OK, finally got the missing lag screws in hand and went back to work on the greenhouse trailer. (Actually over a week ago but things have been busy and I'm just now getting around to posting this.)
I marked out and drilled holes in the top rails of the trailer, one hole for each end of each deck-board. 24 holes in all. Yep, 24, half of 48, not the 14, generously rounded up to 20 that I somehow calculated - miss-calculated - when I bought the first batch of lags. Of course that actually made 48 oily, messy holes I had to drill since the first hole was with a well oiled 1/8th bit which I then had to follow with an even well-er oiled 1/4 bit. Those top rails are thick!!
A wipe down and a quick shot of grey primer down the top rails will, hopefully, slow down the corrosion between steel and wood.
Being careful to measure out the same amount of overhang on each end, I laid the first 2x10 deck board even with the front rail of the trailer and screwed it down to the joist I previously placed down the center of the bed using 3 deck screws. Then I had to crawl underneath and drill pilot holes up through the pre-drilled top rail at each end of the deck-board for the lags. Getting smart, (Quick, use it while you've got it because it doesn't seem to last very long anymore!!) I drug out my air powered ratchet, which I very rarely think to use, and drove the lags home tight. (If you remember, I now have an air compressor that can keep up with me.)
Using a couple scraps of wood as 3/4" spacers between boards, I laid the second board in place, making sure one end was flush with the previous board, and repeated the fastening routine.
12 boards later I had a deck for my greenhouse! I did have to remove the tires, one at a time since I only have one floor jack, in order to reach the lags for the one board right over the fenders so it wasn't like I finished in 10 minutes or anything, but I did get it done. Boy there sure was a lot of getting down on the ground and back up again along the way!!
The next step was to measure and mark the first and last boards to exactly 10 feet in length, 5' to either side of the centerline. All the boards were anywhere from 10' 1/4" to 10' 1/2" in length. After snapping a chalk line to these marks on each side I ran my saw down the lines and evened everything up.
The final step for the day was cutting and fitting the skirt-boards around the perimeter of the deck, leaving a 4 foot opening at the rear of the trailer which will be the door. This also required a lot of getting down and back up again since most the deck screws had to be installed from the bottom up.
Now, some measuring and fine tuning of my drawing and I will make a materials list for the walls and roof for the next time I take the van into town. (Our other vehicle is way too small to carry things like lumber!)