Built up roofing (BUR) systems are a popular choice for flat or low-slope roofs due to their durability and multi-layer protection. A crucial factor in BUR design and installation is the overall built up roofing weight, which affects structural load, installation cost, and long-term roof performance. This article delves into the components influencing BUR weight, how to calculate it, and its implications for building owners and contractors.
| Component | Typical Weight per Square Foot (lbs) | Impact on Total Weight |
|---|---|---|
| Asphalt (Hot Mop Bitumen) | 2.0 – 3.5 | Provides waterproofing and adhesion |
| Roofing Felts (Layers) | 1.5 – 3.0 (each layer) | Strengthens the membrane and durability |
| Gravel or Aggregate Surfacing | 5.0 – 8.0 | Protects the membrane from UV and physical damage |
| Insulation Boards (per inch thickness) | 1.0 – 2.5 | Impacts structural load depending on thickness |
What Is Built Up Roofing and Its Standard Components?
Built up roofing is a multi-ply roofing system made by layering asphalt and roofing felts (or ply sheets) on a roof deck. The layers are bonded together with hot asphalt or coal tar to form a durable, weather-resistant membrane. BUR systems are finished with a protective aggregate surfacing such as gravel or slag.
The standard BUR components influencing weight include:
- Bitumen (Asphalt or Coal Tar): Acts as the adhesive and waterproofing agent.
- Felt Layers: Multiple layers (typically 3 to 5) of fibrous roofing felts reinforce the membrane.
- Surfacing (Gravel or Aggregate): A protective coating that adds weight and UV resistance.
- Insulation: Usually installed beneath the BUR system, insulation adds to the total roofing system weight.
How Is Built Up Roofing Weight Calculated?
The weight of a BUR system is cumulative and depends on the number of felt layers, thickness of asphalt application, and the type of surfacing used. Calculating the weight involves adding the individual weights of each component per square foot.
| Component | Weight per Square Foot (lbs) | Example Details |
|---|---|---|
| Bitumen | 2.5 | Approximate for 3-ply application |
| Felt Layers | 4.5 | 3 Layers × 1.5 lbs each |
| Gravel Surfacing | 6.5 | Typical aggregate weight |
| Insulation | 3.0 | 1.5-inch Polyiso or similar |
| Total System Weight | 16.5 | Sum of components |
Note: Actual weights may vary depending on materials and installation thickness.
Why Does Built Up Roofing Weight Matter?
The weight of a BUR system affects various aspects of building design and roofing project management. Understanding BUR weight is critical for:
Structural Load Considerations
The roof structure must support live loads (people, equipment) and dead loads (roof material weight). BUR systems are heavier than single-ply membranes, meaning roof decks and supporting structures must be designed or assessed to bear the additional weight safely.
Installation and Handling
Heavier roofing materials require more effort and equipment during installation. Proper logistics and handling must be planned to avoid damage and ensure safety.
Cost Implications
The increased weight can impact the cost of materials, labor, and potentially structural reinforcement. Budgeting for these factors in BUR projects leads to better resource allocation.
Longevity and Durability
BUR weight correlates with durability; heavier systems typically include more reinforcing layers and better surfacing, enhancing resistance to weather and mechanical damage.
Factors Influencing Built Up Roofing Weight
Several factors determine BUR system weight:
- Number of Ply Layers: Increasing the layers of felt adds weight but also strengthens the roof.
- Bitumen Type and Amount: Using coal tar versus asphalt or varying thickness alters weight.
- Surfacing Material: Gravel weight varies by size and type, substantially adding to total weight.
- Insulation Thickness: Thicker insulation boards increase the load on the roof deck.
- Roof Size and Pitch: Large or less-sloped roofs may require heavier layers for water shedding and durability.
Comparison: Built Up Roofing Weight vs. Other Roofing Systems
| Roof System | Typical Weight per Square Foot (lbs) | Key Features |
|---|---|---|
| Built Up Roofing (BUR) | 15 to 25 | Multi-layer, durable, heavy |
| Modified Bitumen | 7 to 15 | Flexible, easier installation |
| Single Ply (TPO, PVC, EPDM) | 1 to 3 | Lightweight, quick installation |
| Metal Roofing | 1.5 to 5 | Long-lasting, lightweight |
Insight: The built up roofing system is among the heaviest, requiring thorough evaluation of existing structures before retrofit or replacement.
Best Practices for Managing Built Up Roofing Weight
Structural Evaluation Before Installation
Engage a structural engineer to assess existing roof support capacity. Reinforce beams or decking as necessary to safely accommodate BUR weight.
Material Specification and Quality Control
Select quality materials with known weights and verify supplier data. Maintain consistent asphalt application thickness and felt layering during installation.
Use of Lightweight Insulation Options
When possible, choose insulation materials like polyisocyanurate or expanded polystyrene with a high strength-to-weight ratio to reduce overall roof weight.
Routine Maintenance
Regular inspection and maintenance help identify early signs of structural stress or roof deterioration linked to heavy weight, enabling corrective measures.
Environmental and Regulatory Considerations
BUR weight can affect compliance with local building codes, especially in seismic or high-wind zones where load limits are strictly regulated.
- Building Codes: Most codes require calculations of dead loads, including BUR weight, during permit application.
- Energy Codes: Heavier BUR systems may influence insulation strategies to meet energy efficiency standards.
- Green Roofing Initiatives: Incorporating built up roofing with vegetative roofs may increase total system weight, affecting structural design.
Innovations Affecting Built Up Roofing Weight
Modern advancements aim to reduce BUR weight without sacrificing durability:
- Lightweight Roofing Felts: New synthetic fibers reduce ply weight.
- Improved Bitumen Formulations: Modified asphalt products provide strong adhesion at thinner layers.
- Composite Insulation Panels: Offer thermal performance with less mass.
These innovations assist in providing durable, compliant BUR systems with lower structural impact.