Linn Westcott's classic book 'How to Build Model Railroad Benchwork' says: "As a rule, install a riser under roadbed every 2 feet or so ... for longer spans use thicker material to prevent sagging " (2nd ed. pg 40). Their typical subroadbed examples are 1/2 inch plywood.
I'm starting to build open grid benchwork with wall brackets tied into the studs instead of legs, so 32 inches would be a natural joist spacing. Although I could still build wall brackets at every other stud and then frame in the intermediate joists, I am concerned that 16 inch joist spacing would be a little 'dense' making it too hard to stand between joists during construction and hard to avoid the classic 'switch machine under the joist' problem. (I've used L-girder construction in the past, but because of height constraints, this time I can't afford the thickness of joists on top of L-girders.)
Is 3/4 inch plywood an OK thickness sub-roadbed for 32 inch joist spacing?
Not sure I understand your construction method, and why you'd need to stand between the joists during construction, but if you're using open grid, I'd put the grid cross-members 16"oc and ditto for the risers. You can still place the support brackets at every other stud. I don't like the chance of even 3/4" plywood not sagging under its own weight on such widely-spaced supports. If you happen to end up with a switch machine at a riser, it's easy enough to remove the riser and replace it with one offset appropriately.
Wayne
If, like me, you are among the less than svelte, 16 inches on centers is a leetle close. OTOH, what are you planning to do back there among the joists? I've designed my layout to be a reach-in operation - no tracks beyond 20 inches from the aisleway, no scenery beyond 32 inches, all electricals run along the fascia lines....
I put my joists on 'sorta 16 inch' centers, but I'm using L girder construction, so moving one (without attached risers) involves removing two screws and sliding it out of the way. As things have shaped up in the Netherworld, there's one 32 inch gap spanned by four steel stud 'through girder bridges' and two wide pine boards. So far, I haven't had a valid reason to install a joist there. However, that's a special case. I don't think I'd space all of my joists 32 inches apart.
My solution to saggy cookie-cut plywood has been to stiffen it up by screwing steel angle iron to the underside. Most of mine is 3/8 inch, but some is thinner. The layer of 9mm foam on top doesn't stiffen it up much - but it does kill unwanted noise.
Chuck (Modeling Central Japan in September, 1964)
grinnell I'm starting to build open grid benchwork with wall brackets tied into the studs instead of legs, so 32 inches would be a natural joist spacing. Although I could still build wall brackets at every other stud and then frame in the intermediate joists, I am concerned that 16 inch joist spacing would be a little 'dense' making it too hard to stand between joists during construction and hard to avoid the classic 'switch machine under the joist' problem. (I've used L-girder construction in the past, but because of height constraints, this time I can't afford the thickness of joists on top of L-girders.) Is 3/4 inch plywood an OK thickness sub-roadbed for 32 inch joist spacing?
I can assure you that 3/4 inch plywood laid on a 2x4 framwork, 32 inches on center, is not going to sag. On an earlier layout, I laid 1/2 inch plywood on 2x4 framing 32 inches on center without any sagging problems. In fact, what I did was to build three 4' x 8' tables, joined together to form a 12' x 8' layout with pop up openings for access inside this large surface area. The cross members were placed every 32 inches.
What I would be concerned about is that the plywood may "bow" upward so that the track will not lay flat. Plywood, whether 3/4 inch or 1/2 inch, needs to be screwed at 12 inch intervals around the perimeter of the frame and down the cross members that are laid at 32 inch intervals.
Having said that I share your concern for the framework getting in the way of trying to mount under table switch machines, even when the cross members are laid 32 inches on center. It is part of Murphy's Law that framing will be in the exact spot where a switch machine needs to be installed. On my current layout, I have 1/2 inch plywood on a 2x4 framework, and the framework always causes me grief when the time comes to install a Tortoise machine under the layout to power a turnout up above.
What I would suggest is to go ahead and build the perimeter of your 2x4 framework and secure it to your wall studs with brackets, as you have described. Then, mount the plywood, screwing it down onto the 2x4 framework at 12 inch intervals. Lay your track work and determine where your turnouts will be located and install your under table switch machines. After all that is done, you can install the 2x4 cross member supports 32 inches on center, taking into account the location of the under table switch machines. You can screw the cross members into the front of the framework from the front of the layout. To secure the cross members at the rear of the framework (which is now secured to the wall studs), you can go under the layout and "toenail" the screws at an angle into the framework.
One other observation. In my opinion, you can use 1/2 inch plywood instead of 3/4 inch plywood. That would be strong enough, and the 1/2 inch plywood would be cheaper, lighter, and easier to drill through when installing under table switch machines.
How deep is the framework going to be? Reach could be an issue unless the depth is 36 inches or less.
Just some thoughts. Hope this helps.
Rich
Alton Junction
Wayne, Chuck, and Rich all make good points. No matter how tight clearance issues get though I just would not space things out more than 16" center to center. Trains are light so the strength of the plywood is not really the issue. To me the main issue is the tendency for wood to 'move around and change shape' over time. If you happen to have 'dry' plywood and live in a dry climate you may not see it. If you end up with slightly greener wood and/or live in a region with huge winter/summer humidity swings even 3/4 can move a little. The best insurance is to do what they say and keep the joint spacing closer to hold the wood where it needs to be.
If you use Tortoiese Switch machines these can be operated facing either frontwards or backwards which gives you essentially two mounting positions in an attempt to clear a joist.
Lance
www.lancemindheim.com
I would be very surprised to find 3/4" plywood of a decent grade sagging between two supports 24" apart. In fact, I would not expect 5/8" ply to sag with the same support configuration.
Crandell
If you are using open grid the "joist" spacing is independent of the stud or support spacing. For example on my open grids, each grid is 6 ft long with cross pieces every 18". The studs in the walls are on 16in spacing. Although the grids are on legs it wouldn't make any difference because the grid joist/crosspiece spacing is independent of the support spacing. For switch machines I just flip the road bed riser to the other side of the cross piece or make the riser narrower and offset it away from the switch.
Dave H. Painted side goes up. My website : wnbranch.com
Not sag due to weight but it could twist, warp, or bow upward depending on the quality of wood and humidity. Shorter spans make it easier to anchor in place.
I live in Maryland. In the summer the humidity is so thick it fogs your eye glasses when you walk outside. In the winter it's so dry your hands crack. Wood doesn't like that type of fluctuation in humidity. Sealing the wood helps some but not entirely.
Thanks for all the good comments. Dr Wayne, you are right, I really don't have to get between the joists in this area. In this section I'll have main line in front, a yard and then both a climbing grade in back and a hidden track below comming up from staging. So until I build the yard it would be very convenient to have close access to the back tracks. In the wider sections with turnback loops against a wall, I have to have pop-ups between the joists and 24 inch spacing is fine for this.
Got away from the computer and had a better idea: build wall brackets at every third stud giving me 48 inch spacing. Then I can frame in a joist at 24 inches.
Grinnell
HoosierLine Not sag due to weight but it could twist, warp, or bow upward depending on the quality of wood and humidity. Shorter spans make it easier to anchor in place. I live in Maryland. In the summer the humidity is so thick it fogs your eye glasses when you walk outside. In the winter it's so dry your hands crack. Wood doesn't like that type of fluctuation in humidity. Sealing the wood helps some but not entirely. Lance www.lancemindheim.com
Steel Studs with Foam on top would eliminate this problem. Plus steel studs are cheap and only require tin snips and/or a hack saw to cut. Steel and Foam combo would be incredibly lightweight also. Just my .
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
And I found out the hard way; attach your risers with cleats from the bottom. If you screw them in from the top, it's a real pain to find the screws if you need to move/change them.
Marlon
See pictures of the Clinton-Golden Valley RR
richhotrain grinnell: .....Is 3/4 inch plywood an OK thickness sub-roadbed for 32 inch joist spacing? I can assure you that 3/4 inch plywood laid on a 2x4 framwork, 32 inches on center, is not going to sag. On an earlier layout, I laid 1/2 inch plywood on 2x4 framing 32 inches on center without any sagging problems. In fact, what I did was to build three 4' x 8' tables, joined together to form a 12' x 8' layout with pop up openings for access inside this large surface area. The cross members were placed every 32 inches.
grinnell: .....Is 3/4 inch plywood an OK thickness sub-roadbed for 32 inch joist spacing?
.....Is 3/4 inch plywood an OK thickness sub-roadbed for 32 inch joist spacing?
You're probably right on that Rich, but it's my understanding that the OP is talking about cut-out sub-roadbed, with support at 32" intervals only along its length - not full sheets with fasteners along the edges, too.
richhotrain What I would be concerned about is that the plywood may "bow" upward so that the track will not lay flat. Plywood, whether 3/4 inch or 1/2 inch, needs to be screwed at 12 inch intervals around the perimeter of the frame and down the cross members that are laid at 32 inch intervals.
Part of the reason that plywood bows up in the middle is due to improper application of the screws or nails holding it in place. I can't count the number of times I've watched supposedly experienced carpenters fastening the edges (all edges) of plywood in place, then going back later to tack down the centre. The proper method is to start at one edge (the one adjacent to already-installed sheets) and work your way across the entire sheet, securing the opposite edge last. This eliminates the chances of trapping stress in the installed sheet, which can cause it to bow upwards later.
Building codes vary, and aren't necessarily applicable to layout construction, but the nailing schedule for plywood in this area is every 6" along edges, and every 12" across the centre of the sheet.
In planning any benchwork, always consider what you will need to reach from underneath after the scenery is installed. If necessary, plan your turnouts so that the operating machine will be well away from a cross-member of the benchwork. Unless your layout is to be flat, plan on using risers for all sub-roadbed, so culverts, canyons, washes, and rivers can be modeled below the basic track level.
Max span also depends on the grade and type of plywood. For example CDx is poor quality while AB is good quality. Many brands have the max recommend span marked right on the sheet. as mentioned by DrWayne, this is for a whole sheet properly nailed.
Personally I wouldn't go 32" on cookie cutter strips but gluing 2" foam on top will add some rigidity if there is no other solution for you.
The mind is like a parachute. It works better when it's open. www.stremy.net