Most deck building pros know the difference between above-ground (UC3B) and ground-contact (UC4A) pressure-treated lumber and where each should be used. But most decks are built by non-pros, who often don't understand that ground-contact PT lumber is required for certain applications, such as stair stringers that extend to ground level. Compounding the problem, ground-contact PT lumber is often not even stocked at many lumberyards, and instead has to be special-ordered. That situation should change, however, following news that the American Wood Protection Association will update its U1 standard for preservative-treated lumber later this year (likely in May or June of 2016).
According to a press release from the Southern Pine Inspection Bureau, modifications to the U1 standard will mean that many - if not most - deck joists, beams, and ledgers will have to treated to the AWPA's UC4A preservative retention levels, even when they're not in contact with the ground. This is a view shared by many in the industry, who point to language in the standard that says: "Joists and beams shall be treated to requirements for UC4A when they are difficult to maintain, repair or replace and are critical to the performance and safety of the entire system/construction". But at least one wood treater interprets the new rules differently, saying that the use of above-ground treated materials is still approved for joists and support beams that don't come in contact with the ground, and that it will continue to warrant its above-ground rated lumber used for deck beams and joists provided the material is properly installed. How the new rules will affect the availability and cost of UC3B and UC4A PT lumber - and therefore the cost to frame a deck - remains to be seen. Read More