Typically post frame (pole) buildings have a pressure treated board at the base of all enclosed walls.
Known also as a Bottom Girt, Grade Girt, Skirt Board or Splash Plank, it is a decay and corrosion resistant girt which is in soil contact or located near the soil surface. It remains visible from the building exterior upon building completion, and is normally two inches in nominal thickness.
A decade ago, after CCA (Chromated Copper Arsenate) pressure treating went by the wayside, many splash planks were treated with ACQ (Alkaline Copper Quaternary). When water was introduced into connections between ACQ pressure treated lumber and steel, a chemical reaction occurred, which could quickly corrode steel siding and fasteners. For the most part, ACQ treated lumber is now rarely used, having been replaced by chemicals which do not interact corrosively with steel.
In efficient post frame construction, building components are positioned and connected in such a way to form a diaphragm. The gradeboard is an integral component of a properly designed shear wall (diaphragm) assembly. Structural wall sheathing (most commonly plywood, oriented strand board or corrugated steel) is attached to the gradeboard by sheathing-to-framing connectors which are located at the bottom edge of the structural sheathing panels.
The sheathing-to-framing connectors must be adequate in size, strength and resistance to pull out to be able to transfer shear loads, which are induced into the building by wind or seismic events, from the sheathing into the gradeboard. Similarly, the gradeboard must be attached to the columns at each end sufficiently to transfer the load.
At one time I had read a posting online, which downplayed the structural significance of gradeboards. In my mind, they are a key and important structural member.
Touted as an alternative to the traditional pressure treated lumber skirt board, is the composite grade board. Manufactured in a fashion similar to composite decking, they eliminate any risk of corrosion, rotting or decay.
The composite grade board features a built-in bottom ledge, which eliminates the need for a steel bottom trim (also known as base trim or “rat guard”).
Composite grade boards expand and contract dramatically with temperature fluctuations. As such, potentially unsightly gaps must be left at each end. For a 60 degree temperature fluctuation, a 3/16” gap is required (based upon a 16’ piece).
One of the “benefits” of the composite grade board is supposed to be price. I recently did a price comparison and found 2×8 #2 and better pressure treated at 97 cents per foot, while the composite grade board from the very same retailer was $2.99 per foot (over three times the cost)!
Having used composite materials for decks and stairs, I’ve found composites to not nearly have the strength of the lumber they are replacing. Furthermore, composites seem to allow a great deal of “give” around fasteners. When loaded in a shearwall assembly, I would have great concerns about fasteners pulling through composite grade boards.
The IBC (International Building Code) gives specifics in tables for shearwall construction, none of which include an approval for use of composite materials rather than dimensional lumber framing. Alternative materials are allowed to be utilized outside of the prescriptive portions of the Code, however testing documentation to prove structural adequacy should be provided.
Me…without some actual proven test results, there is no way on earth I would structurally use, or recommend the use of, a composite grade board.