Section 321.27. Roof design and framing.

 

(1)   Structural design.

(a) General. Roof and roof-ceiling assemblies shall support all dead loads plus the minimum live loads under par. (c) and s. SPS 321.02 .

(b) Applicability of tables. The joist and rafter tables in ch. SPS 325 Appendix A are valid for roofs with a minimum slope of 3 in 12. Lesser slopes require engineering analysis or shall be provided with a ridge beam.

(c) Sloped roof snow loads. Snow loads specified in s. SPS 321.02 (1) (b) 2. may be reduced for roof slopes greater than 30 ° by multiplying the snow load by Cs. The value of Cs shall be determined by the following: - See PDF for diagram

where a is the slope of the roof expressed in degrees.

(2)   Lateral restraint of walls. Provisions shall be taken to absorb the horizontal thrust produced by a sloping roof through the use of wall ties, ceiling joists, beams at the ridge or at the wall or a system designed through structural analysis.

(3)   Uplift and suction forces.

(a) General.

1. Roofs shall withstand a pressure of at least 20 pounds per square foot acting upward normal to the roof surface.

2. Roof overhangs, eaves, canopies and cornices shall withstand an upward wind pressure of at least 20 pounds per square foot applied to the entire exposed area.

(b) Anchorage.

1. Roof framing members spanning more than 6 feet measured from the outermost edge of the roof shall be permanently fastened to the top plate of load bearing walls using engineered clips, straps or hangers.

2. Roof framing members spanning 6 feet or less measured from the outermost edge of the roof shall be permanently fastened to the top plate of load bearing walls using toe-nailing or engineered clips, straps or hangers.

(4)   Roof rafters.

(a) General.

1. Rafters shall be notched to fit the exterior wall plate and fastened to the wall.

2. Collar ties shall be installed on the upper third of every third pair of abutting roof rafters or every 48 inches, whichever is less.

(b) Ridge boards.

1. Where rafters meet to form a ridge, the rafters shall be attached to a ridge board.

2. The ridge board shall have a depth at least equal to the length of the cut end of the rafter abutting it.

3. Where all rafters are placed directly opposite each other or are offset at the ridge board by less than the thickness of the rafter, the ridge board shall have a nominal thickness of at least 1 inch.

4. Where one or more rafters are offset at the ridge board by more than the thickness of the rafter, the ridge board shall have a nominal thickness of at least 2 inches.

(c) Ridge beams. Rafters shall be attached to ridge beams using engineered clips, straps or hangers or the connection shall be designed through structural analysis.

(d) Bearing. The required bearing for wood rafters shall be in accordance with the NDS adopted in Table 320.24-6m, except in no case shall the bearing be less than 1 1/2 inches on wood or metal or less than 3 inches on masonry or concrete.

(e) Ladders.

1. Overhangs at gable end walls of more than 12 inches shall be provided with ladders which extend into the structure a distance no less than the length of the overhang.

2. The ladders shall be fastened at the wall.

3. The interior end of each ladder shall be attached to a rafter or truss with a hanger.

(5)   Ceiling joists.

(a) Ceiling joists shall be nailed to exterior walls and to the ends of rafters.

(b) Ends of ceiling joists shall be lapped at least 3 inches and be fastened either with 3-16d nails or in accordance with the floor joist requirements under s. SPS 321.22 (4) (a) 1. d.

(c) Where ceiling joists are placed at right angles to the rafters, the lookout joist or ties shall be fastened to the parallel ceiling joists or rafters using engineered clips, straps or hangers or the connection shall be designed through structural analysis.

(6)   Valley and hip rafters.

(a) Valley rafters.

1. Where no bearing is provided under valley rafters at the intersection of 2 roof areas, the valley rafters shall be doubled in thickness and shall be at least 2 inches deeper than the required common rafter to permit full bearing at the beveled end.

2. Where ridges are provided at different elevations, vertical support shall be provided for the interior end of the lower ridge board or ridge beam.

(b) Hip rafters. Where no bearing is provided under hip rafters, the hip rafters shall be of the same thickness as common rafters and shall be at least 2 inches deeper than required to permit full contact with the jack rafter.

(7)   Roof trusses.

(a) Metal plate connected wood roof trusses shall be designed in accordance with TPI 1 and the NDS adopted under s. SPS 320.24 .

(b) Truss members shall not be cut, bored or notched, except as allowed under sub. (8) (d) .

(c) If connection is provided to stabilize a non-load bearing wall, a slotted expansion joint or clip shall be used.

(8)   Notching and boring.

(a) General.

1. Notching and boring of beams or girders is prohibited unless determined through structural analysis.

2. Notching and boring of ceiling joists and rafters shall comply with pars. (b) and (c) .

(b) Notching.

1. Notches located in the top or bottom of ceiling joists and rafters are prohibited from all of the following:

a. Having a depth exceeding 1 / 6 the depth of the member.

b. Having a length exceeding 1 / 3 the depth of the member.

c. Being located in the middle 1 / 3 of the span of the member.

2. Where ceiling joists or rafters are notched at the ends, the notch may not exceed ¼ the depth of the member.

3. Bird mouth cuts may not exceed 1 / 3 the depth of the rafter unless the seat cut bears fully on the wall plate.

(c) Boring.

1. Holes bored within 2 inches of the top or bottom of ceiling joists or rafters may not be located in the middle 1 / 3 of the span of the member.

2. The diameter of a hole may not exceed 1 / 3 the depth of the member.

3. A hole may not be bored within 2 inches of a notch or another hole.

4. The distance between adjacent holes may not be less than the diameter of the larger hole.

(d) Engineered wood products. Notching or boring of engineered wood products shall be done in accordance with the manufacturer's instructions provided those instructions were developed through structural analysis or product testing.

(9)   Roof sheathing, boards and planking.

(a) Structural sheathing. The allowable loads and spans for structural sheathing shall be in accordance with the grade stamp on the panel.

(b) Roof boards.

1. Where the rafter spacing is 24 inches on center or less, roof boards may be used that have a minimum thickness of 5 / 8 -inch for solid sheathing and 3 / 4 -inch for spaced sheathing.

2. Where the rafter spacing is greater than 24 inches on center, roof boards shall be tongue and groove, at least 1.5 inches thick.

(c) Roof planks.

1. Roof planks shall be tongue and groove or splined and at least 2 inches, nominal, in thickness.

2. Planks shall terminate over beams unless the joints are end matched.

3. The planks shall be laid so that no continuous line of joints will occur except at points of support.

4. Planks shall be nailed or fastened to each beam.

Cr. Register, November, 1979, No. 287 , eff. 6-1-80; am (3) (a), Register, January, 1989, No. 397 , eff. 2-1-89; r. and recr. (1), am. (3) (a), Register, March, 1992, No. 435 , eff. 4-1-92; r. and recr. (3) (a), Register, November, 1995, No. 479 , eff. 12-1-95; r. and recr. (3) (a) 1. and 2. c., Register, January, 1999, No. 517 , eff. 2-1-99; am. (3) (a) 1. a., Register, March, 2001, No. 543 , eff. 4-1-01; CR 02-077 : r. and recr. (3) (b) Register May 2003 No. 569 , eff. 8-1-03; CR 08-043 : r. and recr. Register March 2009 No. 639 , eff. 4-1-09; correction in (1) (a), (c), (4) (d), (5) (b), (7) (a) made under s. 13.92 (4) (b) 7. , Stats., Register December 2011 No. 672 ; CR 15-041 : am. (4) (d) Register December 2015 No. 720 , eff. 1-1-16.

Note

A roof pitch of 7 in 12 is equal to 30 ° . Microsoft Windows NT 6.1.7601 Service Pack 1 For information on toe nailing, see the fastener schedule table in the ch. SPS 325 Appendix A . Microsoft Windows NT 6.1.7601 Service Pack 1 Collar ties are intended to provide stability to the roof at the ridge. Lateral restraint for the walls must be provided in accordance with sub. (2). Microsoft Windows NT 6.1.7601 Service Pack 1 For the purposes of this section, a ladder is defined as a perpendicular projection extending beyond the face of the wall below. Microsoft Windows NT 6.1.7601 Service Pack 1 See the fastener table in the ch. SPS 325 Appendix A for a nailing schedule for ceiling joists. Microsoft Windows NT 6.1.7601 Service Pack 1