Ceiling Joist and and Rafter Framing Details

Ridge, Valley, and Hip Rafter Framing

1. Install at least () a 1 inch (nominal thickness) ridge board at roof ridges. Install a ridge board that is at least () as deep as the (plumb) cut end of the rafter. Install rafters directly across from each other at the ridge board. You may omit the ridge board if you secure the rafters to each other with a gusset plate.

2. Install at least a () 2 inch (nominal thickness) hip rafter and valley rafter at all hips and valleys, including valleys formed when one roof is framed on top of another.

3. Support hip and valley rafters at the ridge with a brace to a load-bearing wall, or design the hip and valley rafters to bear the load at the ridge.

4. Toe nail rafters to the ridge and to valley and hip rafters using at least () four 16d nails, or face nail rafters using at least () three 16d nails. Toe nail rafters to the top plate using at least () two 16d nails.

5. Design and support ridge, hip and valley rafters as beams, when the roof pitch is less than (<) 3/12.

6. Design and support the ridge as a beam and design the walls supporting the ridge board to bear the ridge board load, when framing cathedral and vaulted ceilings without ceiling joists and rafter ties.

Purlins

1. You may use purlins to support rafters that would otherwise span a greater distance than allowed by the IRC. Example: a properly installed purlin at the center of an 18 feet long rafter would allow you to use 9 feet as the rafter span distance.

2. Install purlins that are at least () the same depth as the rafters they support. Example: use a 2×6 purlin to support a 2×6 rafter.

3. Use at least () one 2×4 purlin brace to carry the purlin load to a load-bearing wall. The purlin brace length should not exceed () 8 feet without additional bracing (usually an additional 2×4 nailed to the brace). Purlin braces should bear on a load-bearing wall and may not slope at less than (<) a 45-degree angle from horizontal. Space the purlin braces not more than () 4 feet apart.

Ceiling Joist Nailing to Rafter

1. Toe nail all rafters to the top plate using at least () two 16d nails regardless of whether the rafter is or is not parallel to the ceiling joist.

2. Connect one rafter to one ceiling joist when the ceiling joists are parallel to the rafters. Face nail these ceiling joists to rafters using the quantity of 16d common nails or 40d box nails indicated in Table R802.3-1.

3. Connect one rafter to one rafter tie and connect the rafter tie to the corresponding rafter on the other side of the roof when the ceiling joists are not parallel to the rafters. Install the rafter ties as close to the ceiling joists as practical. Face nail rafters ties to rafters using the quantity of 16d common nails or 40d box nails indicated in Table R802.3-1. Use 2×4 or larger lumber for rafter ties. Connecting rafters to ceiling joists or to rafter ties helps the walls resist outward thrust pressure from the roof.

4. Lap ceiling joists that meet over interior walls or beams at least () 3 inches and face nail using same quantity and type of nails indicated in Table R802.3-1. Apply this requirement when the ceiling joists are designed to resist rafter lateral thrust. This requirement usually applies when the ceiling joists are attached to the rafters.

5. You may butt the ends of ceiling joists together over interior walls or beams fasten the ceiling joists as indicated in Table R602.3-3 if the ceiling joists are not designed to resist rafter lateral thrust. This exception usually applies when the ceiling joists are not attached to the rafters.

Ceiling Joist Nailing to Rafter Exceptions

1. Note the following exceptions to the quantity of nails required in Table R802.3-1:

(a) you may reduce the required quantity of nails by 25% if the nails are clinched (the pointed ends sticking out from the wood are bent over),

(b) you need not use Table R802.3-1 if you support the ridge board on load-bearing walls or if you design and support the ridge as a beam,

(c) you may use a smaller roof span column in Table R802.3-1 if you install purlins to support the rafters, Example: if you install purlins at the center of rafters with a roof span of 24 feet, you may reduce the roof span by 50% and use the 12 feet roof span column,

(d) you may reduce the actual rafter slope by one-third if you substitute rafter ties for ceiling joists, Example: if the actual rafter slope is 9/12, use 6/12 as the adjusted rafter slope, but because there is no 6/12 slope row, use the nearest more conservative 5/12 row,

(e) increase the quantity of nails in the table if the ceiling joists or rafter ties are not located at the bottom of the attic space, and use Table 802.3-2 to calculate the nail quantity increase. Refer to the Rafter Span Measurement and Span Adjustment for Ceiling Joist Location section for explanations and examples of how to calculate values for Table 802.3-2.

Collar Ties

1. Install 1×4 or larger collar ties or use a 1 ¼ inch x 20 gage ridge strap between rafters to resist ridge uplift by wind force. You may omit collar ties in vaulted and cathedral ceilings when you design and support the ridge as a beam.

2. Space collar ties not more than () 4 feet on center.

3. Locate collar ties in the upper one-third of the attic space.

4. Connect collar ties and rafters using at least () three 10d nails, face nailed.

5. Verify collar tie requirements with the local building official and with the Wood Frame Construction Manual in high wind design areas, and when the roof slope is less than (<) 3/12, and when the roof span is greater than (>) 36 feet.

Rafter and Ceiling Joist Bearing on Support

1. Install rafters and ceiling joists with at least () 1 ½ inches of the rafter or joist bearing on supporting wood members( such as a top plate or a valley rafter) and at least () 3 inches of the rafter or joist bearing on masonry or concrete.

2. Toe nail rafters to the top plate using at least () two 16d nails.

3. Install sill plates with a bearing area of at least () 48 square inches when the sill plate bears on concrete or masonry. This means that the sill plate should have at least 48 square inches of surface area in contact with the masonry or concrete.

Rafter and Ceiling Joist Bridging and Lateral Support

1. Install bridging on rafters and ceiling joists deeper than (>) 2×12 (6 to 1 depth to thickness ratio). Space bridging not more than () every 8 feet. Bridging should consist of solid full depth blocking, wood or metal diagonal bridging, or by nailing at least () a 1×3 wood strip to each rafter or ceiling joist. Bridging is required by some building officials on ceiling joists smaller than 2×12.

2. Install lateral support at bearing points (such as exterior walls and interior support walls) on rafters and ceiling joists deeper than (>) 2×10 (5 to 1 depth to thickness ratio).

Rafter Connection to Walls

1. Attach rafters to the supporting walls using either 3-16d box nails (3½ in. x 0.135 in.) or 3-10d common nails (3” x 0.148 in.). Toe nail the rafters to the wall. This applies when: (a) the rafter is attached to a parallel ceiling joist; and when either (b) roof uplift force is not more than () 200 pounds and the rafters are spaced not more than () 24 inches on center; or when (c) the wind speed is not more than () 90 mph., and the wind exposure category is B and when the roof span is not more than () 32 feet, and the roof pitch is at least () 5:12.

2. Refer to the IRC or engineered plans for wall attachment requirements when the limitations in #1 above do not apply.