April 1, Please note that the following Power Point presentation is solely intended as educational material that is general in nature. This is not intended to be an exhaustive review of the Department of Public Safety’s laws and regulations and is not intended to provide legal advice. Materials presented in this presentation should not be considered a substitute for actual statutory or regulatory language. Always refer to the current edition of a referenced statute, code and\or regulation for precise language.

April 1, CMR 7 th Edition: Rafter Span Tool Guideline Example Cases for Snow Loads

April 1, On-Line Tool-- Span Calculator

April 1, AWC Span Tool If you search for this web site: on your computer…

April 1, this screen will appear The link may be entered as a “favorite” in your web browser

April 1, A wide selection of wood species is available, including the four found in the 7 th Edition Table for Rafter Spans. For this example will select Hem-Fir

April 1, Five section sizes (the same as found in the 7 th Edition Table for Rafter Spans) are available. For this example will select 2 x 8

April 1, Several grades are available. For this example will select No. 1

April 1, For “Member Type” Rafter (Snow Load) is selected.

April 1, Six different Deflection Limits are available. For a ceiling attached to the rafter will select L/240.

April 1, One of five different Spacings can be selected. For this example select 16 in.

April 1, “Snow Load” value is typed in (there is no pull down menu). For this example 55 was entered. “Dead Load” value is a pull down menu, will leave at 10. This completes the data entry. One click on “Calculate Maximum Horizontal Span” and ….

April 1, The output contains the Maximum Horizontal Span, E, and stress values. Another feature allows the user to input a Required Horizontal Span. You can get to this feature by clicking here when you first open up the Span Tool link.

April 1, The output for this feature looks like this. As you can see, a 2 x 8 section still is suitable for this design condition.

April 1, The desired span is entered here in Feet and Inches. For this example the same design condition of the previous example will be selected...

April 1, Question: A contractor using a snow load of 25 psf said she is going to use 2x8 No. 2 Spruce-Pine-Fir rafters to span 16 ft. at 16” spacing. Can I use the span calculator as a quick check? Answer: Yes, but refer to 780 CMR as well. Note that a maximum span of 16 ft. 2 in. is allowed.

April 1, Question: The same contractor said her “new” snow load is 35 psf. She wants to know if a 2x8 No. 2 Spruce- Pine-Fir can still span 16 ft.? Answer: Yes. 16 ft. 5 in. is allowed but note the spacing is 12 in. and not 16 in.

April 1, Nomenclature F b : Extreme fiber bending stress (psi) E: Modulus of Elasticity (psi) P f, P g : Snow Loads (uniform) IRC 2003: International Residential Code for One- and Two-Family Dwellings L: span (ft)

April 1, Please note that the following Power Point presentation is solely intended as educational material that is general in nature. This is not intended to be an exhaustive review of the Department of Public Safety’s laws and regulations and is not intended to provide legal advice. Materials presented in this presentation should not be considered a substitute for actual statutory or regulatory language. Always refer to the current edition of a referenced statute, code and\or regulation for precise language.