I'm very new to this great hobbie, and I'm fixing to build the table for my layout.
Is it really nessesary to put homastat over the plywood?
Welcome to the forums.
In short no. However in the larger scheme of things, there are reasons to attach something over the plywood. If you use extruded foam or homasote over the plywood it is much easier to have below grade details such as culverts, drainage ditches and the like. Also, it is more like the real thing, with the rails above the surrounding scenery. That said, if you are just doing a yard, not as critical.
Might I suggest you read a book or couple articles on benchwork before you get to far along. You can find other threads here on the forums by going to the search function to the right. You can also go to Resources above, then click on Index of Magazines to search for magazine articles.
Hope this helps.
Good luck,
Thanks Cowman, I'll do some more homework.
One of the most limiting elements, when planning a layout, is the concept of a "table" for the trains. It tends to force the placement of scenic elements and make you do weird things with track configurations, and it leaves one baffled as to why things don't look quite right when done. Perhaps it has the benefit of being readily understood by the novice modeller, but in my opinion it runs the risk of establishing habits that undercut the enjoyment (long term) of the hobby.
If one does build up from a table surface, though, I suggest that they really build UP from it. Get the tracks elevated onto at least a thickness of homasote, or even better an inch or two of foam, so that there's plenty of room to carve in drainage ditches or even ponds and streams. So many times I've seen water features with their fill line at basically the same height as the rails, and it instantly signals to our brains that this ain't real, this is just a toy. But with an extra layer of something that can be carved, there's room to express realistic topography over which the rails must run.
If using homasote, due to its density I'd suggest just cutting it out where the tracks are gonna go, then using some sort of scenery material elsewhere; i.e. a 2-inch wide (give or take) strip of 'Sote under the rails, and bare table top everywhere else. Then, one could use easily-carve foam boards to infill, or you can do as I do, which is to have a TON OF FUN with expanding foam insulation, such as "Great Stuff". You just spray it out of a can, and a tiny amount grows into big lumps; once cured, you can use an old steak knife to carve it to any shapes or slopes you want. Then, on top of this foam base, apply your choice of plaster; I use a mixture of papier-mache with joint compound (drywall mud), and I add some brown and black tempera paint to this to give it a soil color throughout. The plaster thickness if between 1/8" and 1/2", depending on the bumpiness of the underlying foam, and because of all the randomness in both the foam layer and the plaster, you end up with a ground surface that is naturally un-smooth. I use tempera rather than acrylics because its cheaper, but also because it remains water-soluble, so if things don't look right when it dries you can always just wet it again and work it some more.
The point, though, is that the finished surface one is looking at is not the table at all. The table is just a backbone onto which the scenic flesh has been grafted.
This photo shows an in-progress shot of my N-Trak module, which is built in this manner, with yellow expanding foam and a papier-mache/joint compound top coat. You can see that in some places, there is no foam, it's just the plaster on the plywood. But I still apply the plaster in order to get a uniform look as well as to defeat the utter flatness. While the plaster is still wet, I sprinkle on some dirt mix, to augment the look of bare soil. Once all this is done, I add the grasses and vegetation - but not so thick that all you see is green! I let some of the brown show through. Incidentally, I used HO scale cork under my N-scale track, to give it a little bit more height relative to the surrounding scene; in your case, the homasote under the track would accomplish the same thing.