quantity, and insulation thickness.
- Find manufacturer data sheets for the exact products used: weight per ply, asphalt density, ballast density, and insulation weight per inch.
- Calculate base BUR weight (no ballast): multiply the number of plies by the estimated weight per ply, and add cap sheet weight if applicable.
- Add insulation weight: multiply insulation thickness by its weight per inch (varies by material).
- Add ballast weight: use the ballast psf value and the area of the roof (psf × roof area).
- Sum components to obtain total system weight per square foot, then multiply by the roof area to get total weight.
Sample scenario (illustrative only): A 20,000-square-foot roof with 3 plies, a cap sheet, 1 inch of polyiso insulation, and gravel ballast. Base BUR weight: 5 psf. Insulation: 0.6 psf. Ballast: 15 psf. Total: 5 + 0.6 + 15 = 20.6 psf. Total roof weight: 20.6 psf × 20,000 ft² ≈ 412,000 pounds. Actual values require project-specific data and verified calculations from a licensed professional.
Structural Design Implications And Code Considerations
BUR contributes to the dead load that a roof structure must support. Building codes, including the International Building Code (IBC) and local amendments, require that roof systems be designed to withstand both dead loads and anticipated live loads (including snow). When planning a BUR installation, several considerations matter:
- Structural capacity: The structure must handle the added dead load from BUR, ballast, and insulation without compromising safety or service life.
- Existing conditions: On renovation projects, existing decks or framing may already be near capacity. Re-roofing with heavier configurations can push the system over limits.
- Snow load and climate: In regions with heavy snow, ballast weight and insulation thickness may need adjustment to maintain safety margins.
- Engineer involvement: For complex builds or significant weight changes, a licensed structural engineer should verify design loads and provide necessary calculations.
- Code compliance: Manufacturers’ installation instructions and code-prescribed limits govern how BUR can be installed and layered, including ballast usage and fastening methods.
From a practical perspective, a BUR project should begin with a structural assessment and end with documentation of the expected dead load and load path to the building frame. Clear communication between designers, contractors, and building officials helps avoid surprises during inspection and occupancy.
Maintenance, Replacement, And Practical Considerations
Over time, roof systems may be repaired, retrofitted, or re‑covered. Each option affects weight differently. Consider these points when planning maintenance or replacement:
- Aging BUR: Some BUR systems may require gravel ballast removal prior to overlay or replacement, affecting short-term weight and handling logistics.
- Re-roof decision: Installing a new BUR layer over an existing roof adds weight. In many cases, structural checks are needed to confirm capacity for the additional dead load.
- Material choices: Lighter ballast options (such as ballast substitutes or reflective cap sheets) can reduce weight while maintaining performance. Insulation choices also influence overall weight and thermal performance.
- Inspection and load considerations: Regular inspection to assess ballast settlement, edge detail integrity, and flashing condition helps prevent weight-related failures at joints and perimeters.
- Long-term planning: For aging buildings, anticipate future re-roofing needs and select BUR configurations that balance weight, service life, and energy performance.
In practice, accurate weight estimation supports safer construction practices and informed decision-making. Owners and builders should source data sheets for every material layer, perform calculations with a licensed professional, and maintain records of the final system weight for future renovations or structural assessments.