Decking boards

The three classic oceanic high nutrient, low chlorophyll (HNLC) regimes are now universally recognized by oceanographers as being Fe-limited. A plethora of deck-board and open ocean Fe addition experiments have conclusively demonstrated... 

Construction top deck boards will be evenly spaced so as to give 21 mm gaps. Annular ring nails are to be used for all joints, and each nail must be more than 20 mm from the wood edge. 

Figure 1.2 Pressure-treated lumber (as a reference deck board).

Note As a concrete example, a local lumberyard (Newton, MA) sells in October 2006 2 X 6 pressure-treated boards for 14.24/16 ft. ( 0.89/ft.) and Trex deck boards for 41.60/16 ft. ( 2.60/ft.). Almost a 3-fold price difference. 

The magazine has also noted that the following two deck board brands provide with the greatest range in styles ... 

The rest of the deck board materials were (in the order of residual rating)... 

What about WPC deck boards Chapter 7, in this book covers this issue in detail. As a brief example, let us consider two WPC boards— Trex and GeoDeck. Trex has reported that flexural strength of their boards (solid boards of 5.5" width and 1.25" thickness) is 1423 psi. It means that a Trex board placed on two joists at 16" span would have a break load derived from the formula... 

These examples illustrate that the flexural strength of composite deck boards is quite satisfactory. It is several times higher than the respective building code... 

There are two main situations concerning deflection of boards that may not pass the building code requirements deck boards at a certain span (distance between neighboring joists) and stair tread at a certain span. Let us consider these situations using the same examples Trex composite deck boards and GeoDeck composite deck boards. These examples would illustrate general shortcomings of plastic-based composite deck boards in terms of their flexibility and deflection. 

The building code requires that the maximum load at certain deflection of the test span shall be recorded (ASTM D 7042, Section 5). A common load requirement for measuring deflection of deck boards is uniformly distributed live load of 100 Ib/fC. 

As a result, for Trex boards (5/4 X 6) maximum decking span at 16 in. at 100 Ib/ft is allowed (ICC-ES Report ESR-1190), and for Geodeck (5/4 X 6) at 24 in. is allowed (ICC-ES Report ESR-1369). Only two WPC deck boards, GeoDeck and TimbeiTech (ICC-ES Report ESR-1400) are allowed to employ 24-in. span on decks three more WPC commercial deck boards are allowed to have 19-20-in. span 14 WPC commercial deck boards on ICC-ES record have 16-in. allowable span and one WPC board has allowed only 12-in. span on decks. 

These records show that flexural modulus of commercial WPC deck boards (and the respective span on decks) certainly has room for improvements. This in turn will improve quality of WPC boards and save money and material on deck joists. This conclusion is supported by consideration of support spans for stair treads (see below). 

The building code requires that the maximum deflection of deck boards used as stair treads under concentrated load of 300 lb placed at midspan shall be 1/8 in. (3.2 nun) or l/180th of the span (AC 174, Section 4.1.1 2000 International Building Code, Section 1607.1). For 16-in. span, the allowed deflection is either 0.125 in. or 16"/180 =... 

Overall, for 12 WPG deck board brands for which allowable stair tread span is on ICC-ES record (published in the respective ICC-ES reports), only two (CorrectDeck and GeoDeck) have allowable span of 16 in., six have allowable span of 12 in., and four have allowable span of 10.5", 9", or even 8 in. 

In a similar manner, as it was discussed in the preceding section, we can ask— how stiff a WPC deck board can possibly be, if not filled with mineral fillers We know that wood is very stiff, at least in applications WPCs are intended for. As it is shown in Chapter 7, flexural modulus of wood is about 1,500,000 psi. Polymers are much more flexible, and flexural modulus for HDPE is at best at 150,000 psi (Chapter 2). Again, in a very simplified case, for 50% HDPE - 50% wood fiber composites, in which both principal ingredients are ideally mixed and wood fiber is oriented along the flow, that is, longitudinally, the flexural modulus would be equal to a symmetrical superposition of the flexular moduli of the matrix and the fiber, which is about 825,000 psi. 

The longer the fiber (the higher is the fiber aspect ratio) and the more it is oriented longitudinally, along the deck board, the lower is the coefficient of thermal expansion-contraction. Overall, for different commercial WPC deck boards the coefficient is in the range of 2 X 10 to 5 X 10 1/°F. In other words, some commercial WPC boards can expand-contract by 250% higher than others. These overexpanded decks are very noticeable and sometimes cause complaints from the deck owners. 

Overall, values of expansion-contraction of WPG boards are largely unpredictable and represent highly empirical values. To make composite deck boards with truly minimized coefficients of thermal expansion-contraction is a very challenging task, not resolved as yet in the industry. 

At 200°F (about 90°C), deck boards would further accelerate the rearrangement of polymer chains in 4 = 64 times faster than that at 60°C (see above), which is in about 9 h. This is a common annealing time period for WPC deck boards. 

Figure 1.26 An 1-in. gap due to shrinkage of composite deck boards on a deck.

Generally, WPC deck boards are more slippery than the wood boards. It is easy to verify using a simple experimental setup. Take a 4-ft. conditioned (not wet) board, fix it at a certain angle, place onto the board a leather-sole shoe with a chunk of a heavy metal in it (to increase the weight of the shoe for its stability on the board), and slowly (or step-wise) incline the board until the shoe starts to slide down. With a wood board (such as pressure-treated lumber), it will happen at an angle of about... 

Figure 1.32 Pressure development by a composite deck board (13.5-in. long and 5.5-in. wide) immersed into water, for 2-1/2 month. The initial pressure, caused by holding clamps, was 1000 lb. After 80 days, water was drained and the board was getting dry with the resulting pressure release for the following 2-1/2 months. ...

One can see that commercial WPG deck boards are dramatically more termite resistant than wood lumber. 

Figure 1.40 WPG deck boards before and a few minutes after ignition (by permission from the University of California Forest Products Laboratory).

For comparison, wood fiber-filled HOPE hollow boards have an FSl around 150, solid boards about 80-100, WPC hollow boards containing minerals have an FSl around 100, and PVC-based wood-filled deck boards typically have an FSl between 25 and 60. 

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