I can’t find my Westcott book on benchwork since moving to a new place, so I’ll trouble you for some very basic information. I’m using three-quarter-inch plywood as subroadbed. What’s a reasonable span between joists for this material? Put another way: how far apart should I place my joists to maintain a solid support for the track?
16" or less, same as house framing.
If you’re working in HO or smaller, I’d say 24" spacing is more than sufficient. For larger than HO, go with 16" to support the greater weight of the trains.
I agree. On my HO scale layout, the cross members are spaced 24" apart.
Rich
To expand on my initial reply… your risk is more from plywood’s natural sag over time than it is about weight carrying ability. Trains and scenery weigh virtually nothing relative to the load carrying capability of plywood. Consider snow load on house roof sheathing with trusses on 24" centers. It is the never ceasing pull of gravity and humidity fluctuations that will over time bring about change.
The type or quality of plywood is an important consideration. High quality, cabinet grade, many veneer plies, hardwood plywood will be much more resistant to sag than lower grade pine sheathing construction plywood. Are you using $20 a sheet plywood or are you using $50 a sheet plywood? Makes a difference.
Additionally, the form factor and attachment of the plywood to the benchwork will come into play. Cookie cutter narrow strips of plywood attached only at risers will be far more susceptible to sag than a sheet of plywood that spans the full width and length of the benchwork and is secured all way around its perimeter.
I agree with the others in that if your construction method is the latter (tabletop sheathing) then 24" centers is plenty sufficient. I would add that if you use very high quality plywood larger spans can be utilized. Consider that many 4x8 tables have but two crossmembers making the span 31". I disagree with the others if your construction is the former (cookie cutter or greatly carved up to create landforms). In this situation the spacing of support becomes very critical hence the 16" or less recommendation. Especially when narrow strips (sub-roadbed) are used.
There is also your expectations to consider. How much sag will you tolerate? Model trains will tolerate a lot providing the change is not abrupt. For my own personal tastes, I won’t accept any visual sag and considering the extremely low cost of adding crossmembers, I err on the side of too many rather than too few. Why try to save less than a dollar if it presents a potential i
Yeah, weight of the trains is of little concern for the spacing of supports, as just about any method can hold up the trains by themselves. You must consider the natural tendency of construction materials to move over time, and inadequate support will lead to sagging roadbed.
Getting the crossmembers too close is also a potential problem though. One of my buddies decided that if 16" centers were good, 12" would be even better. The problem is that the narrower spacing makes working between the supports for wiring miserable (in places where the roadbed is on tall risers), and it’s sometimes hard to get a drill or other tool to fit comfortably for things like drilling holes for wire runs or adding risers with screws.
If you read chapter 5 carefully, he mentions spacing of 16 to 20 inches for 1/2 ply subroadbed, however, in several diagrams it looks more like 24 inches. Multiply 3/4 plywood (the $50 type - find it on sale) would probably span longer distances without sagging. I suppose one could at a 1x2 vertically under the roadbed (forming a T girder) to reduce that tendency even more.
Alan
Getting the crossmembers too close is also a potential problem though. One of my buddies decided that if 16" centers were good, 12" would be even better. The problem is that the narrower spacing makes working between the supports for wiring miserable (in places where the roadbed is on tall risers), and it’s sometimes hard to get a drill or other tool to fit comfortably for things like drilling holes for wire runs or adding risers with screws.
Good catch Rob. Excellent point I forgot to mention. Right angle drills help a lot in tight quarters. I have resorted to it many times and my benchwork is built on 16" centers.
I would use 3/4" 13 ply birch plywood with support every 16". I once laminated 2 pieces of cheap 3/4" plywood together for a 3 ft span across a doorway. Over time (15 years) it sagged enough to be visible, fortunately not enough to affect operations.
OTOH I have some bookshelves made with 3/4" 13 ply birch plywood 4 ft long that are 30+ years old and have never sagged even loaded with magazines and books.
Good luck
Paul
I went with basic 16 inch spacing for the joists on my steel stud benchwork. When my narrow, thin subgrade acts up I beat it into submission with steel angle iron (screwed to the underside between risers.)
One of my joists turned out to be unnecessary - the tracks crossing it were all supported inside steel rain gutters (steel studs laid open-side up like through girders.) That joist was removed and recycled for use elsewhere. That’s the beauty of L-girder construction with all fasteners accessible from underneath.
Chuck (Modeling Central Japan in September, 1964)
24" spacing has worked fine in a past 4’x6’ “HO” layout (2’x6’ grids), and 4’x4’ “N” Christmas layout (2’x4’ grids) – Each used 1/2" Homasote (not plywood) – Each could support your weight.
U-shaped CR&T will employ 3/4" cookie-cutter, and the grids will be smaller than 24" to obtain greater “supports” flexibility for the cookie-cutter elevations, and mild L-girder needs. CR&T must use plywood for “no-wiggle” traction poles between live overhead – “Take no chances.”
Foam can always be used between trackage for scenery-carving. Example: See Tony Koester’s July 2013 MR “Dig those ditches” (pages 33-37) incorporating only 3/8" thick insulation board (foam) over plywood. Pelle Soeberg’s preceeding article “How to make track more realistic” (pages 28-32) has one picture (page 31) with L-girder cross-supports that appear to be less than on 24" centers.
Hi,
If you are using 3/4 inch ply for a base, 24 inch spacing is just fine. Do make sure that you attach the plywood securely with screws (coarse thread deck/drywall 2inch) every 12 inches and corners.
If you haven’t already got the 3/4 ply, you may consider 1/2 inch instead.
I used 16" on 5/8" plywood, conservative, but I varied that, plus and minus, on my smallish layout where I wanted to be sure turnout throwbar / Tortoise installation warranted tweaking the joist location. There were also places where, with cookie cutter elevation changes, cross-supports came into play. In one or two cases, I left the joists our initially until drawing the layout on the plywood and/or cookie cutting, so I could add the joists in the best spot. Having the layout planned on XTrackCAD helped me see where issues might arise.
Another item on joists and cross-members: as you work on framing pieces, it’s a lot easier to drill holes for wiring and then assembling, versus adding holes later. I just put holes every 6-8", using those in most cases and adding a few extras as needed when all assembled and adding the (DCC buss, in my case) wiring.
The quality of the ply and the width of the subroadbed will have the greatest effect on how far the joists should be spaced. If using risers and 3/4" x 2- 2 1/2 single track I would not reccomend spanning greater than 16".
To the OP…
BEFORE you attach the plywood, you might want to drill holes in all the joists thru which you can pass wiring. This keeps it up out of the way, neat and organized.
Yes, you can drill holes after the plywood is installed, but it is just making an easy job difficult.
Fiberboard, such as MDF or especially particleboard, will sag as some have described here. Particleboard is particularly unsuited to model RR use, since it sags tremendously even on short spans.
Flakeboard i can’t speak to, since I’ve never used it in a flat installation (only vertical as in walls).
But unless you are in an extreme climate with huge swings in humidity, good old BC plywood will not noticeably sag using the spans I described earlier for 1/2" and 3/4" thicknesses. If your humidty changes enough to cause sagging problems, that will be the least of your issues.
That goes for tabletop and cookie-cutter type installation.
As far as weight of the trains not being a driver - it certainly is! But the issue isn’t the sag induced by the weight - it’s the vibration. The plywood roadbed will oscillate with the passage of trains, due to the non-stationary loads induced by atrain’s passage (think of the roadbed as a drumhead, and the joists are the rim of the drum). A greater gap allows more oscillation. So greater weight on larger spans will induce much greater oscillations (more than just the product of the span delta plus weight delta). And I’m not talking about sound transmission - that a different but related issue. I’m talking about the magnitude of the roadbed “bounce.” If it gets to be too much, it can be problematic.
I use a template to make my joists. Align the tops and ends, then clamp them together. Not only do I drill for wiring, I drill for alignment dowels. Before unclamping them, turn them so what will be the bottom side is face up. Draw a line across the bundle and arrows facing the same direction. Then, when you get ready to use them, they are all aligned before gluing and nailing/screwing them together.
I used to drill holes through joists for wires. Never thought far enough ahead to drill before attaching the subroadbed, though.
Now I attach wires to the bottom of the joists using plastic P-clamps. No need to drill all those holes, and if I need to add wires, I just remove the screw, add the aire and re-attach the clamp to the joist.
Another advantage to using steel studs to frame benchwork - every joist and C (works like L) girder is a wire run. Short lengths of cap stud snapped on every 12 inches or so keep everything inside the joist or girder. I don’t have any ‘flying’ wires between joists - all my wiring runs panel-main girder-joist-end user.
Chuck (Modeling Central Japan in September, 1964)