Table One is an abbreviated version of Table R502.3.1(2), from the International Residential Code for One — and Two — Family Dwellings. This particular table, just one of many in IRC codebook, is for floor joist spans for common lumber species, and assumes that we are designing for a residential living area with a live load of 40 pounds per square foot (40 PSF), a dead (structural) load of 10 PSF and an allowable deflection of 1/360.
Table Two is an abbreviated form of Table R802.5.1(7), from the International Residential Code for One — and Two — Family Dwellings. This table is helpful in designing rafter spans for anticipated 70 PSF snow loads on a 20 PSF dead load.
Lets do a couple of exercises, using Table Two:
Example 1: If I want rafters to be two-feet (24") on-center, what depth of rafter will I need to accommodate 12-foot spans?
Procedure: Go to the bottom — 24" — portion of the chart and look for spans of 12 feet and over. Only six of the strongest 2" X 10" will do it, the four select structural grades of all woods plus the Douglas fir-larch #1 and the Southern pine, #1. With 2" X 12" rafters — the last column — twelve of the sixteen listed woods will do the trick. Only the #3 grades — the weakest stuff — will not.
Example 2: I’ve scored a great deal on some 2" X 10" Southern pine #1 rafters. What is the greatest rafter span I can support?
Procedure’. Go to the 2" x 10" column and look down until you spot Southern pine #1 for 16" and for 24"on-center spacings. At 16" centers, a span of 14 4" is possible. At 24" centers, the span drops to 13′ 1". These possible spans now need to be balanced against the size (length) of the desired building and the actual number of rafters “scored.” The complete version of these span tables, as the appear in the International Building Code, also includes rafter spacings of 12" and 19.2" (0.5 meter).
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Table 1: R502.3.1 (2) Floor Joist Spans for Common Lumber Species
|
|
Douglas fir-larch #1 |
8’8" |
11 ’0" |
13’5" |
15V" |
|
Douglas fir-larch #2 |
84" |
10’3" |
12-7" |
14’7" |
|
Douglas fir-larch #3 |
6’2" |
7’9" |
9’6" |
1Г0" |
|
Hemlock-fir SS |
8’6" |
11 ‘3" |
14-4- |
17’5" |
|
Hemlock-fir #1 |
8’4" |
10’9" |
134" |
15’2" |
|
Hemlock-fir #2 |
741" |
10’2" |
12-5" |
14-4» |
|
Hemlock-fir #3 |
6’2" |
7-9- |
9’6" |
1l-o" |
|
Southern pine SS |
840" |
11 ‘8" |
1441" |
184" |
|
Southern pine #1 |
8’8" |
11 ‘5" |
14’7" |
17’5" |
|
Southern pine #2 |
8’6" |
11 ‘0" |
134" |
15’5" |
|
Southern pine #3 |
6’7" |
8’5" |
941" |
1140" |
|
Spruce-pine-fir SS |
8-4- |
11 ‘0" |
14’0" |
17’0" |
|
Spruce-pine-fir #1 |
84" |
10’3" |
12-7- |
14V- |
|
Spruce-pine-fir #2 |
84" |
10’3" |
12-7" |
14V- |
|
Spruce-pine-fir #3 |
6’2" |
7-9- |
9’6" |
1l-o" |
Table 2: R802.5.1 (7) Rafter Spans for 70 PSF Ground Snow Load
Dead Load = 20 psf
2" x 4" 2" x 6" 2" x 8" 2" x 10" 2"x12"
Rafter Spacing Species & Grade (feet & inches)
(inches)
|
Douglas fir-larch SS |
6’10" |
10’3" |
13’0" |
1540" |
184" |
|
Douglas fir-larch #1 |
540" |
8’6" |
10’9" |
13’2" |
1513" |
|
Douglas fir-larch #2 |
5’5" |
741" |
104" |
124" |
14′ 3" |
|
Douglas fir-larch #3 |
4’1" |
6’0" |
7’7" |
94" |
10’9" |
|
Hemlock-fir SS |
6 6" |
104" |
12’9" |
157" |
18’0" |
|
Hemlock-fir #1 |
5’8" |
8’3" |
10’6" |
1240" |
1440" |
|
Hemlock-fir #2 |
54" |
740" |
941" |
124" |
144" |
|
Hemlock-fir #3 |
44" |
6’0" |
7’7" |
94" |
10’9" |
|
Southern pine SS |
6’9" |
10’7" |
14’0" |
1740" |
21 ‘0" |
|
Southern pine #1 |
6’5" |
9’7" |
12’0" |
144" |
174" |
|
Southern pine #2 |
540" |
84" |
10’9" |
1240" |
154" |
|
Southern pine #3 |
4’4" |
6’5" |
8’3" |
9’9" |
117" |
|
Spruce-pine-fir SS |
64" |
9’6" |
12’0" |
14’8" |
174" |
|
Spruce-pine-fir #1 |
5’5" |
741" |
104" |
124" |
14’3" |
|
Spruce-pine-fir #2 |
5’5" |
741" |
104" |
124" |
14’3" |
|
Spruce-pine-fir #3 |
44" |
6’0" |
7’7" |
94" |
10’9" |
|
Douglas fir-larch SS |
6’5" |
94" |
1140" |
14’5" |
16’9" |
|
Douglas fir-larch #1 |
54" |
7’9" |
940" |
12’0" |
1341" |
|
Douglas fir-larch #2 |
5’0" |
7’3" |
9’2" |
1T3" |
13’0" |
|
Douglas fir-larch #3 |
3’9" |
5’6" |
641" |
8’6" |
940" |
|
Hemlock-fir SS |
64" |
97" |
11 ‘8" |
147" |
15’5" |
|
Hemlock-fir #1 |
5’2" |
7’7" |
97" |
1Г8" |
137" |
|
Hemlock-fir #2 |
441" |
77" |
94" |
114" |
1240" |
|
Hemlock-fir #3 |
3’9" |
5’6" |
641" |
8’6" |
940" |
|
Southern pine SS |
64" |
10’0" |
13’2" |
16’5" |
197" |
|
Southern pine #1 |
541" |
8’9" |
1V0" |
134" |
157" |
|
Southern pine #2 |
54" |
7’7" |
940" |
1T9" |
13’9" |
|
Southern pine #3 |
4’0" |
541" |
7’6" |
840" |
107" |
|
Spruce-pine-fir SS |
541" |
8’8" |
11 ‘0" |
1315" |
157" |
|
Spruce-pine-fir #1 |
5’0" |
7’3" |
92" |
1T3" |
13’0" |
|
Spruce-pine-fir #2 |
5’0" |
7’3" |
9’2" |
1T3" |
13’0" |
|
Spruce-pine-fir #3 |
3’9" |
5’6" |
641" |
8’6" |
940" |
The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method
of resisting the outward push of the rafters on the bearing walls, such as rafter ties, is provided at that location.
• Books. Seven of the books in the Bibliography have useful span tables, indicated by the notation (ST) before the entry The International Residential Code for One — and Two-Family Dwellings has span tables that you know will be approved by code.
• The Internet. It is possible to find all sorts of span tables on the world wide web. I searched for span + tables on the popular Google search engine and came up with these excellent websites, among others:
www. southernpine. com/. This is the Southern Pine Council’s website. Click on “Span Tables” for a list of over 40 different floor joist, ceiling joist, and roof rafter span tables using various grades of southern pine. Very comprehensive.
www. cwc. ca/design/design_tools/calcs/SpanCalc2002/index. php/. This is the Canadian Wood Councils Span Calc 2000 program. You can select the member type (rafter, floor joist, etc.), species of wood, grade, dimensions, spacing, and loads. Press “calculate” and the program instantly returns the maximum span. Despite being a Canadian website, the SpanCalc results are only valid in the United States.
www. wwpa. org/. This is the Western Wood Products comprehensive website. Click on “Span Tables Online,” then “Individual Span Tables” and you will have access to dozens of span tables for rafters, floor joists, and ceiling joists. Various western wood species and grades are covered. You will need Acrobat Reader to download these.
