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Roof Joists vs.
Roof Rafters
Can you advise the difference between a Roof Joist and a Roof Rafter? If possible, in relation to the Ontario Building Code. I believe a Roof Joist has a finish other than OSB or Plywood on the bottom where a rafter has no finish.
The terms “roof joist” and “roof rafter” have long been used when roofs are constructed on-site from dimension lumber, as opposed to prefabricated roof trusses that are commonly used today.
The 1968 Canadian Wood Frame Construction Manual defined a rafter as “one of a series of structural members of a roof usually of 2-inch nominal thickness designed to support roof loads, but not ceiling finish.” Rafters are the usually sloped members that support the roof sheathing and finish.
A joist was defined as “One of a series of horizontal wood members, usually 2-inch nominal thickness, used to support a floor, ceiling, or roof.” Ceiling joists tie the roof together and usually support the ceiling finish and insulation.
According to the latest Wood Frame Construction Manual, “Where rafters also serve as ceiling joists, the term “roof joist” is used. Both roof and ceiling loads determine their size.” So, with flat roofs or cathedral ceilings, where the rafters support insulation and drywall, the member is both a rafter and a roof joist. However, as a roof joist, it is carrying extra load and may be bigger. The Wood Frame Manual also notes that “rafters chosen for structural adequacy may not, however, provide adequate depth for insulation and ventilation of the roof space. In such cases, wider lumber or engineered wood products should be used.”
The Ontario Building Code Table 9.23.3.4 lists nailing for framing of “roof rafter, roof truss, or roof joist.” -it depends on the roof design which one will be present at the roof-to-wall bearing location.
So, in summary: if it is the sloping part of a roof supporting the sheathing and roofing alone, it is a roof rafter. If it also supports insulation and ceiling finish, it’s a roof joist. If it is horizontal and ties the roof rafters together, as well as supporting the ceiling finishes,
it’s a ceiling joist.
How can one determine the Fire Resistance Rating (FRR) and Sound Transmission Coefficient (STC) ratings for wood stud shear walls that include plywood on one or both sides and are finished with gypsum? To achieve a 1-hour rating, is it possible to use an existing tested gypsum assembly to meet both FRR and STC requirements and simply add plywood without affecting the ratings? Is there a better way to determine the FRR and STC ratings in this scenario?
ASK AN EXPERT
Determining Fire Resistance Rating and Sound Transmission Coefficient
How can one determine the Fire Resistance Rating (FRR) and Sound Transmission Coefficient (STC) ratings for wood stud shear walls that include plywood on one or both sides and are finished with gypsum?
To achieve a 1-hour rating, is it possible to use an existing tested gypsum assembly to meet both FRR and STC requirements and simply add plywood without affecting the ratings?
Is there a better way to determine the FRR and STC ratings in this scenario?
As only a one-hour FRR is required, the component additive method (CAM) spelled out in the National Building Code (D-2.3, Appendix D) can be used. The CAM is used to determine the fire resistance rating of building components by adding together the fire resistance times assigned to individual components within an assembly.
CAM does not address shear panels, but a National Research Council Canada (NRC) report ‘Solution for mid-rise wood construction: full-scale standard fire resistance tests of wall assemblies for use in lower storeys of mid-rise buildings: report to Research Consortium for Wood and Wood-Hybrid Mid-Rise Buildings’ shows that adding an OSB shear membrane to a light wood frame wall assembly (test assembly #1) didn’t negatively affect the rating; it increased it, when comparing it to the wall assembly without any shear membrane (wall assembly #4).
Based on the results of these tests, we can assume that one can add a sheathing membrane to a wall with a 1-hour FRR without it negatively affecting the rating, and in some cases, it might even improve it a bit.
With respect to sound transmission, footnote 13 from the Part 9 Fire and Sounds tables indicates that adding sheathing reduces the STC rating.
NRC Report: Solution for mid-rise wood construction: full-scale standard fire resistance tests of wall assemblies for use in lower storeys of mid-rise buildings: report to Research Consortium for Wood and Wood-Hybrid Mid-Rise Buildings
Where bracing material, such as diagonal lumber or plywood, OSB, gypsum board or fibreboard sheathing is installed on the inner face of one row of studs in double stud assemblies, the STC rating will be reduced by 3 for any assemblies containing absorptive material in both rows of studs or in the row of studs opposite to that to which the bracing material is attached. Attaching such layers on both inner faces of the studs may drastically reduce the STC value but enough data to permit assignment of STC ratings for this situation is not available.
An outside engineering judgement is recommended to determine any affect that a shear panel will have on the assembly’s STC rating.
This is an example of a 1 hr FRR assembly that would also have a 45 STC:
1. Loadbearing wood stud framing at 400 mm o/c.
2. Wall filled with cellulous fibre insulation.
3. Resilient metal channels spaced at 600 mm o/c.
4. One layer of 15.9 mm type X gypsum board on the interior side.
Here is an option with 2 layers of gypsum board that has a 65 min FRR and 55 STC:
1. Loadbearing wood stud framing at 600 mm o/c.
2. Wall filled with glass fibre insulation.
3. Resilient metal channels spaced at 400 mm o/c.
4. Two layers of 12.7 mm type X gypsum board on the interior side.
1. Loadbearing wood stud framing at 600 mm o/c.
2. Wall filled with glass fibre insulation.
3. Resilient metal channels spaced at 400 mm o/c.
4. Two layers of 12.7 mm type X gypsum board on the interior side.
ASK AN EXPERT
Roof Joists vs.
Roof Rafters
Can you advise the difference between a Roof Joist and a Roof Rafter? If possible, in relation to the Ontario Building Code. I believe a Roof Joist has a finish other than OSB or Plywood on the bottom where a rafter has no finish.
The terms “roof joist” and “roof rafter” have long been used when roofs are constructed on-site from dimension lumber, as opposed to prefabricated roof trusses that are commonly used today.
The 1968 Canadian Wood Frame Construction Manual defined a rafter as “one of a series of structural members of a roof usually of 2-inch nominal thickness designed to support roof loads, but not ceiling finish.” Rafters are the usually sloped members that support the roof sheathing and finish.
A joist was defined as “One of a series of horizontal wood members, usually 2-inch nominal thickness, used to support a floor, ceiling, or roof.” Ceiling joists tie the roof together and usually support the ceiling finish and insulation.
According to the latest Wood Frame Construction Manual, “Where rafters also serve as ceiling joists, the term “roof joist” is used. Both roof and ceiling loads determine their size.” So, with flat roofs or cathedral ceilings, where the rafters support insulation and drywall, the member is both a rafter and a roof joist. However, as a roof joist, it is carrying extra load and may be bigger. The Wood Frame Manual also notes that “rafters chosen for structural adequacy may not, however, provide adequate depth for insulation and ventilation of the roof space. In such cases, wider lumber or engineered wood products should be used.”
The Ontario Building Code Table 9.23.3.4 lists nailing for framing of “roof rafter, roof truss, or roof joist.” -it depends on the roof design which one will be present at the roof-to-wall bearing location.
So, in summary: if it is the sloping part of a roof supporting the sheathing and roofing alone, it is a roof rafter. If it also supports insulation and ceiling finish, it’s a roof joist. If it is horizontal and ties the roof rafters together, as well as supporting the ceiling finishes,
it’s a ceiling joist.
Determining Fire Resistance Rating and Sound Transmission Coefficient
CAM does not address shear panels, but a National Research Council Canada (NRC) report ‘Solution for mid-rise wood construction: full-scale standard fire resistance tests of wall assemblies for use in lower storeys of mid-rise buildings: report to Research Consortium for Wood and Wood-Hybrid Mid-Rise Buildings’ shows that adding an OSB shear membrane to a light wood frame wall assembly (test assembly #1) didn’t negatively affect the rating; it increased it, when comparing it to the wall assembly without any shear membrane (wall assembly #4).
Based on the results of these tests, we can assume that one can add a sheathing membrane to a wall with a 1-hour FRR without it negatively affecting the rating, and in some cases, it might even improve it a bit.
With respect to sound transmission, footnote 13 from the Part 9 Fire and Sounds tables indicates that adding sheathing reduces the STC rating.
NRC Report: Solution for mid-rise wood construction: full-scale standard fire resistance tests of wall assemblies for use in lower storeys of mid-rise buildings: report to Research Consortium for Wood and Wood-Hybrid Mid-Rise Buildings
Where bracing material, such as diagonal lumber or plywood, OSB, gypsum board or fibreboard sheathing is installed on the inner face of one row of studs in double stud assemblies, the STC rating will be reduced by 3 for any assemblies containing absorptive material in both rows of studs or in the row of studs opposite to that to which the bracing material is attached. Attaching such layers on both inner faces of the studs may drastically reduce the STC value but enough data to permit assignment of STC ratings for this situation is not available.
How can one determine the Fire Resistance Rating (FRR) and Sound Transmission Coefficient (STC) ratings for wood stud shear walls that include plywood on one or both sides and are finished with gypsum? To achieve a 1-hour rating, is it possible to use an existing tested gypsum assembly to meet both FRR and STC requirements and simply add plywood without affecting the ratings? Is there a better way to determine the FRR and STC ratings in this scenario?
An outside engineering judgement is recommended to determine any affect that a shear panel will have on the assembly’s STC rating.
This is an example of a 1 hr FRR assembly that would also have a 45 STC:
Here is an option with 2 layers of gypsum board that has a 65 min FRR and 55 STC:
1. Loadbearing wood stud framing at 600 mm o/c.
2. Wall filled with glass fibre insulation.
3. Resilient metal channels spaced at 400 mm o/c.
4. Two layers of 12.7 mm type X gypsum board on the interior side.
ASK AN EXPERT
A:
As only a one-hour FRR is required, the component additive method (CAM) spelled out in the National Building Code (D-2.3, Appendix D) can be used. The CAM is used to determine the fire resistance rating of building components by adding together the fire resistance times assigned to individual components within an assembly.