Low-Slope Membrane Roofing (Built-Up, Modified Bitumen, and Single-Ply)

Low-Slope Roofing (Built-Up, Modified Bitumen, and Single-Ply Membrane)

Chapter Introduction

This chapter covers the three main families of low-slope roofing systems: built-up roofing (BUR) systems, modified bitumen (MB) membranes, and single-ply synthetic membranes (PVC, TPO, EPDM). For the Red Seal exam, you must master not only installation techniques, but also design principles, slope calculations, the requirements of the National Building Code (NBC) , and applicable CAN/CGSB standards. Mastering these systems is essential, as they represent over 80% of commercial and industrial roofing in Canada.

2. Fundamentals of Low-Slope Roofing

2.1 Definition and Slopes

A low-slope roof is a roof with a slope of less than 1:4 (14° or 25%). In practice, we distinguish:

Slope Type	Ratio	Percentage	Angle
Flat roof (called "membrane")	1:50 to 1:100	1% to 2%	0.6° to 1.1°
Minimum low slope (BUR)	1:50	2%	1.1°
Low slope (modified bitumen)	1:25 to 1:50	2% to 4%	1.1° to 2.3°
Moderate slope	1:12 to 1:4	8% to 25%	4.8° to 14°

Essential Rule: The NBC requires that every roof have a minimum slope of 1:50 (2%) towards drains or valleys, unless the membrane is specifically designed for ponding water (e.g., 60-mil EPDM with vulcanized seams). The slope is calculated as follows:

Slope (%) = (Height Difference ÷ Horizontal Distance) × 100

Example: A roof with a 20 m span and a height difference of 0.4 m has a slope of (0.4 ÷ 20) × 100 = 2%. This is the minimum acceptable slope.

2.2 Drainage and Water Evacuation

Drainage is the most critical factor for the longevity of a low-slope roof. The NBC requirements (Article 9.26.2.1) state:

At least two drains must be installed for each roof or roof section, unless the area is less than 200 m².

Drains must be spaced no more than 15 m apart in each direction.

The minimum diameter of drains is 100 mm (4 in).

Overflow (scuppers) must be sized to evacuate the excess if the main drains become blocked.

Drainage Calculation: The capacity of a 100 mm drain is approximately 4.5 L/s under a 50 mm head of water. For a heavy rainfall of 50 mm/h on a 500 m² roof, the flow rate is:

Flow Rate (L/s) = (Area × Precipitation) ÷ 3600 = (500 × 0.05) ÷ 3600 = 0.007 m³/s = 7 L/s

You would therefore need at least two 100 mm drains for this area.

2.3 Classification of Systems According to the NBC

The NBC classifies roofs according to their fire performance (Article 3.1.5.1):

Class	Description	Examples
Class A	Resists fire without propagation	BUR with gravel, fire-retardant PVC membrane
Class B	Resists fire with limited propagation	Modified bitumen with mineral surface
Class C	Resists fire with controlled propagation	Unprotected EPDM

Exam Trap: The NBC requires that roofs of buildings over 3 storeys or over 600 m² have a Class A or B classification, depending on proximity to property lines.

3. Built-Up Roofing Systems (BUR)

3.1 Composition and Materials

The traditional built-up roofing system consists of alternating layers of roofing felt and hot bitumen. The components are:

Vapour barrier: 6-mil polyethylene sheet or self-adhesive bituminous membrane, installed on the warm side of the insulation.

Thermal insulation: Polyisocyanurate (PIR) boards, extruded polystyrene (XPS), or mineral wool, mechanically fastened or adhered.

Felt layers: Glass felt (Type G) or polyester felt (Type P), in 2 to 4 layers.

Bitumen: Type I to IV bitumen (ASTM D312), applied hot at a temperature of 180°C to 230°C (verify the temperature with a probe thermometer).

Protective surface: Gravel (16 to 25 mm river stone), mineral cap sheet, or aluminium coating.

3.2 Installation Procedure (Hot Method)

36.Deck preparation: The deck (concrete, steel) must be dry, clean, and free of ice or debris. Maximum concrete moisture content is 6% (measured with a moisture meter).

37.Vapour barrier installation: Unroll the membrane with a 150 mm overlap at the seams, adhered with hot bitumen or self-adhesive.

38.Insulation installation: Mechanical fastening (screws and plates) at a rate of 4 fasteners per board minimum, or adhered with hot bitumen (full or strip mopping).

39.Bitumen application: Bitumen is applied with a squeegee at a rate of 1.0 to 1.5 kg/m² per layer. The application temperature must be checked at the kettle outlet and at the point of application.

40.Felt layer installation: Each layer is unrolled into the hot bitumen, with a side lap of 75 mm and an end lap of 150 mm. The seams of successive layers must be staggered by at least 300 mm.

41.Surface layer: Application of the final layer (gravel, cap sheet, or coating) within 24 hours of the last bitumen layer.

3.3 Quantity Calculations

Number of Layers: For a 1,000 m² roof with a 3-ply system:

Felt: 1,000 m² × 3 plies = 3,000 m² of felt (allow 5% for waste = 3,150 m²).

Bitumen: 1,000 m² × 1.2 kg/m² × 4 applications (3 plies + surface layer) = 4,800 kg of bitumen.

Gravel: 1,000 m² × 20 kg/m² = 20,000 kg (20 tonnes).

Exam Trap: Felt overlaps increase the actual surface area. For a 75 mm side lap on a 1 m wide roll, the usable area per roll is 0.925 m² per linear metre, representing a loss of 7.5%.

3.4 Common Defects and Repairs

Defect	Probable Cause	Corrective Action
Blistering	Trapped moisture or bitumen too hot	Cut open, dry out, re-adhere
Cracking	Aging, thermal movement	Cover with a modified bitumen layer
Wind uplift	Poor adhesion or insufficient fastening	Re-fasten mechanically, add strips
Alligatoring	Excessive UV exposure of bitumen	Apply a protective coating

4. Modified Bitumen Membranes (MB)

4.1 Types of Membranes

Modified bitumen membranes are prefabricated membranes composed of bitumen modified with polymers, reinforced with polyester or fibreglass. There are two main families:

Type	Modification	Characteristics	Installation Temperature
SBS (Styrene-Butadiene-Styrene)	Elastomeric	Flexible in cold weather, ideal for cold climates	Torch or adhesive
APP (Atactic Polypropylene)	Plastomeric	Rigid, heat resistant	Torch only

4.2 Installation Methods

4.2.1 Torch Application

The most common method in Canada for APP and SBS membranes:

58.Unroll the membrane and position it without adhering (pre-positioning).

59.Heat the underside of the membrane with a propane torch (blue flame, flame temperature of 1,100°C to 1,200°C).

60.Melt the bitumen over a width of 100 to 150 mm ahead of the roll.

61.Unroll the membrane into the melted bitumen, ensuring the bitumen flows out 25 to 50 mm ahead of the roll (sign of proper fusion).

62.Broom the membrane (press down) with a squeegee to eliminate air pockets.

63.Side laps must be 75 to 100 mm and end laps 150 mm.

Minimum Ambient Temperature: Torch application is prohibited if the temperature is below -10°C (some standards require -5°C). Below this temperature, the bitumen cools too quickly and fusion is incomplete.

4.2.2 Cold Application (Adhesive)

Use of a cold adhesive (solvent-based or water-based) applied with a squeegee or roller.

Coverage rate: 0.5 to 0.7 L/m² for adhesive.

The membrane must be broomed immediately after adhesive application.

Drying time before installing the next layer: 24 to 48 hours.

4.2.3 Mechanical Fastening

Fastening with screws and plates in the side lap.

Fastener spacing: 200 mm in the field, 150 mm at the edges.

The membrane must have a polyester reinforcement to resist tear-out.

4.3 Flashings and Details

Flashings (parapets, walls, penetrations) are the most critical points:

Minimum flashing height: 200 mm above the finished roof level (NBC, Article 9.26.2.2).

Wall return: The flashing must extend up 200 mm minimum and be mechanically fastened at the top with a metal counter-flashing.

Sharp angles: A cant strip or angle piece (wood or insulation) must be installed to avoid right-angle bends.

Penetrations: Penetrations larger than 300 mm in diameter must have an individual flashing; smaller ones can be grouped in a curb.

4.4 Calculations for Modified Membranes

Number of Rolls: A roll of SBS membrane is typically 10 m × 1 m (10 m²). For a 500 m² roof with overlaps:

Gross area: 500 m²

Waste for overlaps (10%): 50 m²

Waste for flashings and details (15%): 75 m²

Total area: 625 m²

Number of rolls: 625 ÷ 10 = 63 rolls (round up to 65 for unforeseen waste).

Exam Trap: Flashings must be included in the total area calculation. A 300 mm high flashing on a 100 m perimeter adds 30 m² of membrane.

5. Single-Ply Membranes (Synthetic)

5.1 System Comparison

Characteristic	PVC (Polyvinyl Chloride)	TPO (Thermoplastic Polyolefin)	EPDM (Ethylene-Propylene-Diene)
Type	Thermoplastic	Thermoplastic	Thermoset (vulcanized)
Common thickness	1.5 mm (60 mils)	1.5 mm (60 mils)	1.5 mm (60 mils)
Seaming method	Hot-air welding	Hot-air welding	Adhesive or tape
UV resistance	Excellent	Good	Good (with protection)
Chemical resistance	Excellent	Good	Poor (oils, solvents)
Ponding water resistance	Excellent	Good	Excellent (if 60 mils)
Welding temperature	400°C to 500°C (air)	400°C to 500°C (air)	N/A (adhesive)
Estimated lifespan	20-30 years	15-25 years	20-30 years

5.2 PVC and TPO – Hot-Air Welding

Principle: Thermoplastic membranes are welded by thermal fusion with hot air. The weld is made with an automatic welding machine (for long seams) or a hot-air gun (for details).

Critical Welding Parameters:

Parameter	Typical Value
Air temperature	400°C to 500°C
Travel speed	2 to 4 m/min (automatic welder)
Welded seam width	25 to 40 mm
Minimum overlap	50 mm (side), 75 mm (end)

Weld Testing: After welding, the seam must be checked with a flat-blade screwdriver (probe test): the weld must not peel apart. A peel test must show failure within the material, not at the seam.

Fastening:

Mechanical fastening: Screws and plates in the overlaps, spaced 300 mm in the field and 150 mm at the edges. The membrane must be tensioned before fastening (pre-tension of 1 to 2%).

Ballasted: 50 mm of gravel (50 kg/m²) or concrete pavers (40 kg/m²).

Fully adhered: Adhesive applied at a rate of 0.3 to 0.5 L/m² (contact adhesive) or 0.8 to 1.2 L/m² (trowel-grade adhesive).

5.3 EPDM – Adhered Installation

EPDM is a thermoset (vulcanized) membrane that cannot be welded. Seams are made using:

Contact adhesive (neoprene): Applied to both surfaces, drying time of 10 to 15 minutes, then pressing together.

Double-sided splice tape: 75 mm wide, applied under the overlap.

Primer: Application of a cleaning primer on both surfaces before bonding.

Minimum Seam Widths: 75 mm for side laps and 150 mm for end laps.

Exam Trap: EPDM must never be installed in direct contact with hot bitumen or petroleum solvents. A separator layer (200 g/m² geotextile) is required between EPDM and polyisocyanurate insulation.

5.4 Critical Details for Single-Ply Membranes

Flashings: Minimum height of 200 mm, mechanically fastened at the top with a metal termination bar and counter-flashing.

Inside corners: A cant strip at 45° (wood or foam) must be installed to avoid sharp bends.

Penetrations: Use prefabricated pipe boots in EPDM or PVC, sealed by welding or adhesion.

Expansion joints: Membranes must be interrupted at building expansion joints with a bellows with an amplitude of ± 25 mm.

6. Applicable Standards and Codes

6.1 CAN/CGSB Standards

CAN/CGSB standards (Canadian General Standards Board) are the national references for roofing materials:

Standard	Title	Application
CAN/CGSB-37.51-M	SBS Modified Bitumen Membrane	Specification for SBS membranes
CAN/CGSB-37.52-M	APP Modified Bitumen Membrane	Specification for APP membranes
CAN/CGSB-37.54-M	PVC Membrane	Specification for PVC membranes
CAN/CGSB-37.55-M	EPDM Membrane	Specification for EPDM membranes
CAN/CGSB-37-GP-56M	Fibreglass Roofing Felt	Specification for BUR felts

6.2 National Building Code (NBC)

Relevant NBC articles (2020 edition):

Article 3.1.5.1: Classification of roofs according to fire performance.

Article 3.2.3.7: Protection of roofs against sparks (combustible-frame buildings).

Article 9.26.2.1: Minimum roof slope (1:50).

Article 9.26.2.2: Minimum flashing height (200 mm).

Article 9.26.4.1: Vapour barrier required for heated roofs.

Article 9.26.6.1: Thermal insulation of roofs (minimum R-value according to climate zone).

Minimum R-Values (NBC, Table 9.26.1.1) :

Climate Zone	RSI (m²·K/W)	R (ft²·°F·h/BTU)
Zone 4 (Vancouver)	4.67	26.5
Zone 6 (Toronto, Montréal)	6.67	37.9
Zone 7 (Ottawa, Québec)	7.41	42.1
Zone 8 (Edmonton, Winnipeg)	8.67	49.2

Conversion: R (imperial) = RSI × 5.678

6.3 Safety Standards – Working at Heights

The Canada Occupational Health and Safety Regulations (COHSR) require:

Guardrails: 1.1 m minimum height, with intermediate rail and toe board.

Lifeline: Certified anchorage for a load of 22 kN (2,244 kg).

Safety harness: Mandatory if the slope exceeds 1:4 (14°) or if the fall height exceeds 3 m.

Personal Protective Equipment (PPE): Safety glasses, heat-resistant gloves (for hot bitumen), respirator (for solvent vapours).

7. Advanced Calculations and Technical Considerations

7.1 Thermal Resistance Calculation (RSI Value)

The total thermal resistance of a roof is the sum of the resistances of each layer:

RSI_total = RSI_insulation + RSI_membrane + RSI_vapour barrier + RSI_deck

Example: Roof with 100 mm of polyisocyanurate (RSI = 0.035 m²·K/W per mm):

RSI_insulation = 100 × 0.035 = 3.5 m²·K/W

Add the membrane (RSI = 0.01), vapour barrier (RSI = 0.005), and steel deck (RSI = 0.001):

RSI_total = 3.5 + 0.01 + 0.005 + 0.001 = 3.516 m²·K/W

R_imperial = 3.516 × 5.678 = 19.96 (rounded to R-20)

7.2 Dew Point Calculation in the Roof Assembly

The dew point is the temperature at which water vapour condenses within the roof assembly. It is calculated using the psychrometric chart or the approximate formula:

T_dew = (RH/100)^(1/8) × (112 + T) - 112

Where RH = relative humidity (%) and T = air temperature (°C).

Example: Interior air at 21°C with 50% relative humidity:

T_dew = (0.5)^(0.125) × (112 + 21) - 112 = 0.917 × 133 - 112 = 122 - 112 = 10°C

If the temperature at the underside of the membrane is below 10°C, condensation will occur. The vapour barrier must therefore be placed on the warm side (interior side) of the insulation.

7.3 Wind Load Calculation

Wind pressure on a roof is calculated according to the NBC (Appendix C):

p = q × C_e × C_g × C_p

Where:

q = dynamic wind pressure (kPa) = 0.5 × ρ × V²

ρ = air density (1.225 kg/m³)

V = reference wind speed (m/s)

C_e = exposure coefficient (1.0 for open terrain)

C_g = gust coefficient (2.0)

C_p = pressure coefficient (varies from -1.5 to +0.8 depending on zone)

Example: 100 km/h wind (27.8 m/s) on a roof edge zone (C_p = -1.5):

q = 0.5 × 1.225 × (27.8)² = 0.5 × 1.225 × 773 = 473 Pa = 0.473 kPa

p = 0.473 × 1.0 × 2.0 × (-1.5) = -1.42 kPa (suction)

The membrane must resist this suction. For mechanical fastening with screws spaced 300 mm apart, the load per fastener is:

Load per fastener = p × tributary area = 1.42 kPa × (0.3 m × 1 m) = 0.426 kN

The fastener must have a pull-out resistance greater than 0.426 kN (standard fasteners resist 1.5-2.0 kN).

8. Quality Control and Inspections

8.1 Control Points During Installation

Step	Inspection	Acceptance Criteria
Deck	Cleanliness, dryness	Moisture < 6% (concrete)
Vapour barrier	Continuity, overlaps	Overlap ≥ 150 mm, sealed seams
Insulation	Fastening, alignment	4 fasteners/board, staggered joints
Hot bitumen	Temperature	180°C to 230°C (depending on type)
BUR felts	Adhesion, overlaps	No blisters, overlap ≥ 75 mm
Modified membranes	Bitumen fusion	25 mm bitumen bead at the leading edge
Single-ply membranes	Welding	Probe test, no delamination
Flashings	Height, fastening	≥ 200 mm, fastened at max 300 mm spacing

8.2 Watertightness Testing

Flood test: Fill the roof with 50 mm of water for 24 hours. Any drop in level indicates a leak.

Electrical testing (leak detection): Use of a high-voltage generator (10,000 to 40,000 volts) to detect perforations in synthetic membranes.

Infrared thermography: Detection of moisture in BUR roofs by analyzing temperature differences.

9. Maintenance and Repairs

9.1 Preventive Maintenance Program

Frequency	Action
2 times per year (spring, fall)	Visual inspection, drain cleaning
1 time per year	Check seams, flashings, counter-flashings
After each storm	Inspection for damage (hail, wind)
Every 5 years	Complete watertightness test

9.2 Common Repairs

Blister in a BUR: Cut an X, remove moisture, re-adhere with hot bitumen, close with a membrane patch.

Tear in a single-ply membrane: Clean, apply primer, install a patch of the same membrane (welded or adhered) with a 100 mm overlap on each side.

Separated seam: Clean, reheat (for thermoplastics) or re-adhere (for EPDM), then re-test.

10. Pitfalls to Avoid

186.Confusing SBS and APP: SBS can be installed cold or with a torch; APP is torch application only. SBS is flexible in cold weather; APP is rigid.

187.Forgetting the 1:50 minimum slope: The NBC requires this slope for all systems, except specific exceptions (60-mil EPDM with vulcanized seams).

188.Neglecting the vapour barrier: In heated buildings, the absence of a vapour barrier on the warm side causes condensation in the insulation and blister formation.

189.Welding a PVC membrane with a torch: PVC is thermoplastic but is welded only with hot air (400-500°C). A propane torch will damage the membrane.

190.Confusing slope units: A 2% slope equals 1:50, not 1:25. Always verify the ratio.

191.Forgetting overlaps in quantity calculations: Waste for overlaps and details represents 10 to 20% of the total area.

192.Installing polyisocyanurate insulation directly on EPDM: A separator layer (geotextile) is mandatory to prevent plasticizer migration.

193.Ignoring minimum installation temperatures: Torch application is prohibited below -10°C; cold adhesive below 5°C.

194.Not checking bitumen temperature: Bitumen too hot (> 260°C) loses its properties; too cold (< 180°C) will not adhere.

195.Forgetting overflow drains: The NBC requires properly sized overflow drains to prevent collapse in case of drain blockage.

11. Summary

Low-slope roofs have a minimum slope of 1:50 (2%) according to the NBC.

Three families of systems: BUR (built-up bituminous), modified bitumen (SBS/APP), and single-ply (PVC/TPO/EPDM).

Drainage is critical: at least 2 drains per roof, spaced 15 m maximum apart.

Flashings must have a minimum height of 200 mm with a metal counter-flashing at the top.

CAN/CGSB standards specify materials; the NBC specifies installation requirements.

Minimum R-values range from R-26.5 (Zone 4) to R-49.2 (Zone 8).

Working at height safety requires guardrails of 1.1 m and certified anchorages rated at 22 kN.

Wind load and dew point calculations are essential for design.

Watertightness tests (flood, electrical, thermography) validate installation quality.

Preventive maintenance (2 inspections per year) extends the roof's lifespan.

12. Self-Assessment Questions

210.What is the minimum slope of a membrane roof according to the NBC?

211.What is the minimum height of a roof flashing?

212.What is the difference between an SBS and an APP membrane?

213.At what temperature should hot bitumen be applied for a BUR?

214.What is the minimum overlap for a PVC membrane weld?

215.What is the minimum anchorage load for a lifeline?

216.What is the minimum RSI for a roof in Zone 7?

217.How do you calculate the slope of a 30 m roof with a height difference of 0.6 m?

218.What test verifies the quality of a PVC weld?

219.What is the approximate capacity of a 100 mm drain?

Answers: 1) 1:50 (2%); 2) 200 mm; 3) SBS = elastomeric (cold or torch application), APP = plastomeric (torch only); 4) 180-230°C; 5) 50 mm (side); 6) 22 kN; 7) 7.41 m²·K/W (R-42); 8) (0.6 ÷ 30) × 100 = 2%; 9) Probe test (screwdriver); 10) 4.5 L/s.