used for the Decking on west coast decks. 1) Pressure Treated Lumber (PT) derived from a mixture of softwood trees (SPF) Spruce, Pine, Fir 2) Yellow & Red Cedar. Red cedar originates from coastal rain forests where yellow cedar comes from inland areas. 3) Foreign hardwoods like Batu & Ipe come from oversees rainforests like the Amazon and Congo basin.
Pressure Treated Lumber (PT) is milled wood that has preservative pressurized around the circumference of the boards that penetrate the lumber by about 1/8". Once these boards are cut they expose untreated areas of lumber that Need to be treated onsite before assembly. Spruce, Pine, Fir (SPF) hemlock are the species of softwood lumber that is used for pressure treatment.
Main reasons we only use PT lumber for deck framing/building and not the decking or railings is that chemical off gassing will lasts 6 months to a year that delays staining processes which can cause the allready less stable SPF wood to split, warp, splinter and cup even more due to the nature of the species used. Surface toxicity may also be absorbed by hands and paws when touched by humans and pets and every time it rains small amounts may be washed into the ground water.
Most lumber available from the lumber stores is 'green lumber' (not pressure treated) that describes the fact that it hasn't been 'dried' and 'cured' in a controlled environment like kiln dried wood. All wood decking needs some sort of finish coatings to preserve, protect and stabilize the wood. Proper application methods require a relative moisture level of no more than 15% before applying the finish to seal the wood otherwise trapped moisture will cause wood rot, premature stain failure and other undesirable imperfections.
When lumber dries to fast the outer layer will dry quicker than the inner core and this is the main cause for wood to split or crack. Typically most woods take 6-18 months to fully cure depending on relative humidity, temperature, moisture under deck, diameter, sun exposure and species. Stain should always be used instead of paint for any new exterior wood and if a 'solid color' is desired a 'solid stain' or a proven product that will breath better should be used.
The undesirable traits of wood can be further reduced with kiln/pre drying and pre-staining each plank on all 4 sides to seal and protect the planks by reducing moisture absorption that causes wood to expand and contract every time that it rains or when there is high humidity. This stabilizes the planks and reduces 'nail lifting' and splitting.
The pre-stain process will extend the maintenance, appearance longevity and service life of the decking and railings that is applied before assembly because the 3 sides (other than the side you see) of the planks are not fully accessible after construction and are the areas where most of the moisture lingers.
Example: If you seal just the top of the plank and it rains, the water and the relative humidity in the air will cause the top surface to expand or swell much less than the rest of the plank causing it to cup upwards. When the plank dries to the same relative humidity as the top surface the board flattens. This constant and uneven expansion and contraction causes nails to 'Lift' and if face screws are used it may cause the boards to split along with other undesirable traits.
There are two variations of wood that can be purchased for your deck and railing project. One is called 'Tight knot' and the other is 'Clear' (clear of knots) This refers to the area of the tree where the boards were cut from. The outer layer of the tree is less valuable where knots are formed from the branches of the tree. Select Tight Knott (STK) is the grade used where the knots shouldn't fall out of the plank leaving a hole. STK has a more 'rustic' appearance as the knots are darker than the surrounding areas of the board.
Clear wood is quarter cut from the center of the tree where the vertical edge grains do not have knots and provide a more uniformed color and grain structure. Clear lumber is usually double the cost of STK lumber and offers the most lumber stability from its grain characteristics and is usually kiln dried and/or stored in a low humidity covered area. In the case of 'clear cedar' an unstained S4S 2x6 plank (Smooth 4 Sides from re-milling) is about the same cost per linear board foot as an averaged price composite deck plank. Note: Cedar is a softwood and may scratch or dent easier than composites or hardwoods.