NOTE TO READER: Where applicable, the content of this page is replicated in the RoofStar Guarantee Standards sections of this Manual
Securement of the roof system to the roof deck is necessary to prevent damage caused by wind uplift (possibly resulting in a blow-off) and lateral movement of the roofing materials due to thermal and moisture variations. Traditionally, the roof insulation and roofing are secured to the roof deck by means of mechanical fastening, adhesive, or a combination of both.
RoofStar Guarantee Standards requires mechanical fastening of the insulation wherever practical. Over concrete and other non-nailable decks, or where mechanical fastening is not feasible an adhesive may be used, usually bitumen.
In the past, roofing was installed over wood decks using mechanical fasteners. As construction technology progressed, many new types of decks developed bringing with them different securement requirements.
Concrete and other non-nailable decks dictated that an adhesive, usually bitumen, be used to apply the roof insulation and roofing. On sloped roofs where securement is required to prevent slippage, wood nailing strips must be cast into or fixed to the deck to permit mechanical attachment.
On steel decks, the initial use of bitumen as the adhesive has proven to be less successful than adhesion to concrete. Steel deck deflection can result in breaking the adhesive bond between the insulation and the deck. In addition, the use of bitumen may contribute to the fire hazards associated with steel decks.
Several adhesives successfully achieve adequate adhesion against wind uplift and lateral movement resistance. Many factors can influence the strength achieved, including substrate characteristics and preparation, bonding area, chemical compatibility of adhesive and insulation and moisture and temperature conditions at time of application. To properly evaluate an adhesive for use in a roof assembly, the design authority must
- Know if it is acceptable to the membrane manufacturer
- Understand the adhesion properties of the product, and
- Consult the list of tested assemblies compliant with the CSA A123.21-14 Standard test method for the dynamic wind uplift resistance of membrane roofing systems.
3 Mechanical Fasteners
Mechanical fastening to steel decks provides more effective wind uplift resistance and greater horizontal shear resistance than adhesives (thereby decreasing potential membrane splitting). Fastening patterns and rates that have proven to be successful are published in the RoofStar Guarantee Standards and should be enforced as a minimum. In addition to insufficient fasteners, the type of fastener can prove critical. For example, failure may occur from nails with inadequate head size to pull through the roofing felts.
The correct fastener should be specified by the design authority for each component of a roof system.
The design authority should consult with the fastener manufacturer to determine the fastener's pullout strength in specific deck types to determine the number of fasteners required.
Wind uplift has been shown to be greater at the perimeters and therefore fastener requirements vary between field and perimeter areas. In addition, mechanical fastening may be required at the perimeter detail to restrain lateral movement of the membrane.
4 Fastener Corrosion
Responsible fastener manufacturers have developed sophisticated fastener coatings and metal alloys to counteract or slow fastener corrosion. These fasteners are usually tested for corrosion resistance through a method called the Kesternich Chamber. This method stipulates that the fastener be exposed to sulphurous acid for a minimum of 15 cycles and show less than 15 percent rust after testing. Proof of successful testing at least gives an indication of corrosion prevention but the designer must still consider the other corrosion contributors discussed.
5 Roofing Nails
Manufacturers of roofing nails that conform to CSA Standard CSA B111, suitable for use in roofing systems include:
- National Nail Inc.
- Simplex Nails
These are available through selected RCABC Associate Members.
6 Wind Uplift
|Qualifying for a RoofStar Guarantee|
|To qualify for a RoofStar Guarantee on low-slope roof systems, and to comply with the RoofStar Guarantee Standards in this Manual, mechanically attached and adhered insulated membrane systems must be secured to the roof deck according to each manufacturer’s tested specifications conforming to CSA A123.21 Standard (latest edition) for assembly wind ratings.
Any roof system must conform to the complete tested assembly requirements of CSA A123.21 and a calculation for the building wind load requirements using the Wind-RCI calculator.
6.1 Design Wind Loads
Design Wind Loads are forces caused by wind, which a structure is built to withstand, whether they are negative or positive forces. On the roof the forces are typically negative much like the negative pressure acting on the top of an airplane wing providing lift. Therefore wind acting against the wall of a building passes over the roof exerting uplift on the roofing system. Factors affecting uplift forces are wind speed, building height, roof slope, wall openings, roof overhangs and ground roughness. The roof is divided into three zones: the field; the perimeter, and the corner.
For more about design loads and zones, see Wind-RCI.
6.2 Wind Speed
Designing for wind speed requires specific data. The historical data of weather patterns determines the anticipated wind speeds a structure should be designed to withstand. Wind speed for a wind chart is typically measured at an elevation of 9 m - 11 m (30 - 35 feet) above ground level based on intervals of 50 - 100 years (Mean Recurrence Interval). The u>Regional Wind Speed diagram (see below) gives a general location of high wind areas in the province of B.C.
For more about the impact of wind speeds on building height, see Wind-RCI.
6.3 Ground Exposure
Refer to Wind-RCI for Canadian standards pertaining to structural wind exposures.
RoofStar Guarantee Standards fastening requirements take into consideration the worst-case scenario for a wind speed of up to 137 kph (85 mph) and building height up to 18 m (60’) and are intended to meet or exceed the requirements set out in the test methods of CSA A123.21. For buildings exceeding the parameters of the RCI wind calculator, the design authority must perform calculations for appropriate wind loads, membrane systems and attachment requirements.
All wind load calculations for buildings in Canada should now reference the Wind-RCI wind calculator.
6.5 Fastener Pull-out Strength
Fastener pull-out is the force required to remove the fastener from the substrate without unscrewing. Pull-out test results depend on the deck material, fastener material, shank size and length, and thread pitch and depth. A typical #14 screw designed for insulation attachment (as an example) in a 22 ga. steel deck has a pull-out resistance of 230 kg (507 lb).
The total fastener pull-out strength of all fasteners in a sheet must be greater than the total uplift for each sheet being attached. Distribution is also important.
6.6 Fastener Distribution
Fasteners must be distributed evenly throughout the sheet material being attached. The actual strength of the material between fasteners is the issue. If a roof requires 1.7 kPa (35 psf) uplift resistance, a 1.2 m x 2.4 m (4’ x 8’ or 32 ft²) sheet of insulation would require a total force of 508 kg (1120 lb) to resist uplift. A single fastener with 522 kg (1150 lb) pull-out strength would cover the uplift force but clearly would not be sufficiently distributed throughout the sheet. In most cases the distribution of fasteners takes precedence over pull-out strength.
6.7 Deck Material
Fastener requirements are not the same for all deck materials. As plywood thickness and steel gauge increases, fastener pull-out strength increases, hence fewer fasteners are required, as shown in the tables found in the RoofStar Guarantee Standards for Low Slope Roof Systems
6.8 WIND UPLIFT DESIGN CRITERIA
The reader is urged to consult the RoofStar Guarantee Standards for Low Slope roof assemblies.
6.9 Regional Wind Speed
The province of British Columbia is divided into three regions:
Region 1 - High wind region
Region 2 - Moderate wind region
Region 3 - Low wind region
- Region 1 is located near the coast while most of Region 3 is located at the inland area.