Wood Decking Grades and Terminology
Wood is affected by environmental elements, insects, and fungus. When building a deck, it’s important to choose a wood grade durable enough to withstand damage from these influences over time. But decks are more than something to stand on; they are an addition to your home. The wood you choose should be structurally sound, suitable for the environment you live in, and aesthetically pleasing – functional, durable, and beautiful.
Lumber grading considers several wood characteristics. There are three primary grades of wood, 1, 2, and 3, and the lower the number the better the wood. Within each grade are different varieties that describe specific characteristics but to not affect the grade of the wood, for example #1 Select Tight Knot, #1 Standard Tight Knot or #1 Clear (no knots). Different wood grades are recommended for different projects, and even for different purposes within the same project – the supports for a deck should be structurally sound, but not necessarily attractive.
Wood Characteristics that Affect Grade
To understand wood grades, some background in terminology is helpful. Here are some common terms you might encounter.
Wane is a thickening of the lumber where a tree branch grew. Structurally, wane is not a significant factor, but could affect precise measurements when nailing a deck together.
wood knots are whorls that form as part of the wood grain. Knots can be tight and tiny or so large they affect the structural integrity of the wood. They are part of what makes each section of wood unique and beautiful. The wood grain, knots and cracks that occur naturally as trees grow can be a factor in stability, but not always.
Knots on the edge are more likely to affect the structural stability of the plank. Spike knots bore all the way through a plank and can cause the wood to splinter and break when nailed. Since spike knots can be quite small, they may escape the grading process. Inspect individual planks for knots that extend through both sides of the wood.
The size of the wood helps determine what it’s used for. Size includes length, width, and thickness.
Wood for decking should be straight and true. Check for bowing or uneven thickness.
Techniques used to dry and cure lumber enhance the durability of the finished wood.
Twist refers to the natural warp in the wood.
The Best Lumber Grades for Decking
The best wood available is usually cut from older growth. The grain is tight and smooth, with more heartwood. It’s more resistant to the elements, but it is also the most costly option. It should be noted that more heartwood is a good choice for some woods and not recommended for others. To save money while showcasing the beauty of a wood deck, use #1 grade sparingly on railings and other highly visible areas.
The next best wood grade contains more knots but is structurally sound and less expensive than #1 grade. Most building codes specify #2 grade and higher. It is durable and serviceable, and many people prefer the rustic look as a decorative enhancement.
The inferior quality of #3 grade wood makes it a poor choice for structural elements of construction, and in general, it is not sufficiently attractive for use on visible areas of decking. It can be used judiciously for non-structural areas that are not easily visible.
I recently had a pergola built. The wood they used was very full of knots, huge cracks ran the length of the 4 X 4 posts and the 4 X 6 top beams, and some of the 2 X 4s used for the top row were rough. It looked horrible! Yet, they kept insisting that it was normal. I agreed to let them fill the cracks since I was going to use a solid weatherproofing stain. Could you tell me if I was unreasonable? Would you use cracked and knotty wood for a pergola?
Thank you so much!