In the 1980's I acquired a set of original irons for a wood pickup box. Now I have the irons cleaned up, painted and ready to make a pickup box for my 1916 runabout. The plans Lang's sold in the past for the Cleveland Hardware kit calls for a floor that is but jointed. I am thinking of joining the floor boards with either tongue and grove or lap joints. This seems like it would make the floor tighter if the boards swell and contract. Any feedback or recommendations from someone that has done this or with woodworking knowledge that could advise?

As a retired truck body manufacturer, I would recommend kiln dried ship lapped dense grain southern pine.

The expansion and contraction of the wood is why typical wood pickup floors have wide gaps with metal strips covering the gap.

Trust me you do NOT want to put any wood boards side by side tight unless the boards are super green and wet because if they are very dry then the slightest moisture will make the boards swell and warp. Green and wet wood will shrink when it dries up and leave you wide gaps which you won't like but it is better than dry and tight joints that for sure will absorb moisture from the outdoors. Trust the engineering of those that made truck beds of wood during the T era and space the boards accordingly and cover with metal strips if you don't want any gaps. The wood is going to move and if you try to stop it with anything that limits its expansion/contraction then it will swell up at that junction and look awful.

The instructions in the Cleveland Hardware instructions says to square cut and but the floor boards, no mention of ship lap or metal strips over the joints. The wood I am using is elm that has a good looking grain and has been drying for several years since sawed. In the day I expect that what ever was affordable was the wood that was used. Other than fir from the lumber yard, the elm or cottonwood fit the bill for less expensive. The wood monger here in town thought I should use his beautiful walnut or maple, but I was afraid to even ask his price. The elm only cost $165 for plenty to make the bed and he even planed it to 7/8" as specified in the instructions.
The one thing I don't want is to have big gaps between the boards or boards that are cupping or bowing from expansion/contraction. I will likely do a ship lap unless someone thinks that would be a big mistake. This way I can incorporate a 1/16" or so gap on a joint that can "move"

Dale, shiplap is the way to go. An oldtimer taught me to use a yard long steel rule between each board to make the gaps. He used to make the lap such that the same gap was between the horizontal faces of the shiplap too. That way water could run straight through the floor, but the floor still looked as it should.

Hope this helps.
Allan from down under.

Maybe '16 is different from '25. My wood floor in my pu is southern hard pine. The metal strips that go between the boards/planks are bolted down so the wood is NOT tight but has a space. Been that way since about 2010 and still looks great. I got this construction info from Larry Smith and Steve Coniff so I think it is correct. My wood is painted black.

It's not a matter if: "Wood will move but how much"

A gap will of course always be required due to expansion and contraction of the wood planks themselves. You can play cute and look up expansion v. moisture content/temperature for a given grade of wood but it is not that critical.

I "get" the idea of ship-lap but it makes the hardware strip mounting a little more difficult, and leaves a 'shelf' at a low point for water to pool, yes?

More critical to me would be in selecting boards that have grain lines more perpendicular to the long side. If the grain lines are cupped or actually run more horizontal to the flat side, then in use the wood will relax along those lines and cup either up or down which you don't want either.

I agree with Allan Bennett's method. I also recommend that you assemble the bed, then take it apart and paint all surfaces of all the boards (especially the ends) with good-quality oil-based primer and paint, to seal up the wood and keep moisture penetration to a minimum. That's how I did mine, using Poplar. The "good looking grain" won't matter when it's painted, as it should be. Forget about varnishing the wood; it would not have been done that way back in the day, and varnished wood out in the weather is a constant maintenance chore.

Just a note : the wood expansion is greater on the length of the board than the width. The tighter the and denser the wood less expansion. MG

Maybe try checking with a quality cabinet maker... The local cabinet guy gets oak wood for for his cabinets, sorting out the 'red' oak which is used sparingly, preferring the 'white' oak. With the non-standard length & thickness of the TT bed, he gave me a very favorable price to cut new boards, even using his CDC machine. Side boards are less tedious, just to correctly connect all the iron-ware. Take as many pictures as possible for later reference before disassembly! ('Half-Zeimers' i.e. half-way there, seems to kick in when least expected.)

White oak is much more water resistant than red oak. If you get quarter-sawn (also called vertical grain) it will cup less and is more stable across the width of the boards. (and looks better) I'm lucky to have a big hardwood dealer not too far away where I can go pick what I want for many, many species from their huge warehouse.

This has become a very interesting "discussion" and the following may be a bit "OT", but bear with me,.....it may add something of use to this discussion:

I'm reminded of something I read about many years ago which pertained to a boat building company by the name of RYBOVICH. (The history of this company is a very interesting read by the way,....)

The company, started in the '20's I think, by the mid '50's, had developed a very interesting boat-building process that very aptly demonstrates the consistent property of the expansion of wood planking due to moisture content:

Rybovich became world famous for building high-end sport fishing boats that had a very unique characteristic different than other wood hulled boats. Accepted wooden boat-building practice has always been to caulk the seams between carvel planked boats with cotton, driven into the seams with a "caulking iron" and caulking hammer, pretty much an "art" in itself by boat builders. Rybovich actually developed a process by which their world-class sport fishing boats were built with NO caulking between planks at all! By using Philippine Mahogany, and carefully drying the lumber specifically to a moisture content of between 12-15%, and then precisely spacing the planks so as to leave EXACTLY the perfect width seam between planks, the boat, when launched, would swell exactly the right amount for the hull to become water-tight, with NO cotton caulking or caulking compound of any kind! And again, that is how PREDICTABLY wood will expand due to moisture!

Again, a bit "OT" maybe, but certainly proof of what is being discussed here about the fact that wood will certainly swell (expand) and/or contract, due to variation in moisture content,.....FWIW,.....harold

MG,

I believe you have that backwards, all I see in practice and read is that wood is quite stable in length but expands and contracts with moisture content in width and thickness across the grain.

Walt

I used to own a very traditional wood boat. I kept salt water in the bilges to keep the wood swollen between launches (salt water to avoid it freezing in winter). Still, it would leak for about 15 minutes of being launched, then it would tighten up and we could go all day without any leakage--actually days, as I would keep it at a dock over weekends. Wood swells! The reason cotton caulking was use was that it would give as the wood swelled up and relax as it dried out to keep things relatively water-tight.

One of the best things about "messing" with Model T's is the opportunity to explore the wide range of crafts that went into building 'em ! Castings,forgings and stampings of different metals and high craftsmanship with leather, wood, glass, rubber and fabrics is what made the Model T endure !

Back to the pickup bed, you'll find square butted planks in wagon boxes because the bed planks flex against one another with the load and the vehicle's movements over the road, and tongue-in-groove or lap joints will eventually split. It's not a boat, so there's no need of making the bed leak-proof. In fact, it's better if it drains right off. Use well-seasoned, clear, straight-grain lumber and allow a little for swelling. Quarter-sawn planks would be really great if you can get 'em; sometimes you can come close if you can high-grade pieces from the bunk.

Iron bed strips are there mostly to prevent undue wear from dragging loads in and out of the bed or from shovelling.

What's the best wood species to use ? That's a really interesting topic. Seems most Model T bodies I've examined were framed with poplar. In the day, custom coach builders and the expensive marques seemed to prefer ash. Pickup beds were floored with wood well into the '50's, I wonder what species came to be preferred if serviceability was the main consideration ?

Last, paint will help preserve wood from the elements, but it's really quite impossible to "seal" wood against changes in ambient humidity with paint (or anything else). Sun and weather (water) rapidly degrade wood, so to preserve it, keeping it painted or well-oiled is a durn good idea.

FWIW.... I used ash on mine... (free Lumber from my father-in-law's back yard) and spaced them an 8d with apart. I want rain to drain through. I used shellac....looks like, but will oil it when the shellac wears off.... Personally I would think ship lapping would be somewhat doomed for failure... that pattern is designed to lay vertical, with the overlap on the outside, sheeting off the elements. Horizontal.... Likely wouldn't dry well ...or consistently, under the laps. Guess that's why decks have gaps. I hope it works for b you, though, as no doubt it would look nice!!

But... if you wish to throw the towel in on the project... I could use me some pickup irons! ;-)

Over the last 35 years i have built two wagons,one 7x14 and one 8x20. I used 2x6 pressure treated toung & Grove pine. Not a nail to be found i used carriage bolts. The treated lumber was installed wet but the tounge & grove makes the racks grain tight even when dry as a stone. I would use 5/4 tounge & grove treated deck boards.Bud.

All interesting info. I have a few weeks to mull it over before i start cutting.

I recommend that you assemble the bed and then separate it and paint all the surfaces of all the plates, especially the ends, with a good oil-based primer and paint to seal the wood.https://www.sunrisingbed.com/