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Section SPS 321.27 allows reduction of the snow live load for roof slopes greater than 30 degrees (7/12 slope) based on the formula Cs = 1 - (a-30)/40, where “a" is the slope of the roof expressed in degrees. Following is a table of tabulated values for certain roof slopes. - See PDF for table PDF
Use the Design Value tables following the Span Tables to determine the acceptable species and grades to satisfy minimum Fb values obtained from the Span Tables. The Design Value tables assume at least three members spaced no more than 24‶ on center. Use the Normal Duration column Fb values for joists and the Snow Loading column Fb values for rafters.
See the following examples for further guidance.
chap_SPS325_APP_B
Tables are reprinted courtesy of American Forest & Paper Association. - See PDF for table PDF
*Deflection criteria are optional. For roof rafters with drywall on the underside, use the stricter L/240 tables to limit deflection.
Example 1. Floor Joists. Assume a required single span of 12‵-9‶, dead load of 10 psf and joists spaced 16‶ on center. Table F-2 (see following highlighted tables) shows that one solution is a grade of 2x8 having an Fb value of 1255 would allow a span of 12‵-10‶ which satisfies the condition. (Note that the recommended E value to limit deflection would be 1,600,000.) Going to the Design Value Tables, we find that as an example, 2x8 Hem Fir grade No.1 has an Fb value of 1310 for normal duration. (It also has an E value of 1,500,000 which does not satisfy the recommended deflection criteria.)
Example 2. Rafters. Assume a horizontal projected span of 13‵-0‶, a live load of 40 psf, dead load of 10 psf, a roof slope of 4/12 and rafters spaced 16‶ on center. Since the slope is shallower than 7/12, there is no allowable reduction of the snow live load. Table R-3 shows that a 2x8 having an Fb value of 1300 would allow a span of 13‵-1‶ which satisfies the condition. (Note that the recommended E value to limit deflection would be 1,120,000.) Going to the Design Value Tables, we find that as an example, 2x8 Douglas Fir-Larch grade No.2 has an Fb value of 1390 for snow loading. (It also has an E value of 1,600,000 which satisfies the recommended deflection criteria.)
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- See PDF for diagram PDF
- See PDF for diagram PDF
- See PDF for diagram PDF
chap_SPS325_APP_C
TABLE F- 2
FLOOR JOISTS WITH L/360 DEFLECTION LIMITS
DESIGN CRITERIA:
Deflection - For 40 psf live load.
Limited to span in inches divided by 360.
Strength - Live load of 40 psf plus dead load
of 10 psf determines the required bending design value.
Joist  
Size   Spacing
(in)   (in)                 Modulus of Elasticity, E, in 1,000,000 psi
    0.8   0.9   1.0   1.1   1.2   1.3   1.4   1.5   1.6   1.7   1.8   1.9   2.0   2.1   2.2   2.3   2.4
  12.0   8- 6   8-10   9- 2   9- 6   9- 9   10- 0   10- 3   10- 6   10- 9   10-11   11- 2   11- 4   11- 7   11- 9   11-11   12- 1   12- 3
  16.0   7- 9   8- 0   8- 4   8- 7   8-10   9- 1   9- 4   9- 6   9- 9   9-11   10- 2   10- 4   10- 6   10- 8   10-10   11- 0   11- 2
2x 6   19.2   7- 3   7- 7   7-10   8- 1   8- 4   8- 7   8- 9   9- 0   9- 2   9- 4   9- 6   9- 8   9-10   10- 0   10- 2   10- 4   10- 6
  24.0   6- 9   7- 0   7- 3   7- 6   7- 9   7-11   8- 2   8- 4   8- 6   8- 8   8-10   9- 0   9- 2   9- 4   9- 6   9- 7   9- 9
  12.0   11- 3   11- 8   12- 1   12- 6   12-10   13- 2   13- 6   13-10   14- 2   14- 5   14- 8   15- 0   15- 3   15- 6   15- 9   15-11   16- 2
  16.0   10- 2   10- 7   11- 0   11- 4   11- 8   12- 0   12- 3   12- 7   12-10   13- 1   13- 4   13- 7   13-10   14- 1   14- 3   14- 6   14- 8
2x 8   19.2   9- 7   10- 0   10- 4   10- 8   11- 0   11- 3   11- 7   11-10   12- 1   12- 4   12- 7   12-10   13- 0   13- 3   13- 5   13- 8   13-10
  24.0   8-11   9- 3   9- 7   9-11   10- 2   10- 6   10- 9   11- 0   11- 3   11- 5   11- 8   11-11   12- 1   12- 3   12- 6   12- 8   12-10
  12.0   14- 4   14-11   15- 5   15-11   16- 5   16-10   17- 3   17- 8   18- 0   18- 5   18- 9   19- 1   19- 5   19- 9   20- 1   20- 4   20- 8
  16.0   13- 0   13- 6   14- 0   14- 6   14-11   15- 3   15- 8   16- 0   16- 5   16- 9   17- 0   17- 4   17- 8   17-11   18- 3   18- 6   18- 9
2x10   19.2   12- 3   12- 9   13- 2   13- 7   14- 0   14- 5   14- 9   15- 1   15- 5   15- 9   16- 0   16- 4   16- 7   16-11   17- 2   17- 5   17- 8
  24.0   11- 4   11-10   12- 3   12- 8   13- 0   13- 4   13- 8   14- 0   14- 4   14- 7   14-11   15- 2   15- 5   15- 8   15-11   16- 2   16- 5
  12.0   17- 5   18- 1   18- 9   19- 4   19-11   20- 6   21- 0   21- 6   21-11   22- 5   22-10   23- 3   23- 7   24- 0   24- 5   24- 9   25- 1
  16.0   15-10   16- 5   17- 0   17- 7   18- 1   18- 7   19- 1   19- 6   19-11   20- 4   20- 9   21- 1   21- 6   21-10   22- 2   22- 6   22-10
2x12   19.2   14-11   15- 6   16- 0   16- 7   17- 0   17- 6   17-11   18- 4   18- 9   19- 2   19- 6   19-10   20- 2   20- 6   20-10   21- 2   21- 6
  24.0   13-10   14- 4   14-11   15- 4   15-10   16- 3   16- 8   17- 0   17- 5   17- 9   18- 1   18- 5   18- 9   19- 1   19- 4   19- 8   19-11
Fb   12.0   718   777   833   888   941   993   1043   1092   1140   1187   1233   1278   1323   1367   1410   1452   1494
Fb   16.0   790   855   917   977   1036   1093   1148   1202   1255   1306   1357   1407   1456   1504   1551   1598   1644
Fb   19.2   840   909   975   1039   1101   1161   1220   1277   1333   1388   1442   1495   1547   1598   1649   1698   1747
Fb   24.0   905   979   1050   1119   1186   1251   1314   1376   1436   1496   1554   1611   1667   1722   1776   1829   1882
Note: The required bending design value, Fb, in pounds per square inch is shown at the bottom of each table and is applicable to all lumber sizes shown. Spans are shown in feet-inches and are limited to 26‵ and less. Check sources of supply for availability of lumber in lengths greater than 20‵.
TABLE C-1
CEILING JOISTS WITH L/240 DEFLECTION LIMITS
DESIGN CRITERIA:
Deflection - For 10 psf live load.
Limited to span in inches divided by 240.
Strength - Live Load of 10 psf plus
dead load of 5 psf determines the required fiber stress value.
Joist    
Size   Spacing
(in)   (in)                 Modulus of Elasticity, E, in 1,000,000 psi
    0.8   0.9   1.0   1.1   1.2   1.3   1.4   1.5   1.6   1.7   1.8   1.9   2.0   2.1   2.2   2.3   2.4
  12.0   9-10   10- 3   10- 7   10-11   11- 3   11- 7   11-10   12- 2   12- 5   12- 8   12-11   13- 2   13- 4   13- 7   13- 9   14- 0   14- 2
  16.0   8-11   9- 4   9- 8   9-11   10- 3   10- 6   10- 9   11- 0   11- 3   11- 6   11- 9   11-11   12- 2   12- 4   12- 6   12- 9   12-11
2x 4   19.2   8- 5   8- 9   9- 1   9- 4   9- 8   9-11   10- 2   10- 4   10- 7   10-10   11- 0   11- 3   11- 5   11- 7   11- 9   12- 0   12- 2
  24.0   7-10   8- 1   8- 5   8- 8   8-11   9- 2   9- 5   9- 8   9-10   10- 0   10- 3   10- 5   10- 7   10- 9   10-11   11- 1   11- 3
  12.0   15- 6   16- 1   16- 8   17- 2   17- 8   18- 2   18- 8   19- 1   19- 6   19-11   20- 3   20- 8   21- 0   21- 4   21- 8   22- 0   22- 4
  16.0   14- 1   14- 7   15- 2   15- 7   16- 1   16- 6   16-11   17- 4   17- 8   18- 1   18- 5   18- 9   19- 1   19- 5   19- 8   20- 0   20- 3
2x 6   19.2   13- 3   13- 9   14- 3   14- 8   15- 2   15- 7   15-11   16- 4   16- 8   17- 0   17- 4   17- 8   17-11   18- 3   18- 6   18-10   19- 1
  24.0   12- 3   12- 9   13- 3   13- 8   14- 1   14- 5   14- 9   15- 2   15- 6   15- 9   16- 1   16- 4   16- 8   16-11   17- 2   17- 5   17- 8
  12.0   20- 5   21- 2   21-11   22- 8   23- 4   24- 0   24- 7   25- 2   25- 8
  16.0   18- 6   19- 3   19-11   20- 7   21- 2   21- 9   22- 4   22-10   23- 4   23-10   24- 3   24- 8   25- 2   25- 7   25-11
2x 8   19.2   17- 5   18- 1   18- 9   19- 5   19-11   20- 6   21- 0   21- 6   21-11   22- 5   22-10   23- 3   23- 8   24- 0   24- 5   24- 9   25- 2
  24.0   16- 2   16-10   17- 5   18- 0   18- 6   19- 0   19- 6   19-11   20- 5   20-10   21- 2   21- 7   21-11   22- 4   22- 8   23- 0   23- 4
  12.0   26- 0
  16.0   23- 8   24- 7   25- 5
2x10   19.2   22- 3   23- 1   23-11   24- 9   25- 5
  24.0   20- 8   21- 6   22- 3   22-11   23- 8   24- 3   24-10   25- 5   26- 0
Fb   12.0   711   769   825   880   932   983   1033   1082   1129   1176   1221   1266   1310   1354   1396   1438   1480
Fb   16.0   783   847   909   968   1026   1082   1137   1191   1243   1294   1344   1394   1442   1490   1537   1583   1629
Fb   19.2   832   900   965   1029   1090   1150   1208   1265   1321   1375   1429   1481   1533   1583   1633   1682   1731
Fb   24.0   896   969   1040   1108   1174   1239   1302   1363   1423   1481   1539   1595   1651   1706   1759   1812   1864
Note: The required bending design value, Fb, in pounds per square inch is shown at the bottom of each table and is applicable to all lumber sizes shown. Spans are shown in feet-inches and are limited to 26‵ and less. Check sources of supply for availability of lumber in lengths greater than 20‵.
TABLE C-2
CEILING JOISTS WITH L/240 DEFLECTION LIMITS
DESIGN CRITERIA:
Deflection - For 20 psf live load.
Limited to span in inches divided by 240.
Strength - Live Load of 20 psf plus
dead load of 10 psf determines the required bending design value.
Size   Spacing  
(in)   (in)                 Joist Modulus of Elasticity, E, in 1,000,000 psi
    0.8   0.9   1.0   1.1   1.2   1.3   1.4   1.5   1.6   1.7   1.8   1.9   2.0   2.1   2.2   2.3   2.4
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Published under s. 35.93, Stats. Updated on the first day of each month. Entire code is always current. The Register date on each page is the date the chapter was last published.