History of pressure treat lumberModern day lumber chemical treatments first began in the late 19th century to help preserve railroad cross-ties withstand all weather conditions and direct ground contact. Most formulas infused the wood grain with chromated copper arsenate (CCA), a powerful preservative that contains arsenic. Arsenic is a Group-A carcinogen and well known toxin that causes adverse short and long term problems, including heart disease, diabetes and cancer.
CCA's primary uses were typically industrial and utilitarian applications, with the concerns of long term exposure, especially to children, taking a back seat to maximum longevity. The concern didn't really begin until the 1970s where homeowners began incorporating arsenic based treatments into outdoor decks, planters and play-sets, as this was cheaper than using naturally more weather resilient material.
Where is PT lumber today?Toward the end of 2003, the United States' Environmental Protection Agency and the pressure treated wood industry as a whole mutually decided the move away from arsenic based formulas and toward something safer for children and adults. The solution was ACQ (Alkaline Copper Quaternary) treated lumber. Although ACQ's preservative properties can't quite match CCA's, the substantial risk reduction of toxin exposure was worth the switch. While CCA is still sold for very specific applications, all the lumber you'll find in hardware stores and residential lumber yards use the ACQ or similar treatment.
In order to closely replicate the protective abilities of arsenic, tremendous levels of copper are instead infused into today's PT lumber. This technique allows the wood (typically Pine) to withstand moisture, fungus and ground contact for many years. While this process is safe for humans to touch and interact with, it does have very corrosive properties when in contact with metallic building materials, such as nails, screws, bolts and joist hangers. Be sure to use galvanized, stainless or coated fasteners when constructing with PT lumber.
What lumber should I use to build my playhouse?If cost is your biggest concern, and the design you chose does not have a roof covering some of the structure, pressure treated lumber is recommend. It is perfectly safe for children to interact with, and when compared to cedar, red wood, composite or other types of soft and hardwoods, PT lumber is the most rigid and weather resistant material for the dollar. If your playhouse has lumber protected by a roof and plywood walls, normal KD is recommend in order to further save.
If what you're looking for is longevity, composite decking and railings may be something you want to consider. While PT can maintain a nice look for 10 or so years, composite often can last 20 to 30 years without splintering or requiring too much maintenance, especially in high traffic areas. Cedar and red wood decks are beautiful, but are expensive and more prone to splintering and indentations than PT.