Building first HO open grid layout. Cork roadbed will sit on 2 1/4" wide1/2" plywood. Any recommended distance between risers. I’m thinking 2 feet between risers maybe closer on curves to eliminate any twisting. Thanks for your suggestions.
With 1/2" plywood, I’d go closer than 24" for structural reasons. Probably in 16" to 18" max. Of course, I use 3/4" on that spacing or closer, so I tend to be in the overkill crowd on benchwork. But then I can get up on top of it if I need to[Y]
Another issue comes up with grades. If your spacers are closer together, it’s easier to get a smooth vertical curve transition. So you may want risers closer together at the start and end of grades.
I think something a little tighter than 2 feet would be best for the thickness you mention. I am sure someone with experience using plywood sub-roadbed will pipe up urging you to consider something more along the lines of 16-18" tops.
Crandell
For the same basic set of materials I prefer >16< inch riser spacing. In places where that isn’t practical (no understructure to fasten a riser to) I provide longitudinal reinforcement under the plywood.
Chuck (Modeling Central Japan in September, 1964 - in a Southern Nevada garage)
Using the same materials that you described 1x4 benchwork, I chose 13" spacing and I’m well satisfied with the results. This turned out to be a good compromise between me being able to squeeze (barely) up between the joists (inhale!), and the structural stiffness I wanted. I believe 16" is pushing it and 24" is better left for barn framing applications. At the end of the day, unless you’re building a club-sized layout, the savings from wider spacing in time and material is insignificant. But then, why use a 2x4 when a 2x10 will work?
Jim
I believe that 24" spacing for supports of 1/2" plywood is close enough. To satisfy yourself, simply take a 24" long strip of plywood and try bending it. Note how much force it takes (if you can bend it at all(. Remember that it will carry almost no weight. Properly securing it to risers should eliminate possible twisting (which can also be minimized by painting the strip, especially if your layout is in a non-climate controlled environment subject to high humidity).
Dante
Most important, is the manner in which the risers are constructed. Assuming that your longitudinal boards are against the wall, and along the aisle, spacers between them should be as widely spaced as will provide rigidity; in no case closer thgan three feet. This will allow ample room to come up undeneath after the scvenery covers the benchwork.
Risers should be constructed in an inverted “U” shape, with the vertical parts attached to the wall and aisle longitudinal boards, and a cross piece between them. This will allow you to come up underneath, without having to squeeze between close-set cross-pieces to reach wires or switch-machines.
It is also possible to attach the vertical portions of the inverted “U” to the spacers between the wall- and aisle- boards, at a slant to provide a grade.
I made the mistake of placing too many braces and cross-pieces in my own benchwork, and it is difficult now to reach through them to adjust , repair or replace switch machines. Once scenery is in place, under-layout work takes on a very different character.
Given the quality of today’s plywood, I would narrow the spacing of the risers to 16". Over time, I have seen 1/2" plywood sag between 24" supports. Linn Westcott published photos of it where it happened to him on his home layout. The sag doesn’t have to be much to be disruptive - 1/8" sag in 24 " is more than a net 1% grade each side of the low point. Many an experienced model railroader has switched to 3/4" plywood subroadbed because of sagging issues (over time) with 1/2 " plywood.
Ironically, where the plywood is bent in a curve in vertical transitions, it needs less support. Introducing a bend stresses the plywood and makes it more rigid.
The plywood is not twisting on curves and grades - it is attempting to untwist. A twist has to be induced to hold it level cross-ways on a curving grade. The sharper the curve and the steeper the grade, the more pronounced the natural positive and negative super elevation will be in the untwisted plywood. Again, inducing the twist actually makes the plywood more rigid, but you need more numerous supports to put the twist where you want it. I learned this the hard way using cookie cutter 1/2" plywood subroadbed with 18" radius curves and 4% grades.
my thoughts, your choices
Fred W
I agree with Fred: while the weights that need to be supported are generally of little consequence, 1/2" plywood not well-supported will sag, over time, under its own weight. I used 3/4" plywood on risers generally 16"o/c or less. For vertical curves, place a riser just before the spot where you want the grade to begin and another where you want it to end, then place a third at the mid-point between the two, adjusting the height to keep the overall grade constant. Continue to sub-divide the spaces and adjust the height until you get somewhere near the 16" spacing. The exceptions should be at the bottom and top of the grade, where the plywood will have formed vertical easements of its own accord: when adding these risers, do not adjust their height.
Here’s my method for adding superelevation when using risers on open grid benchwork, copied and pasted from a Word document:
If you use open grid or L-girder benchwork, superelevation is easy to add, including the vertical easements into and out of the curve.
I use 3/4" plywood as a sub-roadbed, but any similar-type material, or even spline roadbed should also work. Install the straight roadbed on either side of the curve by fastening the risers to the joists - leave the last riser beyond both ends of the curve unfastened. Install risers to the underside of the curved roadbed, but don’t fasten them to the benchwork just yet. If your curve is on a grade, as most of mine are, raise the roadbed through the curve to the proper height, then mark a pencil line on each riser which corresponds to the top of the benchwork to which it will eventually be fastened. If the track through the curve is to be level, adjust the risers accordingly, then make the lines. Next, choose the riser closest to the mid-point of the curve, raise it to the proper height, then push the bottom end of the riser towards the outside of the curve. Re-align the height line on the riser so that its inner end cor
Slightly [#offtopic], but pertinent.
To keep from having problems with switch machine mounting and access under the layout, move the switch machines out to the fascia line and use a tension cable and weight, in conjunction with an Anderson link, to move the points. Likewise, if the wiring is run along the fascia (or close inside it) the only wires you will need to get at under the table will be newly-installed drops pushed down from above. I fish them out with a bent clothes hanger.
Try making risers from steel stud material. You can bend attachment flanges at the top (no separate cleats or pocket drilling) and they can be anchored to anything with those little steel stud screws. The only tools needed are a good pair of tin snips, vice grips and a power drill with a crosspoint screwdriver bit. Just mount one flat side against the joist or stringer - it doesn’t even have to be the wide side. Some of my steel stud risers have stepped tops and support plywood subgrade at two different levels.
If all the fasteners are driven from below, changes can be made without disturbing finished track or scenery above.
Chuck (Modeling Central Japan in September, 1964 - with steel stud benchwork)
HI All: When I built my layout table (5’ X 9’) back in the 70’s, I used 1/2 inch plywood for the yard area. It is supported on 20 to 22" centers. It now has very small sags between the supports. I didn’t notice it until I started putting metal wheels on my rolling stock. When I would uncouple a car on a siding, it would coast to the low point between the supporting girders under the plywood. At first I thought the table was not level, but when I put a 3’ level on the table between the yard tracks I saw about a 1/8 " gape under the middle of the level, but the bubble in the level showed the ends were level. I am trying to figure how to shim the tracks in the yard so the cars won’t roll after they are uncoupled. I don’t have all the yard tracks laid down yet, but I have a sheet of foam rubber laid down (glued with caulk) for roadbed, about 14" wide. This layout sat unused for about 27 years, before I started working on it again. I started on it in 2002 and it wasn’t until this year that I noticed the sag.
Sam
The problem with sagging 1/2" plywood is not due to the weight on top of it, but due to the weight of the 1/2" plywood itself. It WILL sag if you go beyond supports on 16" center.
Gravity – it’s the Law.
[I]
For 3/4" plywood, 36" is too wide, also. This is especially so if you plan anything except dead flat grades, as it will be difficult to get your vertical curves right with such wide spacing.
While access underneath is important, the places where I need to get up between the crosspieces are planned out specifically as pop-ups. If everything is on 3’ centers, you’re going to regret it.
While one may hope underneath-work will not be required, 62+ years in the hobby convinces me that things will break, or fall out of adjustment. (Murphy’s law, y’know). To each his own, however.
To Mr. Lehman, Urbana, Ill. Plywood on edge is unlikely to bend downwards. Thinner track-supporting material can be placed on top. Detailed planning ahead is always advisable, to leave plenty of access from beneath in case it becomes necessary.
Absolutely correct. Plywood used vertically is stronger than the same size solid wood.
But we were talking the effects of potential droop on plywood installed horizontally as roadbed between risers.
Yes, for any potential maintenance, be sure to provide access. For instance, I have a water meter under one corner, which the water company replaces every 10 years. So far, they haven’t sent the big guy who used to play college football to change it yet[:)]
So I agree you should always plan for access. However, in most cases there are better design practices for doing that than using risers that are 3’ apart. Pop-ups are one way, just making sure the reach is not beyond your arm is another.