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I'm a woodworking novice with the goal of putting together a simple convertible sofa / bed. My goals are to keep it simple and affordable to build. I'm planning to buy a couple sheets of plywood and have the lumber yard cut them and then hopefully put it together with just a drill. I made a design in SketchUp and am trying to use the Sagulator to understand what thickness/type of wood I need to use to make sure it has acceptable sag.

I am mostly looking for a gut check on whether I am using the Sagulator correctly, particularly the load inputs.

The sofa and bed will be used by two people 180 lbs and 120 lbs.

For sofa mode (2" thick) I used the following values and got acceptable sag:

180 lbs / foot center load - i assume a person sitting is maybe 1.5-2 feet wide, so 180lbs per foot should be plenty?

350 lbs / foot uniform load - seems more than enough?

For bed mode (1" thick) I used:

100 lbs / foot uniform load - again guessing based on how a person weight is distributed when laying down

Questions:

  1. Am I estimating the load correctly? Is there a rule-of-thumb lbs/ft to use for a sofa / bed, e.g. I guess when a furniture designer designs a piece for sale, there must be some typical standard load rating? I searched for this but couldn't find anything.

  2. The design is very simple, two boxes each missing 2 faces. Stack them for sofa, set them side by side for bed. This means the thickness of the span in bed mode is 1" and in sofa mode it is 2" (2x 1"). I therefore assumed I can input 2" in the Sagulator, but not sure if that is the case?

  3. I am still researching how to attach the "legs" to the "shelf". I was thinking to use pocket holes and some bolts of some kind. I'm most curious about the sag, but would also appreciate any advice on this.

Thanks in advance!

Picture of the "design":

design



Sagulator "sofa mode" - person sitting in center:

sagulator sofa mode person sitting in center



Sagulator "sofa mode" - people sitting distributed evenly:

sagulator sofa mode people sitting distributed evenly



Sagulator "bed mode":

sagulator bed mode



Dimensions:

83x32x10 inches is the bounding volume for each box.

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  • You have entered wrong figures in many fields but does not matter, you can't use Sagulator for this :) See 'width' says vertical plane. Sagulator thinks your edging strip is 9" wide on horizontal plane!
    – Volfram K
    Jun 20, 2022 at 8:33
  • Do you plan to use fir plywood? You should look at what is available to you ASAP! #1, you will probably find 3/4" is thickest available except for junk sold for flooring.
    – Volfram K
    Jun 20, 2022 at 8:45
  • @VolframK thank you for this, I definitely got the edging dimensions mixed up! What else did I mess up :) are the values I used for load reasonable?
    – woodnewbie
    Jun 20, 2022 at 15:27
  • @VolframK good question, no specific plan to use Fir Plywood, it was the only plywood I could see in the list. I'd like to go with the cheapest wood that looks good that won't sag with 1" thickness (1" thick in bed mode, so 2" thick in sofa mode). I posted in another comment below the look I am going for: maidenhome.com/collections/the-muir-sofa which I assume is solid ash
    – woodnewbie
    Jun 20, 2022 at 15:28
  • "it was the only plywood I could see in the list" Yeah, it's a bummer this is all that's listed. This is one (of a few) limitations of the Sagulator. While you can find some halfway decent fir ply I believe none of it will be 1" thickness so straight out of the gate the numbers you enter have that error built in, but much more importantly fir plywood (perhaps more than any other plywood type) is by no means a single entity and the quality varies enormously from one example to another.
    – Graphus
    Jun 20, 2022 at 18:18

1 Answer 1

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The weak point is the front, in the middle of the sofa (and correspondingly, the middle of the bed on the seam). You could fix the sofa with lumber on edge underneath the ply, but that doesn’t fix the other half of the bed.

You could put hinges on the assembly, which would let the weak point share the support of the lumber on edge, or you could make the lumber stick out half its width so that the weak half rests on it. (Neither of these seem like good solutions to me.)

Alternately, you could build torsion boxes out of something like 1/2” ply and 2x2s. You’d need to either find a resource that acts like the sagulator to test deflection, or just build one half as an experiment. My intuition is that it would be plenty stiff, but I have no data to back that up.

Edit:

Knowing that you want a fairly modern look, I’d try this torsion box.

torsion box

Use a stiff hardwood (ash would be good; poplar not so much) for the front and interior supports. Glue and screw.

Have a backup plan to add the optional support to the front of the sofa if needed.

And per the excellent idea of Graphus, stick a block underneath the other side to support bed mode without sagging or additional buildup.

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  • Thanks for the answer! I'll look into torsion boxes. With 1/2" ply and 2x2 I guess it will increase the thickness? I was hoping to keep it to 1" thick in bed mode so that it looks a max of 2" thick in sofa mode, the general look I'm trying to go for is like this here: maidenhome.com/collections/the-muir-sofa which I guess is solid ash. Question: Is two pieces of 1" thick ash stacked on top of each other (as in my design) equivalent to one piece of 2" thick ash for sag calculation purposes?
    – woodnewbie
    Jun 20, 2022 at 15:25
  • Sorry — I meant to address the question of stacking materials for sag calcs. If the materials aren’t glued or screwed together, they aren’t going to perform as one unit for the sagulator. (In other words, they will help each other a little, but not as much as you’d hope.)
    – Aloysius Defenestrate
    Jun 20, 2022 at 16:46
  • 1
    My first idea was a pair of wooden I-beam like things that could be slid under in two configurations for sofa and bed mode, but I think something more akin to a torsion box is probably the way to go to be on the safe side. My later thought was a good starting point would be code for distance between centres on flooring. Without going for a much nicer type of plywood (even assuming the cost would be acceptable to the OP and it might not be given current prices) this should essentially remove all worries about deflection.
    – Graphus
    Jun 20, 2022 at 18:41
  • @AloysiusDefenestrate thank you for confirming about the stacked planks, it seems obvious in hindsight now that I think about it!
    – woodnewbie
    Jun 21, 2022 at 15:05
  • @Graphus insightful thank you!
    – woodnewbie
    Jun 21, 2022 at 15:06

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