Can you tell what is done here by its looks?

[sofadoc]

You still need a bottom rail that will create a crevice for your padding and fabric to pull through and staple. Am I making sense?

You don't want the back webbing to go all the way to the seat deck.

[Boxduh]

OK, we are getting somewhere.

I raised the rail part that picks up the bottom ends of the ziggys for the back.  Here is a back view.



And a section cut.  You can see the two rails inset down into the seat box to pick up the ziggys for the seat platform.



So is this getting close to right?

[sofadoc]

I'd say that you're pretty close.

It's possible that this curve isn't cut out deep enough. If you're going to use webbing in the seat deck, you'll probably still feel this board when you sit down.



But overall, I don't see anything else that your upholsterer can't work with.

[mike802]

Although plywood can be used successfully in furniture if used correctly, it would be much easier to build your piece from solid hardwood.  I would suggest 5/4 soft maple for unexposed frame members, if you do not have the equipment to dress rough lumber, I am sure a local woodworker would be able to do this for you rather inexpensively.  Springs put an enormous amount of stress on a frame, an 8 way hand tied coil spring system will normally put a bow in 4/4 hardwood frame on a club, or wing chair and Zig Zag springs should be installed using a special tool to help the upholster use some leverage, otherwise installing them can be dangerous, if not imposable.  Plywood flexes easily in the exact direction that you need it to be stiff to resist the tension from the springs, not to mention several hundred pounds of people flopping around on it. I have repaired many plywood sofas because of issues listed above, but also because today's plywood has a tendency to delaminate.  You are copying methods used in production furniture, these methods are not for increasing quality, but for making a cheaper product that will last out its warranty period. With all the time and effort you will have invested in this project, I think it is in your best interest to build it from materials that will last several generations.

[byhammerandhand]

To add some engineering to Mike's comments:

Beam strength is proportional to the cube (third power) of the dimension bearing the load.  
For example, if you have a beam that is 2x2 (full dimension, not nominal)
If you change it so the beam is twice as wide (2" thick and 4" wide)  you have twice the beam strength
If you lay that beam on edge so it's now 2" wide and 4" thick, you have eight times the beam strength (i.e., 4 x as much as laying on its side)

So for your application, if you use 3/4" thickness wood for your outer frame,   .75^3 = .42
change it to a full 1" thick                                                                                    1^3  = 1.0   (more than twice as much)
change it to 1.25 thick                                                                                      1.25^3 = 1.95 (more than 4x as much)

If you want to play around with sag factors, http://www.woodbin.com/calcs/sagulator.htm  Unfortunately, it does not include plywood (probably because it's not a uniform product -- baltic birch will not behave he same as CDX, or Chinese birch plywood or poplar core cherry, etc.)

As Mike said, plywood is not going to have the stiffness of lumber because about half the plies are going cross-grain and are weaker in that dimension.

Disclaimer : My wife always accuses me of overbuilding stuff and making it too heavy.   I say, "If it's worth engineering, it's worth over-engineering."

[Darren Henry]

We're dang close to excellent----BUT, I'd like to toss in a couple of thoughts;

Plywood vs. dimensional hard wood ( or > 1.5 X measurement soft wood);
     > As Kieth said, go big or go home on the spring rails. The ones  I have to repair on a usual basis are usually too small for the task and/or have knots.

     >I have on occasion been forced to reinforce the warped spring rail with angle iron on the corner that still allows all stapling. Even 3/4X3/4X 1/8th would be sufficient an save trying to find acceptable hardwood if you wanted to go all plywood. I like screws to fight gravity and stove bolts to fight the springs. Stagger the holes in the angle iron of course.

Load bearing;

     >I would rotate the bottom spring rail in the back as Keith pointed out.

     >That frame member across the bottom of the unit---I would lay on it's flat instead of on edge. It's not really load bearing (although it "helps" keep the frame from warping) , so turning it increase what it does, allows easier access to the spring rails for your upholsterer, and gives you a wider footprint to mount legs.


Spreader cut out;


     >Yes will need to double check that before you go too far, but it can be trimmed in place before you attach the deck.

     >If I 'm using furni-web, I double up the webbing next to the spreader an crank it a little tighter. By that I mean 2 strips side by each like a pair of toast, not folded.

[byhammerandhand]

I had a repair last week, the guts fell out of a sofa.   The front frame was 3/4" Oriented Strand Board (OSB).  There were two front-to-back supports (the parts that fell out).   The spring rails were just 1x2 (laid flat, thankfully)

However the whole thing was stapled together.   The spring rail had one staple about every 8-10 inches.    No wonder it fell apart.

Sad thing is that the frame has a 7-year warranty.   Didn't last quite two years.  I did what I could with it.