Thursday, February 28, 2013

Example Specification of Heavy Timber Decking for a Roof

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.

These values are added to the applied Dead load to give us (using 2 in. decking):

Live (Snow) load of ... 50 psf (given), and

Total load of ... 50 + 9 = 59 psf (Snow plus Dead plus `self’ weight).

Page 11 gives us the so-called `Load Duration factor’ to use.  Since our load is `Snow’ we will use 1.15.

(Wood is 15% `stronger’ under snow load than `normal’.) 

Page 13 gives us design stress and stiffness values for various species and grades.

For Ponderosa Pine, Commercial Grade,

Fb (Design Bending stress) = 1250 psi (pounds per square in.), and

E (Modulus of Elasticity) = 1,100,000 psi.

On Page 13 we are also given the Size Factor, CF, applied to the Design Bending stress:

for 3 in. nominal ... 1.04, and

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

Fb = 1250 psi x 1.15 x 1.10 = 1581 psi.

The Modulus of Elasticity, E, doesn’t get adjusted; it stays at 1,100,000 psi.

To determine whether or not the 2 in. decking will work at Controlled Random Layup we need to make sure it is STRONG enough and STIFF enough.

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.

Since our total applied load is 59, and the Allowable is 108, the 2 in. decking is strong enough.

Now let’s see if it is stiff enough.

The decking stiffness determines how much the decking will deflect, or sag, under load.  If the decking is not attached to crack-able materials, such as sheetrock, the limits on deflection for a roof application are L/240 due to Live load and L/180 due to total, where L is the decking span.  Since, in our example, the decking is also the ceiling, we will investigate deflection using these two limits.

Scrolling down the left side of Table 5 on Page 15 to a Modulus value of 1,100,000 psi, and then over to the right for Controlled Random, 6 ft, we get Allowable loads of:

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. 

In any particular application the `edge groove’ condition should also be specified, as well as any stains, etc.

In this example we came out `good’, `okay’, `perfect’ ... with the decking under investigation.  Had our investigation turned up that the decking was `not good’ (not stiff enough or not strong enough), the following possibilities could have been (further) investigated:

1.  use a different layup (stronger or stiffer),

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).



AITC 112-93, Standard for Tongue-and-Groove Heavy Timber Decking, American Institute of Timber Construction (now managed by the West Coast Lumber Inspection Bureau, Portland, Oregon).













nutrendmedia said...

Great Stuff!! I am just wondering by seeing the huge difference after following this article. I do appreciate your post. Thanks a ton.

Timber Decking

Frank Walmsley said...

Thank you for sharing and writing
for Deck