Okla. Admin. Code § 748:20-6-13

Current through Vol. 42, No. 3, October 15, 2024

Section 748:20-6-13 - IRC[®] 2018 Chapter 8 Roof-Ceiling Construction

Chapter 8 of the IRC® 2018 is adopted with the following modifications:

(1) Section R801.3 Roof drainage. This section has been stricken from the code.

(2) Section 802.3 Ridge. This section has been modified to clarify a ridge beam shall be designed in accordance with acceptable engineer practices when the roof load exceeds specific criteria to carry one-half of the tributary load. This section has been modified to read: R802.3 Ridge. A ridge board used to connect opposing rafters shall be not less than 1 inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. Where ceiling joists or rafter ties do not provide continuous ties across the structure, a ridge beam shall be designed, in accordance with acceptable engineer practices, and supported on each end of the wall or girder, when roof loads exceed 10 per square foot pound dead loads and 20 per square foot pound live loads. In the case where rafters are used to support roof and finished ceiling (also known as a cathedral ceiling) with no connection of opposing rafters, ridge beam shall be designed, in accordance with acceptable engineer practices to carry one half of tributary load of the roof.

(3) Section R802.4.1 Rafter size. This section has been modified to provide guidance for builders framing rafters above the top sill of the wall system and provide an exception to require collar ties to be sized not less than the required size of the rafters they are connected to. This section has been modified to read: 802.4.1 Rafter size. Rafters shall be sized based on the rafter spans in Tables R802.4.1(1) through R802.4.1(8). Rafter spans shall be measured along the horizontal projection of the rafter. For other grades and species and for other loading conditions, refer to the AWC STJR. The tabulated rafter spans in Tables R802.4.1(1) through R802.4.1(8) assume ceiling joists are located at the bottom of the attic space or some other method of resisting the outward push of the rafter on the bearing walls, such as rafter ties is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter span in these tables shall be multiplied by the following rafter reduction factors: Where ceiling joists or rafter ties are located at one third the span of the rafter the adjustment factor is 0.67, at one quarter of the span of the rafter the rafter adjustment factor is 0.76, at one fifth the span of the rafter the adjustment factor is 0.83, at one sixth of the span of the rafter, the adjustment factor is 0.90, and at two fifteenths of the rafter or less, there is no need for adjusting the rafter capacity. Exception: Collar Ties. Installation of the collar ties to reduce the span of the rafters is permitted as shown in Figure R802.4.5. Collar ties shall be installed at every rafter and sized not less than the required size of the rafters they are connected.

(4) Section R802.4.2 Framing details. This section has been modified to change the rafter framing details. This section has been modified to read: R802.3 Framing details.

(A) Rafters shall be framed to a ridge board or to each other with a collar tie, gusset plate or ridge strap in accordance with Table R602.3(1). Rafters shall be nailed to the top wall plates in accordance with Table R602.3(1) unless the roof assembly is required to comply with the uplift requirements of Section R802.11. Ridge board shall not be less than 1-inch (25 mm) nominal thickness and not less in depth and one size greater than the rafters attached to it.

(B) Where a 1-inch (25 mm) nominal thickness ridge is used, all rafters shall be framed not more than 1.5 inches (38 mm) offset from each other at the ridge board or if no ridge is used they should be framed directly opposite from each other with a gusset plate as a tie. When a nominal 2-inch rafter is used they may be offset with no limitations.

(5) Section 802.4.3 Hips and valleys. This section has been modified to provide an exception for the use of a "Blind Valley," and provide a definition of a brace. This section has been modified to read: R802.4.3 Hips and Valleys. Hip and valley rafters shall be not less than 2 inches (51 mm) nominal in thickness and not less in depth than the cut end of the rafter. Hip and valley rafters shall be supported at the ridge by a brace to a bearing partition, or beam, or be designated to carry and distribute the specific load at that point. Exception: The use of a "Blind Valley" also known as a "Farmers Valley" or "California Valley" will be allowed. In this type of valley, the main roof is framed as usual, it may or may not be sheathed and the intersecting roof is framed on top of the main roof. The two valley plates or sleeps lie on the top of the main roof rafters or sheathing and provide a nailing base for the jack rafters and the ridge board of the intersecting roof. A definition of a brace includes:

(A) Item 1. A triangular configuration of framing members with a horizontal tie and rafter members.

(B) Item 2. King post or similar.

(6) Section R802.5.1 Purlins. This section has been modified to include an exception for spacing the braces at not more than 6 feet (1829 mm) when certain conditions are met. The section has been modified to read: R802.5.1 Purlins. Installation of purlins to reduce the span of rafters is permitted as shown in Figure R802.5.1. Purlins shall be sized not less than the required size of the rafters that they support. Purlins shall be continuous and shall be supported by 2-inch by 4-inch (51 mm by 102 mm) braces installed to bearing walls at a slope not less than 45 degrees (0.79 rad) from the horizontal. The braces shall be spaced not more than 4 feet (1219 mm) on center and the unbraced length of the braces shall not exceed 8 feet (2438 mm). Exception: Braces may be spaced not more than 6 feet (1829 mm) on center if:

(A) Item 1. The purlin brace is 2-inch by 6-inch (51 mm by 153 mm),

(B) Item 2. Purlins shall be sized one nominal size larger than the rafter they support, and;

(C) Item 3. Unbraced length of braces shall not exceed 8 feet (2438 mm).

(7) Section R802.5.2 Ceiling joist and rafter connections. This section has been modified to reflect current framing practices. This section has been modified to read: R802.3.1 Ceiling joists and rafter connections. Where ceiling joists run parallel to rafters, they shall be connected to rafters at the top wall plate in accordance with Table R802.5.2. Where ceiling joists are not connected to the rafters at the top of the wall plate, they shall be installed in the bottom third of the rafter height in accordance with Figure R802.4.5 and Table R802.5.2 Where ceiling joists do not run parallel to the rafters, the ceiling joists shall be connected to top plates in accordance with Table R602.3(1). Each rafter shall be tied across the structure with a rafter tie spaced 4 ft (1219 mm) on center. Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineer practices.

(8) Section R802.7.1.2 Ceiling joist taper cut. This section has been modified to include an exception to the section requirements for ceiling joists not carrying more than a 25-pound live load for limited attic storage. This section has been modified to read: R802.7.1.2 Ceiling joists taper cut. Taper cuts at the ends of the ceiling joists shall not exceed one-fourth the depth of the member in accordance with Figure R802.7.1.2. Exception: For ceiling joists not carrying more than 25 pounds of live load (limited attic storage) then taper cut at end of joist may be able to be increased to D/2.

(9) Section R806.5 Unvented attic and unvented enclosed rafter assemblies. This section has been modified to provide guidance on where the air permeable and impermeable insulation should be installed and provide exceptions to the section related to when air supply to the attic is not required. This section has been modified to read: R806.5 Unvented attic and unvented enclosed rafter assemblies. Unvented attics and unvented enclosed roof framing assemblies created by ceilings that are applied directly to the underside of the roof framing members and structural roof sheathing applied directly to the top of the roof framing members/rafters, shall be permitted where all of the following conditions are met:

(A) Item 1. The unvented attic space is completely within the building thermal envelope.

(B) Item. 2. Interior Class I vapor retarders are not installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof assembly.

(C) Item 3. Where wood shingles or shakes are used, a minimum 1/4 -inch (6.4 mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.

(D) Item 4. In climate Zones 5, 6, 7, and 8, any air-impermeable insulation shall be a Class II vapor retarder or shall have a Class II vaper retarder coating or covering in direct contact with the underside of the insulation.

(E) Item 5. Insulation shall comply with Item 5.3 and either Item 5.1 or 5.2:

(i) Item 5.1. Item 5.1.1, 5.1.2, 5.1.3, or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.

(I) Item 5.1.1 Where only air-impermeable insulation is provided it shall be applied in direct contact with the underside of the structural roof sheathing.

(II) Item 5.1.2 Where air-permeable insulation is installed directly below the structural sheathing, ridge board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control.

(III) Item 5.1.3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

(IV) Item 5.1.4 Alternatively, sufficient ridge board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45 degrees Fahrenheit (7 degrees Celsius). For calculation purposes, an interior air temperature of 68 degrees Fahrenheit (20 degrees Celsius) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.

(ii) Item 5.2. In Climate Zones 1, 2, and 3, air-permeable insulation installed in unvented attics shall meet the following requirements:

(I) Item 5.2.1. An approved vapor diffusion port shall be installed not more than 12 inches (305 mm) from the highest point of the roof, measured vertically from the highest point of the roof to the lower edge of the port.

(II) Item 5.2.2 The port area shall be greater than or equal to 1:600 of the ceiling area. Where there are multiple ports in the attic, the sum of the port areas shall be greater than or equal to the area requirement.

(III) Item 5.2.3 The vapor-permeable membrane in the vapor diffusion port shall have a vapor permeance rating of greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96.

(IV) Item 5.2.4. The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the house.

(V) Item 5.2.5. The vapor diffusion port shall protect the attic against the entrance of rain and snow.

(VI) Item 5.2.6. Framing members and blocking shall not block the free flow of water vapor to the port. Not less than a 2-inch (51 mm) space shall be provided between any blocking and the roof sheathing. Air-permeable insulation shall be permitted within that space.

(VII) Item 5.2.7. The roof slope shall be greater than or equal to 3:12 (vertical/horizontal)

(VIII) Item 5.2.8. Where only air-permeable insulation is used, it shall be installed directly below the structural roof sheathing, on top of the attic floor, or on top of the ceiling.

(IX) Item 5.2.9. Air-impermeable insulation, where used in conjunction with air- permeable insulation, shall be directly above or below the structural roof sheathing and is not required to meet the R-value in Table 806.5. Where directly below the structural roof sheathing, there shall be no space between the air-impermeable insulation and the air-permeable insulation.

(X) Item 5.2.10. Where air-permeable insulation is used and is installed directly below the roof structural sheathing, air shall be supplied at a flow rate greater than or equal to 50 CFM (23.6 L/s) per 1,000 square feet (93 square meters) of ceiling. The air shall be supplied from ductwork providing supply air to the occupiable space when the condition system is operating. Alternatively, the air shall be supplied by a supply fan when the conditioning system is operating. Exceptions:

1. Where both air-impermeable and air-permeable insulation are used, and the R- value in Table R806.5 is met, air supply to the attic is not required.

2. Where only air-permeable insulation is used and is installed in top of the attic floor, or on top of the ceiling, air supply to the attic is not required.

(iii) Item 5.3. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.

Okla. Admin. Code § 748:20-6-13

Amended and renumbered from 748:20-5-11 by Oklahoma Register, Volume 39, Issue 24, September 1, 2022, eff. 9/14/2022