PVC

A long time ago I recall seeing in RMC an article for a large D&H(?) layout that planned to use pvc splines. If it was ever built or how that worked out I can't say.

A quick search using the search bar in the upper right found a couple threads I'll link for you and a few others.

https://forum.mrhmag.com/post/building-roadbed-with-pvc-pipe-12184416

https://forum.mrhmag.com/post/pvc-vinyl-moldings-for-spline-subroadbed-construction-12185816

The second link talk about

The second link talked about splines using PVC, and had Joe's comment that I remembered.  Lots of ideas on how to hold the splines together, I was thinking of using thru screws and that was mentioned.  Another idea was to fill the gap between the splines with foam.  Have to think about that, sounds messy.  I think I'll stick width 1/4 drywall as it bends so the vertical transitions should be no problem.  Joe's not a big fan of Homosote.   Next time I go to the home store I'll look at something call "composite lattice", its much cheaper than PVC.

Why?

  I can't see any advantage of plastic over wood for roadbed. Plastic is more floppy than wood so needs better support. Plastic is also harder to shape and fasten to. Plastic doesn't rot and is more weather proof but that's not usually a layout problem ....DaveB

i do not know the name of the

i do not know the name of the "wood" but same stuff as "peg" board, more a paper than wood. just no holes in it, cut to strips 3/4 - 1 in wide, and wrap around a jig, or supports, glue till you get a 1 - 1/2 - 2 in thick "bar" this stuff is only 1/8 thick, so lots of glue and clamps needed.  then sand level.

can easily bend to a 20 rad curve.

i do not know the name of the

That's Masonite, or generically, hardboard. Comes in tempered (dark brown) and un-tempered (light brown) varieties. You want the un-tempered. It's not paper, though - it's sawdust in a binder.

The tempered stuff can be difficult to paint. The substance it's tempered with can be oily. Water based paints sometimes don't stick well.

I use regular 1/4" MDF ripped

I use regular 1/4" MDF ripped to 1-1/2" wide for my splines without spacers, glued 6 layers together.

It's really, really durable.

I did a weight test

One of my goals with my TOMA modules is keeping the weight down, so I made up some sample short sections of spline from different materials and measured the weight:

The results (top to bottom):  pine 2.1oz; Masonite: 5.8oz; and poplar 2.8oz.

I was looking for which material weighed the least – and pine was the hands down winner, being only 36% the weight of Masonite. 

I was considering PVC square molding, but after reading an installation brochure on the PVC molding, I discarded it. The brochure discussed the different thermal expansion coefficient of PVC to wood and how to alleviate the issues by using preventative measures to minimize the expansion/contraction differences.

In other words, PVC expands to temperature changes very differently than wood -- so much so the installation brochure for PVC molding had to discuss it and tell you the special preventative procedures to do so you didn't later have problems. Most of those procedures involved using extra stout fastening methods to force the PVC to stay in place no matter what.

I decided the last thing I need is a track roadbed that was going to expand / contract and give me track distortion headaches later. So PVC was OUT.

what kind of pine?

Your post about weight started me off on a series of web searches and page reading that ended in the realization that I knew less than I thought when I started.

Where I got to was that weights of wood vary considerably due to moisture content and even vary substantially from sample to sample for a given level of moisture within the same species of lumber. So I think getting to a 'bottom line' requires a lot more research than a single data-point test. I'd tend to favor using the lumber weight tables I found by searching that represent averages tested over many samples rather than what is basically anecdotal evidence - but they do panel weights differently than dimensional lumber weights, so comparing wasn't easy. 

By the way, what species of pine was it you measured? Southern yellow pine is supposedly typically heavier than poplar.

Wood weight

Minor weight variations are not important. A slightly heavier wood might be stiffer so would need less wood to do the same job. Other factors such as cost, availability, resistance to warping or splitting, ease of planing or fastening, etc. should be taken into account....DaveB

Your wood moisture...

Your wood moisture needs to be in the sub 5% for wood to be any kind of stable.  Green or fresh cut wood is at about 30%, construction lumber is at 25 - 15%. Wood that im gonna make anything nice out of needs to be at 5ish%. good plywood is near that, that is why i keep harping on plywood rather than Home Cheeop lumber.  Stick with the Masonite splines, they are proven to work.

Michael the Crank

Considering that its for a

Considering that its for a conditioned space, a spare bedroom, why would thermal expansion of PVC lattice be a consideration?