I think a single layer 3/4" top is plenty stout, but you may want to consider edge banding the plywood with some type of hardwood.. How far apart are your supports under the top? A minimum of 24" center should work, but 16" centers would be better.

to answer your question about the 3/8" dado for your fence inlay. you won't need any backing at all if you have adequate supports and the top is screwed down good at 4" oc.

I've got a lot of work benches framed just like that that have served me well for many years, but i typically put formica over the plywood or particle board. it helps keep it flat and i use bondo to fill holes i've drilled into it when it starts looking to cheesy…

 

Supports are spaced about 24". I always kick myself for way overbuilding, so went easy with support and screws (screws about 1 per foot).

 

Well, if it were mine I'd double it up. I like a lot of mass in my workbench tops to take the pounding and other abuses that we ask our workbenches to withstand. When have you ever thought to your self, gee this work bench is too beefy?

 

I think it depends on what exactly you mean by "flat work surface". If that means somewhere to do glue-ups and apply finishes, then see Big Foot Bow's response.

If it means somewhere to hand plane stock and chop mortises, see Bondo's response.

One other consideration is how "flat" you want it. The plywood is going to stay pretty stable but that construction lumber's gonna wanna twist and turn like a belly-dancer. So they're gonna be fighting over how flat the top stays. With screw joints, the 2Xs just might win. Another layer of ply would help make sure they don't.

And for the record, I'm pretty sure you can sleep at least 5 on there.

 

Another consideration to think of is if you plan to put in any dog holes. If so, then double up the thickness for sure.

 

Got enough dog holes in this (but that's also 3/4 on top, so they are more like "clamping holes" than dog holes - I think the plywood would flex if I put dogs in them and then really cranked down):

HokieKen makes a good point with stuff moving. I had thought the one layer of 3/4 was enough to hold the 2×4's straight (they seem to like to twist). I guess I was being optimistic.
I mainly plan to use it for cutting and assembly (and a place to do sanding and finishing). I have plans of mounting a moxon vie on the Paulk workbench, so that would probably be where I ended up planing anything (and doing any other handwork). I mostly wanted a a flat surface that didn't wiggle when I leaned against it. If it's going to take another layer of 3/4 to keep it flat, so it goes.
Should my strategy for attaching the second layer be "slather on glue and then loads of weight on top" or is there some better way? I don't mind seeing screws in the top, if it means it will stay sturdy and last. At some far future time, I plan on putting on a face frame and doors (to keep the dust out)

 

We each have our own workflows, but for me a flat surface like that would be a big temptation to store all sorts of "stuff". To the degree that the surface is used for working, it won't be available to support the long pieces for the miter saw. IOW, I'd engineer it to avoid the temptation of too large a horizontal surface.

I admit a love/hate relationship with my miter saw. I could never allocate that much space for it, so I have mine on a cart with "wings" that flip up, combined with some stands for really long stuff. I keep getting the urge to get rid of it because of the space and the dust, but at the same time I use it all the time. But I digress…

Given where you are, I would be inclined to go with something like Jay Bates or Jackman Works did, where there's a minor ledge for supporting the long pieces, but it's really built into a bunch of cabinets. You have the starting point for it, but it will take a lot more than 2 sheets of plywood to get the rest of the way.

One last thing, an assembly table that is free-standing (i.e. accessible from all sides) will be better than one that is against the wall, if you build any large items.

I realize this isn't exactly what you asked, but this is what came to my mind when I read your inquiry. There are many ways to do it, of course. Just one person's opinion, offered in good faith.

 

I'd glue the next layer if it were me. You can load it down with weights if it's easy. If I were doing something that large, I'd lay the glue on then add screws to clamp the sheets together. I'd put the screws on the bottom so they aren't visible. If that doesn't bother you though, drive them straight down.

 

I am building a miter saw station. Wanting the top to serve as both a flat work surface, and have a fence and stop on it, I had planned to top it with two layers of 3/4 plywood, and then router out a channel for a T track. Now that I have one layer of plywood on the top, I m feeling cheap and lazy, and pondering if I can just reinforce under where I plan to put the track, and then router out the 3/8 deep for the track. I would reinforce by placing 6" wide scrips of plywood glued and screwed under existing top. I can use oak if you think it will be stronger.

Any opinions?

I already screwed down the first top (with the thought that the second layer of top would hide the screws), so if I go with one layer, screws will be obvious (though not a huge concern to me). It takes two sheets to cover the top (each side is about 8 x 36") so we are talking about $70 worth of plywood.

I had planned to band the two layers, so if I only went with one, I would still put a second layer under the front edge (front edge extends 1.5" for clamping) to give me enough to glue and pin-nail a strip of oak or maple onto.

Here is what I have now (yes, it is ugly and way overbuilt. I had one day to build, so lacked the time to be elegant) On the positive side, I think it can sleep four:

If anything I have said is wrong, or a bad idea, pleas let me know. Thanks!

- Forseeme

I didn't read all the replies. I don't see any problem with adding extra 3/4 plywood under the existing piece where the T tracks will go.

 

We each have our own workflows, but for me a flat surface like that would be a big temptation to store all sorts of "stuff"....

Given where you are, I would be inclined to go with something like Jay Bates or Jackman Works did, where there s a minor ledge for supporting the long pieces, but it s really built into a bunch of cabinets….

One last thing, an assembly table that is free-standing (i.e. accessible from all sides) will be better than one that is against the wall, if you build any large items.

HokieKen - I think I will weigh it down and then run some screws from underneath (my nod to aesthetics)

- SignWave

The Jay Bates station was definitely on my wish list, but I got tired just watching him offload the plywood for that project. I will kind of be doing something slightly similar (in that eventually the wall will have cabinets and the base will have drawers and cabinet doors). I've never mounted drawer slides, so I keep pushing it into the future as something to do.

Yes, flat surfaces can be temping as "crap pile zones", but I seem to be keeping the place clean so far (I find as long as I have a place for something I am able to put it away). The rest of my world does not stay nearly so neat…

The freestanding table is useful as well, but I really like the height of the new bench (I have back issues, so the higher the better for work surfaces). For bigger projects - like a porch swing - the large Paulk bench is a huge convenience.