Tools of the Trade

 

Kelly Nelson, our resident expert on trusses, has been passing along tips and tricks of the trade to co-workers and customers for years. We'd like to share a few of these time saving techniques with you.

 

 

The following is a table of total heel heights for a 2x4 top chord with a 3/8" butt cutt 2x4 bottom chord at various roof pitches:

 

 

 

 

 

 

How to match facias of trusses with two pitches by raising the heel height

of the higher pitched trusses:

 

 

You have a house plan calling for the main body of the roof to be 5 on 12 and two wings extending out of the front at 11 on 12. The job calls for a 29" overhang. Unless an adjustment is made in heel heights, the overhangs on the 11 on 12 roof will extend below the same overhang on the 5 on 12 area. The front elevation of the house clearly shows the facias at the same height. How is this done?

 

Step 1. Calculate the total standard heel of the lower pitch. We utilize a 3/8" butt cut as our standard bottom chord heel height, so we can use the table above for this figure - 4-3/16"

 

Step 2. Calculate the rise of the 29" overhang at both pitches. (This can be looked up in Smoley's or determined with your fancy calculator.) The rise of 29" at 11on 12 is 2' 2-19/32" and at 5 on 12 is 1' 0-3/32".

 

Step 3. Subtract the 5 on 12 rise from the 11 on 12 rise to get 1' 2-1/2". Add this to the standard total heel of 4-3/16" to get 1' 6-11/16" total heel for the 11 on 12 trusses.

Your facias will match every time.

 

 

 

 

Span Tables for Southern Yellow Pine

 

 

Often homes and other structures are built utilizing conventional framing methods rather than with manufactured components. The following tables may assist you in making comparisions between the two types of construction methods. You will be able to determine the correct lumber required for your common framing application.

 

 

The spans, given in feet and inches, are calculated with respect to various loading conditions. Please refer to the discussion below the tables for a further explanation of loading. These values have been compiled by the Southern Forest Products Association and provided by the Southern Pine Marketing Council.

 

 

Floor Joists

 

30 or 40 psf live load; 10 psf dead load; L/360 deflection limit

 

 

 

 

Ceiling Joists- Drywall Ceiling

 

 

10 psf live load; 5 psf dead load; L/240 deflection limit

 

20 psf live load; 10 psf dead load; L/240 deflection limit

 

 

 

 

 

 

Rafters - Any Slope- Drywall Ceiling

 

20, 30, or 40 psf live load; 15 psf dead load; L/240 deflection limit

 

 

* Maximum span is 25'-5" or greater.

 

 

 

 

Loading - the short course

 

 

Building codes have developed loading requirements for different types of materials based on established standards in the construction industry. Loads include both live and dead loads and may also include wind and snow loads if local building codes so deem.

The Wood Truss Council of America defines loads to be the "forces or other actions that arise on structural systems from the weight of all permanent construction, occupants and their possessions, environmental effects, differential settlement and restrained dimensional changes."

 

Live loads are produced during construction and during the life of the structure. Live loads are established for both floors and roofs. Construction workers and their materials are considered roof live loads during the construction process. Ongoing maintenance workers and their materials are examples of live loads during the structures' life. Floor live loads are incurred on the floor system during occupancy and use.

 

Dead loads are the weights of the various structural members and objects that are permanently attached to a structure. Dead loads can be broken down into two types - building dead loads and collateral dead loads. A building dead load consists of the actual building system. This will include the framing materials (floor or roof) and covering materials such as decking, felt, and shingles . Collateral loads include the weight of permanent materials other than the actual building system. These items may include drywall, electrical systems, sprinklers, and the like.

 

One interesting property of wood is its ability to withstand greater loads (stress) for short periods of time. To apply this property to loading, most building codes allow modifications of allowable stress based on the time (duration) the load exists. The rafter

table above uses a load duration factor of 1.15 - the two-month duration factor.

 

Another property of lumber that effects loading is it's propensity to sag under the influence of applied weight or outside forces. This is known as deflection. The tables above specify allowable deflections; i.e. L/240 or L/360. The length of the rafter or joist, in inches, is divided by 240 or 360 to determine allowable deflection in inches.

 

 

 

Calculating Rafter Lengths

 

 

Often times we are asked by our customers for rafter lengths to assist them with ordering tin roofing or calculating decking requirements. The following example and accompanying tables will enable you to determine rafter lengths without the aid of a dimensional calculator.

 

 

Example: You have ordered trusses to be built on a 4 on 12 roof pitch that will span 23'-2" with a 12" overhang. How do you calculate the overall rafter length.

 

• To calculate rafter length, divide the span by 2 and add the overhang. Half the span equals 11'- 7", add the 12" overhang, and your result is 12'-7".

 

• Look up the corresponding value for 7" in the table below to convert it to a fraction. You can now convert 12'-7" to 12.583.

 

• To compute the rise of 12'-7" at a roof slope of 4 on 12, simply multiply 12.583 by the appropriate distance one foot rises at the pitch. Checking the second table, look up this multiplier for the 4 on 12 pitch - 1.054092

 

 

                Therefore, 12.538 times 1.054092 equals 13.216 or roughly 13'-3".

 

 

  

Inches	Decimals		Pitch	Rise of 1'
1	0.083		1	1.003466
2	0.167		2	1.013794
3	0.250		3	1.030776
4	0.333		4	1.054092
5	0.417		5	1.083333
6	0.500		6	1.118034
7	0.583		7	1.157704
8	0.667		8	1.201850
9	0.750		9	1.25
10	0.833		10	1.301708
11	0.917		11	1.356568
12	1.000		12	1.414213

If you have any specific questions concerning trusses or truss design, email Kelly at kelly@trusslane.com - We think you will find him quite willing to assist you in any way possible.

 

 

Did we mention Kelly's hobbies? They are (in order of importance) deer hunting, deer hunting, and when he has time, deer hunting. Click on the buck below to share your hunting stories with Kelly.