rate of wood shrinkage

huangyhg

rate of wood shrinkage
not so much a structural engineering dilema here, but i havent heard much on the wood design forum....
i'm cutting stair stringers out of 2x12's for a set of deck stairs.  based on my elevation changes and rise/run requirements, i have planned out all my dimensions and cuts based on an 11.25" width 2x12.
now, i go and get my pt 2x12's, and either they just cut the tree yesterday, or soaked the bundle of 2x12's in the river for a month before putting it out for sale (kd19 my a$$).  the actual width of these soaking wet 2x12's is 11.5".  if i lay out my marks on this 11.5" board based on my 11.25" calculations, i will not have level/straight treads, etc.
so how long will i need to wait before the moisture content of the board gets down to something reasonable and shrinks to the 11.25" width?

i won't claim to be an expert in seasoning wood, but i believe that your question depends on the current moisture content of the wood and the climate conditions of your area.  the wood won't shrink until all the "free water" has evaporated and "bound water" starts to evaporate.  the moisture content when the bound water starts to evaporate (the "fiber saturation point") depends on the species of wood, but an average is ~30%.  an average shrinkage value is 6% over a mc=0 to mc=30.  you can assume a linear fit for your curve and you will have a shrinkage of .002 in/in per 1% change in mc.  that means that you need to lose ~10% of your current moisture content to have an 11.25" board.  
forgetting all the theoretical stuff, i have seen 2"x12"s that have shrunk as much as 5/8" and even 3/4".  it all depends on where they've been and what they've been subjected to.

in a worst case scenario, where wood has been saturated in the exterior environment, frozen before any evaporation of the moisture, and installated in a building (generally a house) where it is enclosed and then dried in service in a heated area, the shrinkage generally causes severe cosmetic problems, especially where drywall and tile exist. it also creates tilting countertops, when the cabinets are along an exterior wall where the rim joist does not shrink as much because it is greatly affected by the exterior environment.
also, keep in mind that the cross-grain shrinkage is 30 to 50 times greater than the longitudinal shrinkage.  i don't think you'll have dimensional problems, as the lumber generally shrinks evenly, but the slope of your treads could change.
try to find a better lumber supplier who may have kiln-dried after treatment lumber, and if possible (and this is a big if) see if you can find treated grade no. 2 or better.
are you using pressure treated wood? if so, the excessive moisture is most likely a combination of the diluted chemical used in the treatment process and natural moisture in the wood. because the chemical is applied at an extremely high pressure, the wood fibers are compressed and the microscopic voids are filled with the treatment chemical and combines with the moisture in the wood. as the wood slowly rebounds from this pressure, excessive chemical and moisture is forced to the surface making it soaking wet. the rebound process will usually continue after the wood has arrived at the local lumber yard. i would suggest letting the wood dry for 2 days in sunlight before using it. as for the shrinkage, for a 2x12 up to 1/2" is typical.
how big is your oven?
just kidding. i dont want to be responsible for you releasing the pt chemicals into your house
in good sunlight with warm temperatures (relatively high humidity, though), it took about 14 days to shrink down to the 11.25" range.