Need advice on laying some wood flooring on a flatbed I am trying to finish up here. What I have is (7) 2 by 12s as my floor. I am using 2 by 2 angle steel to seperate the wood and to also act as runners for things to glide on as I load. I have welded the first 2 by 2 L channel upside down on one end so that the first board will slide underneath it and be held down then I will take two of the two by 2s and place them upside down flat end facing each other and weld them down to my crossmembers so that they also hold the next piece of wood down and so on.
My first quection is how much play should I leave between all of this, I pissed off the people at Lowes/Home Depot by demanding they allow me to go thru their entire racks of 2 by 12 so that I could find knot free timber but it is still pressure treated and looks defanitely green and feels wet so how much shrinkage will I have?
I am thinking it will shrink alot once it dries out so I need to cinch this entire assy real tight and then weld it all in place??
Or will it shrink minimally and I am better off leaving it loose so that when it gets wetter from rain I wont have boards that want to burst upward wherever it can. Thanks for any suggestions....Jason

 

Pressure treated lumber really moves around. It is literally dripping wet at the lumber yards here. I would stack it with some stickers (slats) between each board to allow some air circulation. Inside would be best this time of year. Lumber takes years to completely air dry. I'm just suggesting you not install it at 40% moisture content.

It's anybodies guess how much it will shrink if you install it wet. After it's fairly dry I would give each board about a 1/2" to expand across the width. (1/4" per side). Maybe even a bit more depending on where you live. Length of boards will be fairly stable

 

Actually the boards WILL contract along their length, the extent of which is relative to their overall length and grain orientation. Check carefully to be certain that the CCA preservative used in the lumber is compatable with the L channel steel you plan on using for the structural members of the bed. If it isn't, it will accelerate deterioration of the steel anywhere it comes in contact with the wood. Simpson Strong Tie connectors' website may be able to steer you in the right direction re: compatable steel grades/treatments.

 

I wasn't very clear. I meant after they dry, lumber doesn't typically move too much along it's length. He should be able to slip a piece of weatherstripping at the bed from if a large gap appears. While on the subject, When installing a wet board, how much do they usually contract lengthwise while drying?

Also to 30 Dodge , if you dry the lumber some beforehand, put some weight on it. You may have some warp so bad they aren't usable, but better to find that out now.

 

As the fatfenders mentioned, it will move a lot. PT wood is notorious for warping and twisting quite a bit. So the question in my mind is should you "tie" it down to try to control that movement, or should you let it move and then see what you have to deal with. I'm of the thought that you should tie it down sooner rather than later.

 

I made bed sides for a dump truck years ago using soggy 2" x 8"s like you're describing...Weighed a ton, but straight as an arrow...Bolted them to 2" x4" framing in the stake pockets as tightly as they would fit together...After about a year the gaps in my "stakebed" were about 3/4" wide...

 

Sorry, but that's not quite right. Wood will shrink slightly lengthwise, but the majority of the shrinkage is in the direction of the annular rings of the grain (widthwise in this case).

I don't think you are going to be real happy with your wood bed floor for long if you make it out of pressure treated lumber. Ground contact (cc40) is especially wet as it is soaked thru to the center and will exhibit quite a bit of shrinkage (~1/8"+ per in of width) twisting (suitable for making airplane propellers... ), cupping (warps across the width turning the edges up or down) and checking (cracking), especially in your wide widths. Above grade (cc20) is soaked ~ 1/2 way thru so is a little more stable but will still shrink cup and twist considerably. Twist and cupping is determined by the grain where the board was cut from the tree rather than knots. Most PT wood is cut where they will get the best yield from the log so the end grain looks like "smiles" or "frowns" depending on which side is up. This grain pattern will cup in such a direction as to try to turn a smile into a frown (won't typically cup that much but that image makes it easy to remember) Drying rate and differential drying (such as exposing only one side to the sun and shading the other like your truck bed) will have a great affect on the stability as well. Locking the boards down will not help much, the forces in a piece of wood are surpringly high and are relentless, it can blow welds apart or is held strongly enough the wood will split to relieve the stress. You will get the least shrinkage, cupping and twisting from radial sawn or "straight grained" wood where the grain runs parallel straight up and down from flat side to flat side of the board, but it will not show much grain pattern on it's face. Straight grained wood that is kiln dried to < 20% moinsture content will be the most stable.
As LMS alluded to, the goverment banned arsenic based pressure treatment a few years back, so all PT is now done with salt solutions. The corrosive properties of the salt can rust thru plain steel 10d framing nails in just a couple years, and will corrode any steel it comes in contact with on your truck.
Best thing you can do is PO the clerks at the DIY yard even more by returning those PT boards. Then find a large lumber only yard that handles 2x6 untreated kiln dried clear straight grained Fir, or if you want grain flame pattern, 2x6 red or white oak. It will be a little more expensive but in the long run will be a lot more satisfying and cost effective.
By the way that green color of the PT will soon fade to light grey and finally moldy black when exposed to the weather.

 

You might have to go to a commercial lumber yard to get it, but loblolly pine Scaffold Grade lumber is very good lumber to use for a bed like yours. It is graded for strength and is pretty dense for pine. It seems to stand up to abuse and weathering very well, that's why it's used for life-dependent scaffolding.

 

Both AX and Ross have made good points.

Pressure treated lumber is notoriously corrosive and reacts VERY badly with iron and especially zinc (galvanized) hardware.

Ross' solution is very sound as I believe the Scaffold Grade Lumber is already Kiln Dried, which will minimize or elliminate any shrinkage or warping.

Be absolutely certain to seal the bottoms of the boards prior to installation so they do not pick up any road spray and expand on only one side.

 

Thanks for all the tips, I will print whats needed and take it to a local lumber only yard and see what I can come up with. I do like fir. I paid 17.96 per board and am now curious to see what it might jump too. Id prefer to only do this once and had the feeling that these boards would prob. be trouble sooner than later.

 

Anything I should consider when trying to figure out a way to bolt these things down. I planned on using stainless steel carriage bolts, I have used stainless throughout the truck every chance I was given because it wont corrode and thought Id use it in the bed. On a 2 by 12 board with the 2 inch L channel wrapped over the top on eack side there is approx 8 inches of board showing, underneath the boards I have 2 by 3 square steel for support every 12 inches the entire twelve foot of the bed. Should I consider letting the boards float or should I fasten them as I planned and if so any suggestions on how often to fasten without splitting the wood.

 

If you have trailer manufacturer close look for some Apitong. This is what they use in commercial trailer floors. It has a high resin content and some silica but is very durable. You can proably google for a local supplier.

 

Let the boards float, wood needs room to shrink and grow from humidity changes. Follow the way Ford did it. Rout a step rabbet in each edge for a hold down strip that is 1/4" wider than the combined width of the recessed groove formed by two boards (so there will be 1/4" between boards) drill bolt holes for SS carriage bolts or button head bolts enlarging the holes between the boards 1/8" larger than the bolts so the boards can move. Bolt frequency would depend on thickness of the hold down strip (which should be thick enough to stand proud of the boards when laid in the grooves) but every 16" max is a good spacing.
Whatever sealant you put on the boards be sure to coat all sides equally especially the end grain. If you must have a clear sealant use a penetrating type such as CLR waterproofing or tung oil. A surface film such as laquer or varnish will soon crack and chip no matter what the manufacturer claims. With the penetrating sealer, as the wood ages just give it a light sanding to return it to the original color and apply another coat with a rag or green plastic scrub pad, then scrub off any excess on the surface after a few minutes with a coarse rag such as burlap or canvas.

 

Didnt get anywhere today with calling 4 local lumberyards, kiln dried had to be special ordered and I was told repeatedly that it would serve me no better than pressure treated, fir was priced at 18 dollars a board at one place and 108 at another for a 2by 12by12 and again was reccomended would hold up no better. I will look into the scaffold garde lumber tomm. and also the Apitong.
I need to purchase 1 additional strap gimmick for my bed, I need entire roller and strap assy, I posted pictures of my truck in another post, can anyone tell me what the technical name is for the apperatus so I can search on e-bay or Craiglist or sound intelligent when I go to my local truck surplus store. Thanks

 

That's true when you are talking to wood people about woods and their characteristics wrt shrinkage, warping, etc. HOWEVER, I doubt very seriously that any of the lumber yard guys understand the non-standard-use impacts, as this is not going to frame out a house, but rather is going to be encapsulated in a metal frame, and dependent on large metal bolts for it's structural integrity.

I doubt very seriously that they are of the mindset where they are thinking about a vehicular application, environment, and characteristics.

And what they said supports my point - they share the same basic physical charasteristics - as wood goes. But the difference you are looking at between the PT and the Kiln Dried is chemical, not physical.

By going with Kiln Dried, you are elliminating the corrosive chemicals and their effects on the metal structure of your frame and hardware. I doubt that lumber yard guys are in that thought mode.