Thankyou for the explanation. However, I have been in the Custom Millwork industry for the past 25 years or so and I am perfectly aware of how plywood is manufactured and I am equally aware the plywood is far less stable than either Part. Bd. or MDF. The problem was that I did not understand your previous post. Simply stated it did not make any sense to me. I will have to read it again and see if there is something I missed.
GUB
Plywood comes in odd layer increments (1 layer, 3 layers, 5, 7, etc)
4x8 std sheets have the grain of the wood on the outer layers (the face, not the edges) running the 8' direction. Next layer has the grain running the 4' direction, then the 8', then 4'.......
Because of this the orientation of the grain (more layers with 8' grain directions than 4') the 8' long layers are applying more pull/strain/stress than the 4' layers. So this equates out to an unevenly stressed structure. If you had a sheet with an equal number of 4' and 8 grain runs then you would have a structurally stable sheet.
However, this is when higher quality plywoods come in. The more layers the plywood has the stable it becomes, regardless of total thickness. For example, say you have a sheet of 3 layer ply and a sheet of 7 layer ply. That 3rd layer constitutes 1/3 of the overall strentgh of the sheet whereas the 7th layer on the other ply constitutes 1/7 of the overall strength.
To see how flat the sheet (regardless of layer count or thickness) really is you need to stand it on its end and make your decision there. You will be hard pressed to find any plywood that will stand upright on edge and remain what appears flat.
As for the thinner sheets, they lack enough structural stability to hold themselves flat.
This help at all?
concretelackey wrote:also, the thicker the plywood the more it has a tendancy to appear warped. Plywood has an odd number of layers so there is always one dimension (the length) that wants to pull harder than the width. I have never seen plywood thicker than 3/8" that actually seemed flat on its own.
HUH????
I use 1/2 inch 5ply AC plywood. AC has no plugs or knots on one side. I paid $25 per sheet at local lumber yard.
Wayne
Where does one get good quality 1/2" birch 5-ply cabinet-grade plywood. My local Lowe's and Home Depot only carry 3/4" "hardwood" "cabinet-grade" plywood at approximately $45/ 4' x 8' sheet and it usually looks like junk. I was going to use 1/2" B-C grade pine plywood under 2" extruded blue foam for my N scale layout bench top on "L" girder framework. Even the 1/2" B-C plywood usually looks bad, i.e., badly warped, at Lowe's or Home Depot. I haven't checked local lumber yards yet.
Bob
camaro wrote: I used 1/8 inch lauan glued on top of 1x4 pine frame. I have a around the wall layout that is build in 24 inch by 6 foot sections. I then have 2 inch foam board glued to the lauan. The use of 3/4 inch plywood is overkill. It's not like you are going to be standing on it. My framing has never moved and even it it flexed, the 2 inch styrofoam does not. You need to use common sense when building the framing.
I used 1/8 inch lauan glued on top of 1x4 pine frame. I have a around the wall layout that is build in 24 inch by 6 foot sections. I then have 2 inch foam board glued to the lauan. The use of 3/4 inch plywood is overkill. It's not like you are going to be standing on it. My framing has never moved and even it it flexed, the 2 inch styrofoam does not. You need to use common sense when building the framing.
c:
I think the preference for 3/4" is usually where open-grid benchwork is used with plywood-strip subroadbed. With a solid top, you probably wouldn't need that much strength, and your 1/8" material laminated to foam should work fine. Foam alone would probably be rigid enough, but the wood underneath seems like a good idea, to give a stronger connection to the framework, and to be a nice surface to screw undertable equipment to.
A 3/4" CDX solid top would give greater weight, which could actually be an advantage in certain cases. A heavy table doesn't shudder or skid, spilling the coffee in the diner, when it's bumped, and of course there are those who really do want something they can stand on. Of course, with that kind of strength you introduce the temptation to use the completed, unused table as a workbench, and end up having to build another table for the railroad.
My own railroad has a solid top of 1/2" CDX. It's rigid enough and not too heavy. I'm not using a foam overlay, because it costs a lot per inch of scenery height, and portability isn't a huge concern (though I have broken the table into sections in case it ever needs to move). I do intend to saw out sections cookie-cutter fashion someday, and the 1/2" stock should work for that.
Complaint: Home centers need to teach people what "CDX" means.
As for OSB, I have used it in the past for solid tops without any problems. I even glued ties to it and handlaid track with plier-driven spikes. Don't do that. You'll need a hammer and nail set if you do. However, for prefab track an OSB solid top works fine, though it's splintery and not as pleasing to look at as plywood. I would hesitate to use it cookie-cutter fashion, though it would be strong enough. The glue is waterproof, but the wood chips aren't, so OSB tends to absorb moisture at its cut edges. My understanding is that the factory paint on its edges is there to prevent this, not just to decorate. Moisture absorption causes swelling near the unpainted, cut edges. This isn't a problem if you've plunked down a 4 x 8, since there isn't any track out there, but in thin strips you've got a lot of edge area to absorb water, and that could be trouble. Perhaps the cut surface could be sealed with paint, but I'd rather spend the little extra on plywood, so I did.
I don't know much about MDF, but I think you start turning British if you use it. I tried it for a 1 x 6 switching layout (cheap lumberyard shelf stock), but noticed a faint craving for tea at regular intervals was developing, and I don't make good tea, so I didn't use the board. (Actually, it may have had more to do with the failed experiment involving ceiling-tile roadbed overlay.) MDF seems expensive, and given that it's made from small fibers I'd wonder about that moisture-absorption problem.
Larry
I used 1/2" birch plywood to top my framework.
Don Z.
Research; it's not just for geeks.
**THUMP
3/4 inch plywood.
But I prefer Space Mouse's Chice.. maybe 1/2 inch birch.
3/4 plywood is my choice.
Joe Daddy
I wouldn't use MDF except possibly on edge. Hates water, sags even when dry, heavy, won't take nail easily. I just don't like it.
Plywood is, I think, a better choice than OSB, though there might be some OSB that's better than some plywood. That's a call you can only make by looking at it. Don't confuse OSB with particle board, which is another, completely unsuitable, material.
As far as the bechwork itself goes, L-girder in the Linn Westcott tradition is remarkably study, lightweight, and easy to build. I did not believe most of that until I did it myself. The 1x2s and 1x3s don't need to be perfect, as they pull themselves straight when you make the girders. "How to Build Model Railroad Benchwork" is a classic, and really useful. It is old school, in that is goes a lot deeper than just a basic how to.
Jeff But it's a dry heat!
I would feel more comfortable responding once I understood its intended use. MDF for spline roadbed, and I think it would probably be more quiet than plywood as a layout main surface. It is quite heavy, and very difficult to penetrate with tiny nails and screws.
Depending on the separation of underlying support joists, you could have great success with a quality plywood all the way down to 1/4", maybe less. I used rather heavy 5/8" G1S for my first layout that had 2X4 pony-wall supports running transversely below three of those sheets side by side. The pony walls were seprated by 28", and I didn't have any issues being up on the whole slab doing scenicking.
Plywood doesn't take screws or nails well along its edges, or driven into the edges. If you pre-drill, MDF does a much better job.
Perhaps a description of your bench and where/how you intend to use the material?
I also agree to stay with plywood but thickness is not the only attribute you need to look at in plywood.
You can buy some 3/4" exterior (C-D) sheathing that will give you lots of grief in comparison to 1/2" 5-ply cabinet grade (B-C or better) plywood.
The plywood is only part of a "system." The rest of the system are the joists and other framing members. The strength/quality of the plywood you need will depend upon these other items in the overall system.
How to Build Model Railroad Benchwork is still one of the better books on this subject. It is worth the price.
Good Luck,-John
i agree w/spacemouse. as a builder osb is fine for sheathing, but you never depend upon it for stability. the same applies to 'advantec' and other similar products. use 3/4" plywood. you'll be happy that you paid xtra $, especially compaed to the overall cost of the project.
Erik Fiske
I couldn't fix your brakes, so I made your horn louder
Go with the plywood. A lot less chance of moisture issues.
Chip
Building the Rock Ridge Railroad with the slowest construction crew west of the Pecos.