What thickness of plywood is best for ribbon subroadbed? I'm building a 10'x10' HO scale layout with lots of mountains and 2-4 percent grades (and tight curves--minimum radius 20"). From what I've seen searching the archives, 3/4" plywood may not bend enough to get a good transition into and out of the grades, especially with sharp curves--but then others say that 1/2" plywood (when cut to 2.5-3" wide) may sag. What do you suggest?
If I were building roadbed of the type you describe, I would use some good quality (at least GIS, or good on one side) 1/4-1/2" plywood. It kind of depends on the support you'll have. Will you have it on risers, and if so, how far apart will they be? If on longer solid blocks carved and planed to act as wedges contoured to the grade, then you could just use luan or masonite. The wider the span under any one section of the roadbed, the thicker the product should be, as a general rule.
If you would be more comfortable with a thicker product, then what you can do is use 5/8" for the majority of your roadbed, but use a more flexible, thinner, product to form the transitions you'd like. Glue/screw the thinner stuff flush with the table surface at the commencement of your grade and then bend the end furthest into the grade up to a riser or support....there will be your transition between the two extremes. With the tension imparted by the flexure, you might only need a single small riser block midway under that transition span for support/insurance.
Well, if you use 3/4" and it isn't flexible enough you can always cut ribs through one of the plys on the bottom where it needs to bend.
What roadbed are you going to use? 1/2" plywood with 1/2" homasote glued over it is pretty strong. You could also consider intermediate sizes like 9/16" and 5/8".
It also depends on how far apart the support joists are. The further apart they are, the thicker the plywood needs to be.
I built a 4x6 L girder alyout using plywood ribbon roadbed. I used 1/2" plywood, supported on 18" centers by joists. The layout had 4% grades and 18" radius curves.
I would not have wanted to try that layout with 3/4" plywood. As it was, my issue was not with sag, but the two dimensional bending required to accommodate grades on the curves. With curves that sharp combined with the steep grades, I found that I naturally had negative superelevation on the lower half of the curve/grade and positive superelevation on the upper half of the curve/grade. I had to run additional risers and cleats and forcibly fasten the plywood level across the track (or put in the superelevation I wanted). The 2 way bending with the 1/2" plywood took enough force - 3/4" plywood would have been very difficult.
Note that putting all your joists parallel on a rectangular table sets you up for long distances between risers on curves. L girder allows you to easily skew or orient the joists for better support under curves.
Finally, bending plywood makes it more rigid than leaving it flat. I found 1/2" plywood could go up to 24" between risers pretty successfully, especially on the grades.
just my experiences, yours may differ
Fred W
selector wrote:If I building roadbed of the type you describe, I would use some good quality (at least GIS, or good on one side) 1/4-1/2" plywood. It kind of depends on the support you'll have. Will you have it on risers, and if so, how far apart will they be? If on more longer solid blocks carved and planed to act as wedges contoured to the grade, then you could just use luan or masonite. The wider the span under any one section of the roadbed, the thicker the product should be, as a general rule.If you would be more comfortable with a thicker product, then what you can do is use 5/8" for the majority of your roadbed, but use a more flexible, thinner, product to form the transitions you'd like. Glue/screw the thinner stuff flush with the table surface at the commencement of your grade and then bend the end furthest into the grade up to a riser or support....there will be your transition between the two extremes. With the tension imparted by the flexure, you might only need a single small riser block midway under that transition span for support/insurance.
If I building roadbed of the type you describe, I would use some good quality (at least GIS, or good on one side) 1/4-1/2" plywood. It kind of depends on the support you'll have. Will you have it on risers, and if so, how far apart will they be? If on more longer solid blocks carved and planed to act as wedges contoured to the grade, then you could just use luan or masonite. The wider the span under any one section of the roadbed, the thicker the product should be, as a general rule.
I'm using open-grid L-girder benchwork, so I can put joists and risers almost anywhere (but it would seem best to not clutter things up with too many if possible, especially near turnouts). I hadn't thought of using thinner stuff for the vertical curves, but that's a neat idea--thanks.
Incidentally, I'm planning to use cork roadbed, so that won't add any strength. I thought about Homasote but just can't see any reason to go through cutting out the whole track plan twice (subroadbed and then roadbed), not to mention the dust.
As others have hit on, it's all about support.
My own layout (current and last) uses 1/4" luan with 1/4" foam so I can keep the layers thin. I support this on 12" centers and have had no issues with warpage or sagging. My last layout was up for over 2 years. True, they were both in climate controlled rooms, so that plays into it, but it can be done. It's all about the support though.
fulton wrote: What thickness of plywood is best for ribbon subroadbed? I'm building a 10'x10' HO scale layout with lots of mountains and 2-4 percent grades (and tight curves--minimum radius 20"). From what I've seen searching the archives, 3/4" plywood may not bend enough to get a good transition into and out of the grades, especially with sharp curves--but then others say that 1/2" plywood (when cut to 2.5-3" wide) may sag. What do you suggest?
I used 3/4" plywood (cabinet grade) for my layout. The transitions worked fine - they are gradual, as needed. I did not have any side to side elevation problems. All of my curves are either 20"r or 18"r (most). The picture shows 100' of mountain track work at an average 2% grade (24" climb in three loops).
The following relates to a two car garage-filler with grades to 4% and radii down to 15 inches (very short equipment only. Mainline is 24 inches and larger.) Basic benchwork is L girder, exact location of joists (except for leg-fasteners) negotiable. Subroadbed is 3/8 inch plywood, supported approximately every 16 inches. Roadbed is fan-fold underlayment (thin foam plastic.) Where the surface plies parallel the track I have had to fight side-to-side cupping (handled by two or more screws through wide riser screwing flanges.) Where cookie-cut curves ended up with the surface ply grain across the rails I had a few 'wannabe roller coaster' sections, which I beat into submission with heavy angle iron (screwed on lengthways of the subgrade.) I did find a few places where the plywood didn't take quite the transition curvature I wanted. Those were handled by shimming between the subgrade and the foam roadbed.
So far, the 3/8 ply hasn't sagged - possibly because of the other stress issues I mentioned. Left to its own devices in my dessicated Mojave Desert garage, plywood tends to take on unusual shapes. Forcing it into the desired configuration has been fun.
By way of comparison, the one sheet of 3/4 ply I own has turned itself into a free form salad bowl. I doubt that it could be straightened enough to be useful with anything short of a hydraulic press.
Chuck (modeling Central Japan in September, 1964)
Hi, I used a 2-layer laminate of 1/4" plywood (good-1-side) cut 3" wide and glued together with white glue. That produced the equivalent of 1/2" plywood. Try to keep the grain of the layers parallel to the trackline for more strength. If I were to do it again, (and I will) I'd suggest maybe a bit wider so as to be stiffer between widely spaced riser/supports, but that may interfere with ditching. This gave me great control over vertical easements; by mounting the lower layer and truing it up without the upper layer, it was easy to get a smooth base to glue the upper on later. You'll need a lot of clamps! Do not screw the first layer to the supports from above. After applying the 2nd layer, the screwheads will not be accessible if you want to introduce slope changes by shimming from below.
A further benefit was that in staggering the joints by at least 6", there are no cusps to have to sand down, and the roadbed is in fact a continuous piece of wood. Think of a 11'x14' cookie-cutter on L-girder framing. I topped off with cork. Scenery will be free to do its thing independently of the roadbed, and not carry noise from it.
How to get the 2 layers to be the same curvature and lie neatly on top of each other? I had drawn the trackline 1:1 size on a large piece of the heavy paper that came inside a roll of vinyl flooring and transferred it to tracing paper: once for each layer to mark on the 1/4" ply, so the pieces would be of random lengths to get the joints staggered. It also created very little wastage of plywood. The brown paper is still on my floor and is great to keep slops off the painted concrete. I'll lay cushioned tiles after the messy stuff is done.
Think about it as an idea. It might work for you too.
Have fun, George
Great idea George,
I plan to use two overlapped layers of 1/4" pts ply for my helix (next winter).
For slopes, I used 3/8" BC with supports every 24". I kept it wide though (4-8"). After establishing proper slope, grade and lean, I screwed and glued to the supports then glued on a layer of 1" foam. It's been extremely stable.
The foam adds some rigidity to the thin plywood and transitions were relatively easy to construct too. Here's a image. Unfortunatelyit doesn't show the transitions.
You can see some of the support structure by clicking through the photos at http://stremy.net/SRA/Benchtop/index.htm
You can see there that I experimented a bit with WS foam risers UNDER the subroadbed and with compound angled pieces of cabinet grade 1/8" plywood too.
Experiment a bit Fulton and good luck.
Karl
The mind is like a parachute. It works better when it's open. www.stremy.net
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Thanks for all of the ideas. It's interesting to find there is no single "correct" approach.
George--it sounds like your two-ply method might smooth out the joints and transitions and give an almost seamless result (and possibly stronger for the thickness than plain 1/2" plywood). Perhaps the fact that 1/4" plywood is easier to cut may make the extra cutting more palatable? Maybe I'll give this a try...