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Layout Sub-Structure - Plywood size question

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Layout Sub-Structure - Plywood size question
Posted by ricktrains4824 on Monday, September 13, 2021 3:14 PM

Hi all,

I am getting supplies together to expand my layout footprint, once the final touches are completed in the layout room. 

Now the fun part - My local lumber stores, be they local "Mom & Pop" style or big-box "lowest & home" style, can not seem to keep in stock, at any price point, 1/2" plywood in 4x8 sheets.

Has anyone successfully used 1/4"plywood in HO scale? 

I won't be walking across the layout, and HO trains aren't exactly heavy, but between trains, scenery, etc... it is not light weight either.

I'm trying to figure out if the 1/4' plus support would be better than a 2" foam with support substructure. (The local "home" big-box has all kinds of 1/4" plywood, plus a good supply of straight boards for support structure, but the 2" foam is higher priced currently.)

The original layout is 1" foam scenery base on 1/2" plywood with support structure, save for a 4x4 area of only 2" foam with support structure. (Under layout shelving unit on one end plus shelf brackets along other end.)

Could I get away with 1/4" plywood? Then I could use the 1" foam scenery base like the rest of the layout has already, and still be about the same cost as the 2" foam with a support structure.

Thanks!

Ricky W.

HO scale Proto-freelancer.

My Railroad rules:

1: It's my railroad, my rules.

2: It's for having fun and enjoyment.

3: Any objections, consult above rules.

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Posted by jjdamnit on Monday, September 13, 2021 4:02 PM

Hello All,

I use 1/4-inch plywood over a framework of 1" x 4"s, on 24-inch centered grid-work, for my HO pike.

On top of that is a base of 1-inch foam.

This sits on top of the bed in the computer/railroad/spare room.

It is both lightweight; versus 5/8-inch MDFB, and structurally sound with the right grid work.

The advantage of 1/4-inch ply is it's light and easy to drill through for track feeders and switch machine linkage.

Unfortunately, 1/4-inch ply doesn't have the "bite" to hold under layout devices (ULDs) with just screws; switch machines, circuit boards, boosters, etc.

As with everything there are trade-offs.

If I were able to expand my pike I would still use 1/4-inch ply, in the way I have described, but use 2-inch foam and add reinforcement plates to accommodate ULDs.

Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by wjstix on Monday, September 13, 2021 4:05 PM

I used 1/4" plywood as a backdrop on a prior layout; I would think it would work on the layout but might need more bracing support underneath than 1/2".

Did you check the other way, to see if they had 3/4" plywood sheets in stock? I know a lot of folks prefer that to 1/2" plywood.

You could also use smaller sheets. I've used 1/2" 2' x 8' and 2' x 4' sheets. Works fine, easier to fit in the car than 4' x 8' sheets too!

Stix
  • Member since
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  • From: Bradford, Ontario
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Posted by hon30critter on Monday, September 13, 2021 4:14 PM

ricktrains4824
Could I get away with 1/4" plywood? Then I could use the 1" foam scenery base like the rest of the layout has already, and still be about the same cost as the 2" foam with a support structure.

I used 1/4" plywood under 2" of pink foam, but the only reason for the plywood was to make it easy to attach things like Tortoises to the underside of the layout. I used 1x4s on 16" centers for the frame. The structure is very solid.

To me, 1" pink foam seems to be way more flexible than the 2" stuff, but adding 1/4" of plywood would certainly stiffen it up. My suggestion would be to make a test panel to see if you think the 1" foam/1/4" plywood combo is stiff enough. If you make the test panel the same size as one section of your layout you will be able to use it in the layout if it meets your expectations. If it isn't stiff enough, you could try adding another 1/4" layer of plywood on top. If the layers of foam and plywood are properly glued together that should make a very solid surface.

One of the challenges to gluing the plywood to the foam is getting enough weight on top of the assembly to push the layers tight together. I didn't get that quite right so some of the edges of the foam sheets were higher than others. It wasn't a big deal to sand the foam down. I should have used a notched trowel to spread the adhesive caulking.

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

  • Member since
    November 2006
  • From: NW Pa Snow-belt.
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Posted by ricktrains4824 on Monday, September 13, 2021 4:36 PM

Dave & JJ - Thanks. The whole reason for the plywood was indeed under-layout items like tortii, wiring runs, etc... I will indeed have to think about tring it out. 

Stix - They did have 3/4, but at $82 a sheet..... Way too high still! At least the 1/4 cabinet grade was down below $30 a sheet now. 

 

Ricky W.

HO scale Proto-freelancer.

My Railroad rules:

1: It's my railroad, my rules.

2: It's for having fun and enjoyment.

3: Any objections, consult above rules.

  • Member since
    September 2014
  • From: 10,430’ (3,179 m)
  • 1,639 posts
Posted by jjdamnit on Monday, September 13, 2021 4:38 PM

Hello All,

hon30critter
I used 1/4" plywood under 2" of pink foam, but the only reason for the plywood was to make it easy to attach things like Tortoises to the underside of the layout. I used 1x4s on 16" centers for the frame. The structure is very solid.

Another advantage of using 2-inch foam is the ability to carve features below grade.

I'm limited to what features I can model with the 1-inch foam I used. Using 2-inch foam would have allow me more modeling options.

hon30critter
One of the challenges to gluing the plywood to the foam is getting enough weight on top of the assembly to push the layers tight together.

I didn't glue the foam to the plywood. Instead, I used "Finishing Washers" with "Dry Wall Screws" to attach the foam to the 1/4-inch ply.

Each screw set was placed approximately 12-inches apart along the perimeter and down the centerline.

An advantage of "floating" the foam sheet(s) over the sub-base; whatever material or thickness of choice is, the expansion/contraction of the two differing materials does not transfer to the other.

The plywood can expand and contract at a different rate than the foam with minimal damage to each.

Hint: Don't drive the screws too far into the foam. The tops of the washers should be just below the surface of the foam. Any deeper and you might crack the foam.

Think driving pylons to stabilize the foam to the plywood base rather than "mating" the entirety of the two surfaces. 

Hope this helps.

 

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by 7j43k on Monday, September 13, 2021 7:21 PM

I used 1/4" plywood for the top deck of my modules.  Support is spaced at about 24"-30".  I also attached a spline underneath where the track runs, to eliminate the possibility of sag.  The edges (fascia) are 6" tall and are 1/2" ply.  REMEMBER, these have to be able to be picked up, moved and transported.  They don't have the luxury of just sitting there.

No foam, except for some scenery atop the ply.  To allow for fluctuations in contour, I set the ply at various heights, with some of the ply 2" lower than the zero point.

I've had these for almost 10 years, and there have been no problems.

I would expect you would have to use "proper technique".

 

I expect building materials, including 1/2 ply, to become more available.

 

 

Ed

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Posted by Doughless on Monday, September 13, 2021 8:55 PM

I think the 1/4 inch with a 1 inch foam topper would be fine, but I would use a 16 on center support structure.  You could laminate the foam to the plywood using adhesive to provide the combo its own rigidity as well. 

- Douglas

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Posted by 7j43k on Monday, September 13, 2021 9:35 PM

So there might be a question whether to glue the foam to the plywood to make the assembly stronger. (doughless)

Or to keep the connection loose, to allow for differential expansion. (jjdamnit)

 

Ed

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Posted by hon30critter on Monday, September 13, 2021 11:58 PM

7j43k
So there might be a question whether to glue the foam to the plywood to make the assembly stronger. (doughless) Or to keep the connection loose, to allow for differential expansion. (jjdamnit)

Hi Ed,

I guess part of the issue is whether or not the glue is capable of holding the plywood and foam together over time. The OP doesn't mention where his layout is located. If he is in a basement and uses a decent adhesive I can't imagine things coming apart.

I screwed and glued my 1/4" plywood to the 1x4 subframe hoping that that would make the plywood more stable. My layout is in an unheated but fully insulated garage so I know there will be expansion and contraction. I don't plan on using a space heater except when I'm in the garage in really cold weather (which will make the expansion and contraction worse I imagine). Time will tell if my plan doesn't work!

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

  • Member since
    June 2020
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Posted by Lastspikemike on Tuesday, September 14, 2021 8:39 AM

If you cannot get 1/2" plywood just buy two sheets of 1/4" and glue them back to back. Ordinary carpenters glue will be adequate. You need a bunch of clamps and 2x4 to get a fairly uniform bond. The physics of plywood (or any wooden beam) mean that very little stress occurs through the center ply of the sheet anyway because in that zone tension force changes to compression force of the sheet is loaded from the face. 

If you want more strength for less total weight then just sandwich a sheet of 1/2" foam between and glue them all together face to face. You'd get a very stiff and light structural panel only 1" thick. If I needed that kind of strength I'd glue 1/8" ply sheets to each side of 1/2" foam to get a 3/4" panel, but all that is serious overkill for a model railroad anyway unless you plan on standing on it. 

With the usual type of wooden frame underneath there is no need to use a 3/4" plywood table top. 

Alyth Yard

Canada

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Posted by ricktrains4824 on Tuesday, September 14, 2021 9:09 AM

hon30critter

 I guess part of the issue is whether or not the glue is capable of holding the plywood and foam together over time. The OP doesn't mention where his layout is located. If he is in a basement and uses a decent adhesive I can't imagine things coming apart.

 
Upstairs, climate controlled room. No issues on the prior layout with any warping, delaminating, or expansion/contraction issues.
 
If it was in my basement, it would certainly warp at the very least... I live in a very old farm house (We found a newspaper in the one wall from 1880) where the cellar was dug out and the walls are lined with stone, so it gets very damp when it rains.... We have made drastic improvemetns, but it still gets very damp. (Works fine as a potato cellar, storm cellar, etc..., but not so good with dry usable space...)

Ricky W.

HO scale Proto-freelancer.

My Railroad rules:

1: It's my railroad, my rules.

2: It's for having fun and enjoyment.

3: Any objections, consult above rules.

  • Member since
    December 2008
  • From: Heart of Georgia
  • 4,601 posts
Posted by Doughless on Tuesday, September 14, 2021 9:13 AM

I guess the consequences of the glue not holding over time is that OP will then have a floating foam base that some suggest in the first place.  

16 inches on center would seem appropriate.

Or glue and screw three pieces together to make 3/4 inch ply.

The hobby costs money.  There are only a few options one could have to save $50 in lumber costs.

- Douglas

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Posted by 2ManyHobbeez on Tuesday, September 14, 2021 9:23 AM

Why not just use 1/2" OSB? It's entirely adequate for this application. Mount it smooth side down and paint that side white for better visibility under the layout. Screw foam to it from the top using wood screws and washers (or NBWs). Counterbore those holes or just put enough squeeze on the screws so the washer and screw heads recess themselves. Doesn't take many screws to hold the foam down and flat and it ain't going anywhere. You can take it apart if you want to.

George

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Posted by rrebell on Tuesday, September 14, 2021 9:28 AM

Cured foam dose not expand and contract in any measurable way, plywood dose.

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Posted by 7j43k on Tuesday, September 14, 2021 10:10 AM

rrebell

Cured foam dose not expand and contract in any measurable way,...

 

 

Implication being that if it expands or contracts, you didn't wait long enough.

Since any expansion or contraction with foam is likely to be the same on any axis (unlike plywood), then I would recommend storing the stuff for awhile.  How long?  Well, you very carefully measure the length.  When the length stops changing over time, it must be cured enough to use.

There seems to be two major brands of the foam we use: "pink" and "blue".  It would surely be nice if someone could obtain a fresh piece of each, and chart the shrinkage over time.  That SHOULD give a hint to people how long to wait.

We're talking here, by the way, of non-reversible changes.  As in "shrinkage".  

 

I figure there's also reversible changes:  the foam is a closed cell foam.  There is gas inside each cell.  As it warms up, the pressure inside each cell rises.  This stretches the cell walls, and the cell expands.  ALL the cells expand.  It also goes in the other direction.  

This likely explains the system of cracks that developed in my scenery surface (sheetrock mud) done over foam.  In fact, I noticed the size of the cracks change depending on ambient temperature.  If I had done this in a shallow bowl shape, it might have made a great mud-flat.

The above is likely measurable.  Or was.

 

Ed

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Posted by 7j43k on Tuesday, September 14, 2021 10:45 AM

S&SS of NC, a modular group, builds their modules mostly of 1/4" ply.  I believe.  If you are interested in using the stuff, you can learn a lot from them.

For example, they use it not only for the top, but for the internal "cross bracing" underneath the top.

My big anxiety would be glueing an edge to a sheet (bracing underneath top).  They clearly have pulled THAT off, however.

 

Ed

 

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Posted by Lastspikemike on Tuesday, September 14, 2021 2:51 PM

Edge gluing plywood works fine as long as the face veneer wood is strong enough. Boat builders edge glue ply all the time.

It is better if you can fillet the seam preferably on both sides either with epoxy filler or wooden battens (which can also be made from strips of ply). Modern plywood glue holds to failure of the veneers. The glue never lets go first.

1/4" ply is not stiff enough to use without framing but with framing to hold it rigidly the thickness is more than strong enough for a model railroad.

Frankly, the strength issues are with respect to use of fasteners like screws or nails. 1/4" lacks thread holding depth. Most model railroad strength requirements are well below the failure point for fasteners in wood though. Ironically, the big drawback to 1/4" thick ply is lack of thread holding depth for the supporting wooden framing!

Alyth Yard

Canada

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Posted by danno54 on Friday, September 17, 2021 9:41 AM

Quarter inch plywood is plenty strong for this use. Frame all four sides and mid supports 24" and you'll be fine. I built shelving 30' long floor to ceiling for my basement 35 years ago with this design. It is framed with 3/4 x 2 1/2 inch furring strips. It has upported bins full of books, lps, my bowling and ski equip just fine. It can certainly support my weight of 200#. Perimeter framing spans 48" between vert supports and is still as straight as when installed.

I would probably top it with 2" foam to give me more options for carving out ditches, rivers and such. A close thread wood screw gives plenty of grip for mounting stuff underneath layout.

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