What Is A Built Up Roof? Answered Here
A built up roofing (BUR) system is a durable, versatile flat or low-sloping roof covering made up of alternating layers of waterproofing materials. BUR systems have been used to protect buildings with multiple redundant layers that provide heavy-duty protection for over 100 years.
Read on for the complete lowdown on BUR roofing.
Definition of a Built Up Roof
A built up roof (also abbreviated as BUR) refers to a flat or gently-sloping roof covering consisting of alternating layers of reinforcing fabrics and bitumen. The bitumen waterproofs the layers while the fabrics provide strength and stability.
What is a Built Up Roof
Specifically, a standard built up roof system contains the following layers from bottom to top:
- Rigid roof deck: Plywood, concrete, metal decking or insulation boards.
- Vapor retarder: Asphalt-coated membrane.
- Insulation: Polyisocyanurate foam boards.
- Base sheet: Fully adhered modified bitumen.
- 2-4 Ply sheets: Reinforcing fiberglass mats.
- Gravel surfacing: Top protective layer.
Advanced BUR assemblies may also integrate a cover board above the insulation to enhance fire, hail, and traffic resistance. Rigid cover boards add durability and a separation layer to shield the foam insulation.
Built Up Roof Meaning
The term "built up" refers to the multi-ply redundant layers that make up the roofing system. BUR is also sometimes referred to as "tar and gravel roofing" due to the traditional use of coal tar pitch and gravel surfacing. The bitumen waterproofing agents applied in hot liquid form are colloquially known as "tar", but can also be asphalt.
Built Up Roof Definition
To summarize, a built up roof is defined as:
A commercial flat roof assembly with rigid insulation, multiple hot mopped reinforcing plies, and an aggregate surfacing that resists weathering and moisture infiltration.
Built up roofs rely on having several redundant moisture protection layers to prevent leaks. The components work together to defend against water, temperature fluctuations, ultraviolet radiation, and physical impacts.
Components of a Built Up Roof System
The waterproofing in a BUR system comes from alternating hot mopping layers of bitumen and embedding reinforcing plies. This creates a redundant barrier that stops moisture ingress. Additional components serve specific protective purposes.
Rigid Insulation
Polyisocyanurate or polystyrene foam insulation enhances thermal efficiency and provides a stable substrate over decking ripples. Insulation boards typically range from 1 to 4 inches thick depending on climate location and desired R-value.
Vapor Retarder
Asphalt-coated membranes act as a vapor retarder to prevent interior conditioned air from infiltrating and condensing within the roof assembly. This added moisture protection minimizes risk of blisters or delamination failures.
Base Sheet
Fully adhered base sheets provide a strong foundation for overlaying hot mopped plies. Self-adhering modified bitumen sheets bond firmly with aggressive adhesive backings.
Ply Sheets
Built up roofs gain their name from layers of reinforcing "plies" or membranes between bitumen. Fiberglass mat, polyester mat, organic felt or fiberboard increase durability and redundancy.
Surfacing
Gravel, slag, or mineral cap sheets shield the bitumen coating from weathering degradation. Loose aggregate also provides fire resistance and ballast weight against wind uplift. The optimal surfacing depends on roof location factors and desired aesthetics.
How Built Up Roofs Are Constructed
Installing a BUR system is labor-intensive, requiring hot bitumen mopping equipment and precise layering. Roofers assemble the built up system sequentially from bottom to top.
BUR Installation Equipment
Special tools for installing hot fluid-applied BUR systems include:
- Kettles: Heat and transport hot asphalt or coal tar pitch.
- Mops: Use to spread hot bitumen evenly.
- Roofing felts: Supply reinforcement between moppings.
- Mechanical rollers: Embed each ply fully.
- Aggregate spreader: Applies gravel surfacing.
Steps to Install a Built Up Roof
The basic hot mopped BUR application process includes:
- Install decking material
- Attach vapor retarder
- Adhere rigid insulation boards
- Fully adhere base sheet in hot asphalt
- Heat and mop hot fluid bitumen over base sheet
- Embed reinforcing ply into hot bitumen
- Repeat mopping and embedding for 2 to 4 plies
- Apply flood coat of hot bitumen
- Embed aggregate surfacing into flood coat
Precise controls during heating and application ensure positive adhesion between layers. The redundant plies seal around roofing protrusions for a fully bonded barrier.
Advantages of Built Up Roofs
Built up roofs offer numerous performance benefits including:
Long Lifespan
With multiple waterproofing layers, BUR systems easily achieve 30+ year lifespans. Periodic restoration coatings further extend service.
All Climate Performance
The mass of a BUR assembly provides exceptional thermal inertia against temperature swings. These roofs resist ponding water, intense sunlight, hail impacts, snow loads and rainfall.
Heavy Traffic Resistance
Thick reinforced built up roofs stand up to regular foot traffic, equipment maintenance, and rooftop additions without damage or impairment of waterproofing.
Wind Uplift Resistance
Aggregate provides dead weight ballast while strong ply adhesion prevents wind scour. BUR performs in hurricane zones and high elevations prone to gusts.
In addition to exceptional waterproofing redundancy, BUR systems resist cracking, impacts, expansion and contraction stresses. Their layers work together to shield buildings for decades.
Disadvantages of Built Up Roofs
Despite the strengths, built up roofs also come with downsides such as:
- Challenging leak detection and repairs
- Foul asphalt fumes during hot mop application
- Specialized equipment and crews required
- Dark surfaces increase cooling costs
- Prone to blisters if layers de-bond
Pinpointing and accessing the source of leaks between layers is difficult. Diagnosing problems often requires destructive probing tests. Plus installing and servicing BUR utilizes hazardous hot fluid kettles requiring experienced crews.
While durable, problems can develop on older BUR systems. Preventative inspections coupled with prompt leak repairs are key to maximizing service life. Facility managers should perform spring and fall checks for deterioration with spot repairs as needed.