I need to replace at least a few of the wood floor boards on my 2 horse straight load trailer.  It looks like a simple job, but I know that changes quickly once you get things torn apart, I believe in Murphy.

My question is what is used to secure the boards to the trailer metal frame/cross members? 

Thanks the reply!

My boards are sitting on the frame and attached with what looks like some kind of screws at each end and at two metal crossmembers.  I don't think they're regular screws, they don't seem to have slots in the screw heads.  What is the best fastener to use? I'm also concerned about getting the old fasteners out, I'm sure they're rusted in there and will need to be drilled out.

I believe they are called "carriage bolts." That's what we used when we replaced the floor in our cattle trailer. It was very simple and used I also believe 2x 6 and they were 14footers, that's what the older trailer takes. (If i'm not mistaken)

It's a very simple process, not very hard at all, with two people you can have it done in a few hours maybe less..............

If you are going to remove a couple of boards, I recommend doing them all. It'll actually be easier to remove all the boards than just a few in most cases. Or saw carefully. ;) Before cutting any metal, remove the old boards anyway you can. Power saws are helpful. Use a saw (careful not to saw into your trailer walls or frame) to enable you to do this easily. For the now rusted in screws (they probably had screw guides on them, but corroded away.), easiest way to remove the boards from them is chisel and hammer and pop them from under the boards. And as suggested use the measurements from the old boards total length to cut your new boards. Removal takes longer than installing. The new boards with a little tapping perhaps, should slide into the channel at by the door of the trailer. Get your drill and screws. If you want to use the original holes, get dirty and drill your pilot holes from the bottom of the trailer. Would be a good idea to weight down the boards to keep them in place as you do this. For more clearance, use whatever trailer aid kinda thing you have and could even borrow a friends to lift evenly. Safer than using a jack. If not worried about making new holes, drill from the top. Oh, one way you may try to get rid of the what is left of the old screw if you've broken them fairly flush with the frame is a hammer and a pritchel and tap them out. As for using screws versus carriage bolts, it depends on how big of holes you want to put in your frame and personal preference. They are really there more to keep the boards from slapping around when you travel than anything else. You will want a screw that goes flush with the board.

Have fun and it really won't be that bad.

Oh yeah. And look at the grain on the boards end. You want to put the board with the half moon shape arching up.  Think frown. This will help keep your boards flat in the long run. Otherwise the board will start to smile on you in the future instead of staying flat. And we don't like smiling boards.