Consider a shallow slope roof subject to a Snow load of 50 pounds per square foot (psf) and applied Dead (weight) load of 5 psf. Let’s find, and specify, heavy timber decking to carry the required loads while spanning 6 feet (ft) rafter-to-rafter. By case of example let’s assume that Ponderosa Pine, `Commercial’ grade decking is available, and in lengths up to 12 or 16 ft. Further, the decking will be exposed on the underside and also be the `ceiling’ material. We will use the American Institute of Timber Construction AITC 112-93, Standard for Tongue-and-Groove Heavy Timber Decking, to determine the required thickness.
According to the Standard, a number of `layups’ are typically used for heavy timber decking construction. Some layups are stronger and stiffer than others; while others are more (or less) affordable. Often the `Controlled Random Layup’ is the most affordable (per piece), as it allows the manufacturer to provide decking pieces of various (convenient) lengths. On the other hand, it may result in more trim waste than layups of pieces ordered to `exact lengths’.
Typical heavy timber thicknesses are 2 inch (in.) nominal (1-1/2 in. net), 3 in. nominal (2-1/2 in. net), 4 in. nominal (3-1/2 in. net). And the typical width is 6 in. nominal (5-1/2 in. net), though other thicknesses and widths are available from some manufacturers.
In this example we will investigate using 2 in. nominal (nom.) Commercial Grade Ponderosa Pine with a Controlled Random Layup.
Page 10 of the Standard gives us weights of various decking species. For Ponderosa Pine we are given:
2 in. nominal weighs ... 4.1 psf
3 in. nom. weighs ... 6.9 psf, and
4 in. nom. weighs ... 9.6 psf.
Total load of ... 50 + 9 = 59 psf (Snow plus Dead plus `self’ weight).
(Wood is 15% `stronger’ under snow load than `normal’.)
For Ponderosa Pine, Commercial Grade,
Fb (Design Bending stress) = 1250 psi (pounds per square in.), and
E (Modulus of Elasticity) = 1,100,000 psi.
for 2 in. nominal ... 1.10.
(The way decking is `graded’ the 2-in. and 3-in. thicknesses have a bit more unit strength. The values in Table 3 on Page 13 are based on 4 in. thickness.)
Thus, for our 2 in. `P. Pine’ subject to Snow load, the Bending Stress adjusted for size and load duration, for our application, is
The Modulus of Elasticity, E, doesn’t get adjusted; it stays at 1,100,000 psi.
To deal with `strong enough’ we look at Page 14, Table 4, ... ALLOWABLE ROOF LOAD LIMITED BY BENDING.
Scrolling down the left hand side of the Table to a Bending Stress of 1581 psi ... wait! ... we have 1550 and 1600 ... let’s go to 1550 psi (and be a bit conservative) ... for Controlled Random Layup, Span 6 ft, we get an Allowable load of 108 psf.
l/180 ... 66 psf
l/240 ... 50 psf.
Now let’s check.
The l/180 deals with total load; our total load is 59, and the Allowable is 66: good!
The l/240 deals with (the effect of) Live (Snow) load; our Snow load is 50, and the Allowable is 50. Whoa, good, barely. Perfect! (Close!)
The 2 in. decking works!
Here is our specification: 2 in. Ponderosa Pine Decking, Commercial Grade, Controlled Random Layup.
The Standard goes on to dictate how the pieces must be laid, as well as fastening requirements.
2. use a stiffer or stronger WOOD (different species or grade of decking),
3. try THICKER decking,
4. change the rafter spacing (???), or
5. take into consideration actual roof slope (Page 12 of the Standard).
See also ... http://voices.yahoo.com/weight-board-foot-quantity-calculations-decking-12006629.html?cat=6