Wind Uplift Design and CSA A123.21
Wind Uplift Design and CSA A123.21
|NOTE: The requirements set out below must be interpreted in conjunction with complete RoofStar Guarantee Standards for Low Slope Roof Assemblies. Refer to Modified Bitumen Roof Assemblies, EPDM (Thermoset Membranes) Roof Assemblies, TPO/PVC Roof Assemblies or BUR (Built-up) Roof Assemblies for relevant RoofStar Guarantee Standards.|
|Qualifying for a RoofStar Guarantee|
|To qualify for a RoofStar Guarantee, 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 in keeping with the requirements set out below.|
Click on the gif above to see the full high-definition video, which illustrates why roof system attachment standards matter (NOTE: the system shown in the video represents a mechanically fastened EPDM roof, constructed to RoofStar Guarantee Standards. The membrane "flutter" in wind is normal for this type of roof system.)
- Membrane Roof System - “Consists of a deck and roofing membrane. It includes components such as vapour barriers or retarders, insulation, cover board, etc. It is subjected to a dynamic load sequence that has been developed based on wind pressure records, simulating the effects of wind on membrane-roof assemblies” (CSA A123.21, Scope 1.1)
- CSA Standard - CSA Standard A123.21 Standard test method for the dynamic wind uplift resistance of membrane-roofing systems (latest edition)
- Accepted Membrane Roof System Assembly – an assembly of Accepted Materials that meets or exceeds the Standard
- Mechanically Attached Roof Systems (MARS) – A “system in which the roofing membrane is intermittently attached to the deck using fasteners…” (CSA A123.21, Definitions)
- Adhesive Applied Roof Systems (AARS) – A “system in which the roof membrane is bonded to the substrate using adhesives and all other components below roofing membrane are adhered using adhesives…” This includes “a system in which the roof membrane is bonded to the substrate using adhesives” (CSA A123.21, Definitions)
- Partially Attached (Hybrid) Roof Systems (PARS) – A “system in which the roof membrane is bonded to the substrate using adhesives and a minimum of one component below the membrane is intermittently attached to the supporting structure using fasteners…” This includes “a system in which the roof membrane is bonded to the substrate using adhesives” (CSA A123.21, Definitions)
2 New Membrane Roof Systems
To qualify for a RoofStar Guarantee, and to comply with the RoofStar Guarantee Standards set out herein, all new Membrane Roof Systems that begin with an exposed deck (MARS, PARS and AARS) must be designed and built to meet or exceed the test requirements of the CSA Standard (NOTE: this applies to projects bid after either April 1, 2017 or the issuance of the British Columbia Building Code, 2017 Edition, whichever comes first). Only those Membrane Roof Systems that have successfully met the CSA Standard through tests administered by a qualified test facility/laboratory will qualify as an Accepted Membrane Roof System Assembly for a RoofStar Guarantee (see exceptions noted below). Click here for a list of qualified testing facilities.
When an existing roof system is removed to expose the deck, the replacement roof is considered new and therefore the design and construction of the replacement roof assembly must meet or exceed the minimum requirements set out by the CSA Standard. Furthermore, a replacement roof must conform to existing BC Building Code requirements for performance. For reference, see the BC Building Code (latest edition), Div. A, Part 1, Section 1.1 General (cf. 188.8.131.52. Application of this Code, and 184.108.40.206. Application to Existing Buildings).
When a Membrane Roof System incorporates materials that are not part of the RoofStar Guarantee Program, appropriate substitutions for those materials shall be made from the list of RoofStar-accepted Materials, and test data extrapolated in keeping with ANSI/SPRI WD-1, “Wind Design Standard Practice for Roofing Assemblies” (cf. National Building Code of Canada 2015, A-220.127.116.11.(4) Membrane Roofing Systems). The Design Authority, or in their absence, a Member, shall be responsible for the selection of appropriate material substitutions.
In addition to the above, the CSA Standard also applies to circumstances where “the roof membrane may be designated as the plane of air tightness.”
3 Partial Roof System Replacement
When only a part of a roof system is replaced, in which the roof deck is not exposed, the CSA Standard does not apply. Nevertheless, partial roof replacements shall still conform to all other RoofStar Guarantee requirements for roof system assembly set out in this Manual. Also refer to ANSI/SPRI WD-1, “Wind Design Standard Practice for Roofing Assemblies” (cf. National Building Code of Canada 2015, A-18.104.22.168.(4) Membrane Roofing Systems) and the Wind-RCI calculator. All components utilized in a partial roof replacement shall still be selected from the Accepted Materials section of this Manual.
The Standard does not apply to:
- Membrane Roof Systems with proven past performance for anticipated wind loads (National Building Code of Canada 2015, 22.214.171.124.5 Determination of Wind Load);
- Ballasted roof assemblies or roof assemblies with non-ballast overburdens, where the membrane is secured against wind uplift by the ballast/overburden (National Building Code of Canada 2015, A-126.96.36.199.(4) Membrane Roofing Systems)
See the 'decision tree' illustration below in conjunction with these requirements and guidelines.
5 Using Tested Assembly Reports
- Begin by determining the wind load of the roof using the online calculator available through Wind-RCI (click here for a sample report);
- Choose the method by which the roof will be fastened together:
- Expand the table for the membrane type and look for maximum wind uplift values that exceed the requirements determined through the RCI calculator;
(NOTE: while the reports show two different values for tested maximum assembly strength, the RPM tables display only the values adjusted for the required safety factor of 1.5)
- Open the report for the tested assembly that fits with the design parameters you have chosen and calculated, and examine the report. It will break down the assembly by its components. Ensure all the key components (Secondary materials, such as deck overlay, insulation and insulation overlay) are accepted by the RoofStar Guarantee Program (note that fasteners and adhesives currently are not listed in this Manual - use the materials specified in the wind test report, or consult the membrane manufacturer);
- In cases where a tested Secondary material is not accepted by the RoofStar Guarantee Program, look for alternates which often are published in the each report. In the absence of published alternates, locate a possible alternate in the Accepted Materials section of this Manual. If you are unsure about compatibility, contact the RCABC.
6 Roof Zones and Wind Dynamics
The roof area is divided into three areas (zones). The perimeter area is defined as 10% of the building width or 40% of the building height, whichever is less. In no case will perimeter area be less than 2.0 m (7'). The field area is defined as the remaining area after the perimeter is calculated.
When the roof height is:
- greater than 18 m (60’) and low-slope (slope is 2:12 or less) or
- less than 18 m (60’) and pitched (slope is greater than 2:12).
Perimeter fastening along the entire eave will be enhanced to corner requirements. The corner area is defined by the perimeter in both directions at the corners. In no case will the corner area be less than 2.0 m x 2.0 m (7’ x7’).
For corner enhancement of roofs in high wind locations (higher than 100 mph) or for roofs above 60’.
- Field Area:
- Zone 1
- Perimeter Area:
- Zone 2. The area at the edge of the roof. Perimeter width shall be defined as the lesser of 10% of the building width or 40% of the building height, but not less than 2.0 m (7’).
- Corner Area:
- Zone 3. Corner area dimensions shall be equal to perimeter width in both directions and not less than 2.0 m (7’).
- Wall Openings:
- Contribute to increased wind uplift on roof area. Included in RoofStar Guarantee Standards fastener requirements.
- Adjoining Roof Height:
- Where less than 900 mm (3’), design as if one roof. Where 900 mm (3’) or greater, upper roof should be designed as separate roof.
- Roof Slope:
- Wind uplift changes at all Zones as slope changes.
- Overhangs increase the wind uplift in some roof locations. Included in RoofStar Guarantee Standards fastener requirements.
- Roof Height:
- On slopes under 10° (2’ in 12’ nominal) use eave height for roof height. On slopes 10° (2’ in 12’ nominal) or greater, use mean average roof height (mid-point).
- On slopes over 10° (1:6 or 2" in 12", nominal) the ridge field area should be upgraded to Zone 2 (perimeter).
- Ridge Corner:
- On slopes over 10° (1:6 or 2" in 12", nominal) and on buildings 18 m (60’) high or less ridge perimeter at the corners should be upgraded to Zone 3 (corner).
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.