FYI - nominal lumber dimension

[MarkAndDebbie]

I want to remind everyone when planning a 2x4 and a 2x6 are 1.5x3.5 and 1.5x5.5 respectively...

BUT! By the time you get to a 2x8, the large dimension is less than 7.5. I had to re-cut some of my pre-cut headers pieces and window jacks.

My 2x12s are really closer to 11.25 - making some of my stair calculations a little off (but nothing is cut - so no harm).

[MushCreek]

Thanks for the tip! And to add- those dimensions can vary a LOT, especially in the bigger sizes.

[Don_P]

The dimensions of US and Canadian lumber are on the money at the time of manufacture, the eurotrash is another story. 2x is 1-1/2" thick, widths are 1/2" under through 6" wide and then are 3/4" under. Engineered products do not follow these rules.

The variance in dimension is from moisture gain or loss after it leaves the plant. If the wood is running oversize I would think hard about buying from that lumberyard, they are letting the wood sit in the rain. If the wood is kiln dried, KD on the stamp it was dried to 19%. Wood in unheated storage in my area will slowly dry down to about 12%, in a 2x12 this will undersize it about 1/8"-3/16". In service the moisture content here will drop to about 8% in a heated structure, the 2x12 will shrink 3/16-3/8" total. If the wood is wet when you get it those numbers just increase.

Treated lumber is typically dried, milled to dimension, then treated. It is very rarely KDAT, kiln dried after treatment. Its' dimensions can be all over the road as can its moisture content. The best I can do is realize it was on dimension, will likely return down at least to dimension and so when possible build with that in mind... as my drivers ed teacher pointed out, two objects cannot occupy the same space at the same time.

When you get to final fit and finish, when possible, I like to have heat on and wood acclimated to the conditions of the house, basically using the house as the finishing kiln.

[glenn kangiser]

When making my own shiplap from wet wood I have sawn on my sawmill, I find that most of the softwoods shrink around 5/8 inch per foot of width on the average, so I make my lap cuts 7/8 wide on 1x12's.  That way, in a year or so when it is completely dry it will still be lapping about 1/4 inch.

[MushCreek]

My experience with crazy lumber dimensions had to do with treated lumber. I built an arched footbridge across our pond, and the instructions were very specific that the lumber had to be exact dimensions, or the holes wouldn't line up. Boy, they weren't kidding! I basically had to custom make each piece to fit.

[PEG688]

 Treated wood really can be off. I like to go thru the stack and make smaller stacks of like width stock , then start at one end with the widest joist and work "down" the deck in sequence of width.

This way you at least avoid the up and down humps when you attach your decking.

Which if you are using a composite like Trex can really make a difference in the looks when you're done.

All lumber varies , from unit to unit , you can not assume a 2x8 will be 7 1/4" or 7 1/2"  , they tend to be 1/4" net these days , they used to be 1/2" , thats for all stock except 2x6 and 2x4 , they tend to run in 1/2" widths , although 2x6 can be either 1/2" to 9 /16" , to 5/8" .

It's best to check before you assume

 
 

[davidj]

I got three or four 4x8s from the lumber yard in one order and some were slightly under 7.25" wide and some slightly under 7.5".  I didn't realize this and started seriously doubting my own abilities when a bunch of the studs were 1/4" short when I was sure I'd cut them all the same length.

We needed 2x12 #1 rafters per the drawings, but the yard only had a few and had to special order some better stuff, which ended up being structural select.  By the time the delivery arrived, the #1's had dried down to more like 11 1/8" and the SS lumber was very green and almost 11 5/8".  I thought I was smart using all the SS pieces for the load-bearing rafters, but realized I wasn't when I ended up with a mixture of widths for the eaves.  Fortunately it's all up in the air and covered by sheathing 'cos it wasn't the world's most precise bit of framing!

[glenn kangiser]

Treated wood really can be off. I like to go thru the stack and make smaller stacks of like width stock , then start at one end with the widest joist and work "down" the deck in sequence of width.

This way you at least avoid the up and down humps when you attach your decking.

Which if you are using a composite like Trex can really make a difference in the looks when you're done.

All lumber varies , from unit to unit , you can not assume a 2x8 will be 7 1/4" or 7 1/2"  , they tend to be 1/4" net these days , they used to be 1/2" , thats for all stock except 2x6 and 2x4 , they tend to run in 1/2" widths , although 2x6 can be either 1/2" to 9 /16" , to 5/8" .

It's best to check before you assume

 
 

No fair, PEG.... I'll bet you have done this before......

Thanks for the tips, PEG.  I would never have thought of that in my lifetime.... likely that is why you get the big bucks..