After taking the "Building the BV 1940s Gas Station" diversion, I'm getting back to building the layout itself. I posted a detailed build thread on the gas station in the Buildings and Structures forum, so I won't belabor the point here.
Yesterday I started to actually install joists and risers while the grandsons were occupied working on other projects. I'm building the layout with the table level a little over 43" from the floor. Therefore, the heads of the risers need to be 42.5" inches off the floor. After I installed one, I leveled the next to it and so on, periodically checking the floor to riser height just to make sure there's no collective error creeping in. The joists are 16" on centers like normal stick-built home construction. To level risers at remote distances from this first, "Home base", riser, I'm using a water-tube level that will set the height many feet from this point. It's a bit challenging to use, but it works and ensures that the entire layout is on the same plan, unless I want to change the grade. I'll document it's use when I get to that point.
I already made a slight error that was good to catch at this early stage. For some reason I had in my head that the elevated track portion in the rear most part of the layout was 5.5" inches above the base level and made the first two at that height. But I was doubting my memory so I checked the plans on RR Track. Yup! I was wrong! The height is 5.0" above base level. I was restricted to this dimension if I wanted to keep the grades at 2º or lower. At the termination of both ends of the grade are switches. I didn't want the grades to pass through them so I had to stop one track length in front of each, which created the height restriction. As you guys know, as in real life, railroad design is one compromise after another.
I square up the joists with the girders and square the riser heads to their risers. This isn't critical, but it's just the way I work. Any out-of-squareness in the head would be adjusted as I level the heads when fastening the riser to the joist.
I use clamps all the time, the quick-connect kind. I clamp the joists to the girder before I drill a pilot hole and screw them with a single screw at each end. You only need to stabilize them before the ply goes on top. Once the ply is screwed with two screws to each riser head, and with the load basically straight down, nothing really moves. It becomes a very strong inter-connected network. And it's one of the read advantages of L-girder construction in that it minimizes construction materials about as much as possible while maintaining a light and strong structure. Then I clamp the riser to the girder and use the soft hammer to tap to the correct height and level. I fix one screw and remove the clamp. Then I level it to its neighbor and put in the second screw.
To finish the grade, I'll install the riser at the end of the straight run in the back with it's specific height. Then I'll run a string line from the top riser to this one. All the intermediate risers will be fastened touching the string. This will ensure a nice smooth grade. When it goes around the corner it will be a bit trickier since the string will need something to hold onto while it bends around the curve.
Sometimes, the screw heads on the riser head are obstructed due to other structural pieces blocking the way. For these instances, I bought a flexible shaft adapter for my cordless drill that lets me bend the driver around corners and still drive the screws. Also, in these instances it's good to use the star drive screws since they forgiving when trying to drive them slightly off access. Can't say the same for Phillips head... off access and you'll strip out the screw head.