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Roofing systems guide for construction

Understanding roofing system types, insulation, flashing details, and warranty considerations

The roofing system is a building's primary defense against water infiltration. It encompasses the waterproofing membrane, insulation, vapor retarder, drainage system, and all flashings at penetrations and terminations. Roofing is specified in Division 07 (Thermal & Moisture Protection) and detailed on the architectural roof plans and detail sheets. Understanding roofing systems is critical for reviewing drawings, coordinating rooftop equipment, and preventing the most costly building failures, water leaks.

Roofing System Types

TPO (Thermoplastic Polyolefin)
Single-Ply Membrane
Thickness
45, 60, or 80 mil
Seams
Hot-air welded, strongest single-ply seam method
Lifespan
20–30 years
Attachment
Mechanically fastened, fully adhered, or ballasted
Advantages
Energy-efficient (white reflective), heat-welded seams, chemical resistance
Considerations
Relatively newer track record vs. EPDM, quality varies by manufacturer
EPDM (Ethylene Propylene Diene Monomer)
Single-Ply Membrane
Thickness
45, 60, or 90 mil
Seams
Adhesive or tape, requires careful application
Lifespan
25–35 years
Attachment
Fully adhered, mechanically fastened, or ballasted
Advantages
Proven 50+ year track record, excellent UV resistance, flexibility in cold weather
Considerations
Black absorbs heat, adhesive seams less reliable than welded
PVC (Polyvinyl Chloride)
Single-Ply Membrane
Thickness
50, 60, or 80 mil
Seams
Hot-air welded
Lifespan
20–30 years
Attachment
Mechanically fastened or fully adhered
Advantages
Chemical resistance (restaurants, manufacturing), fire resistance, heat-welded seams
Considerations
Higher cost than TPO, plasticizer migration over time, less flexible in extreme cold
Built-Up Roofing (BUR)
Multi-Ply
Thickness
Multiple plies (3–5 layers)
Seams
Overlapping felts create monolithic membrane
Lifespan
25–40 years
Attachment
Mopped in hot asphalt, cold-applied, or torch-applied
Advantages
Proven redundancy (multiple layers), excellent waterproofing, puncture resistant
Considerations
Heavy weight, hot-applied methods require kettle, difficult repairs
Modified Bitumen
Multi-Ply
Thickness
Cap sheet + base sheet (2–3 layers)
Seams
Torch-welded or adhesive overlap
Lifespan
20–30 years
Attachment
Torch-applied, cold-adhesive, or self-adhered
Advantages
Easier application than BUR, good in cold weather, SBS provides flexibility
Considerations
Torch-applied fire risk, fewer plies than BUR
Standing Seam Metal
Metal Panel
Thickness
22–26 gauge steel or 0.032–0.040" aluminum
Seams
Mechanically seamed or snap-lock raised seams
Lifespan
40–60 years
Attachment
Clip-fastened to purlins or continuous substrate
Advantages
Longest lifespan, handles steep or low slopes, recyclable, aesthetic versatility
Considerations
Highest initial cost, thermal movement management, oil-canning potential
Asphalt Shingles
Steep Slope
Thickness
Architectural (dimensional) or 3-tab
Seams
Overlapping courses with sealant strips
Lifespan
20–40 years (architectural), 15–25 years (3-tab)
Attachment
Nailed to plywood/OSB substrate over underlayment
Advantages
Lowest cost for steep slope, wide availability, easy repair
Considerations
Not suitable for low slopes, wind uplift vulnerability, shorter lifespan

Roof Insulation

Roof insulation provides thermal resistance and, in tapered configurations, creates positive drainage. Insulation is installed above the roof deck and below the membrane in conventional systems, or above the membrane in protected (IRMA) systems.

Polyisocyanurate (Polyiso)
R-5.7 per inch (aged)
Rigid foam board, most common commercial roof insulation. Foil or glass-fiber faced.
R-value decreases in cold temperatures. Multiple layers with staggered joints recommended.
Expanded Polystyrene (EPS)
R-3.8–4.4 per inch
Rigid foam board, lighter weight and lower cost than polyiso. Used in protected membrane (IRMA) systems.
Stable R-value across temperatures. Moisture-resistant but must be protected from solvents.
Extruded Polystyrene (XPS)
R-5.0 per inch
Dense rigid foam, excellent moisture resistance. Used above and below waterproofing.
Blue, pink, or green colored boards. Higher compressive strength than EPS.
Mineral Wool (Stone Wool)
R-4.2 per inch
Non-combustible rigid board, fire-resistant with excellent dimensional stability.
Used where fire ratings are critical. Heavier than foam insulation. Hydrophobic treatment available.
Tapered Insulation
Varies by thickness
Polyiso or EPS boards manufactured with a slope to create positive drainage. Typically 1/4" per foot minimum.
Eliminates ponding water. Cricket and saddle configurations for drain layout.

Critical Flashing Details

Most roof leaks originate at flashings, the transitions between the field membrane and penetrations, edges, or walls. Reviewing flashing details is the most important part of roof drawing review.

Parapet Wall Flashing
Membrane turned up and over parapet, terminated with counterflashing or coping. Critical transition between horizontal roof and vertical wall.
Common Issue: Open seams at coping, inadequate membrane height (min 8" above roof)
Roof Edge / Drip Edge
Metal edge detail at roof perimeter where no parapet exists. Secures membrane edge and directs water off the building.
Common Issue: Insufficient membrane overlap, missing fasteners at wind uplift zone
Pipe Penetration Boot
Prefabricated or field-fabricated flashing collar around pipes (plumbing vents, conduits). Membrane lapped over base.
Common Issue: Cracked boots from UV exposure, insufficient membrane overlap, no clamping ring
Curb Flashing (Equipment)
Raised curb supporting rooftop equipment with membrane wrapped up curb sides. Minimum 8" curb height above roof surface.
Common Issue: Low curbs that dam water, membrane not carried to top of curb, missing counterflashing
Drain Flashing
Membrane clamped into roof drain sump with clamping ring. Tapered insulation directs water to drain.
Common Issue: Ponding water from inadequate slope, membrane not properly seated in drain, debris-clogged drains
Expansion Joint
Flexible joint cover allowing roof movement at structural expansion joints. Raised curb on each side with flexible membrane bridging.
Common Issue: Rigid covers that crack, inadequate curb height, membrane tearing from movement

Warranty Considerations

Material-Only Warranty
10–20 years typical
Covers defects in the membrane material only. Does not cover workmanship, flashings, or system performance.
System Warranty (NDL)
15–30 years, requires manufacturer inspection during installation
No Dollar Limit, manufacturer covers labor and materials for any leak. The gold standard of roof warranties.
Workmanship Warranty
2–5 years typical
Issued by the roofing contractor covering installation quality. Separate from manufacturer warranty.
Warranty Tip

NDL (No Dollar Limit) system warranties typically require that all roofing materials come from the same manufacturer and that installation is inspected by the manufacturer's representative. Mixing manufacturers or skipping inspections can void warranty coverage.

Sources

NRCA, Roofing Manual (Membrane Roof Systems, Metal Panel Systems)

ASHRAE 90.1, Energy Standard (roof insulation requirements)

FM Global Property Loss Prevention Data Sheets (1-28, 1-29, 1-31)

SPRI, Single-Ply Roofing Industry Standards

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