Back panels for wide cabinets

Question

I want to build a free-standing cabinet for the kitchen but since it is my cabinet build, I am not very sure about being able to make a solid/stable cabinet without back panel. It will probably have 4 shelves (fixed) with each shelf divided into 70/30 width with alternating 70/30 divisions (1st "left" shelf is 70%, right shelf is 30% wide, second left shelf is 30% and right shelf is 70% wide and so on), so no vertical board dividing the cabinet in half.

Internet tells me the a 6 mm (quarter inch) plywood board is good enough for the back panel. Planning to make rabbet for back panel with circular saw and sanding. The problem that I am facing is that I cannot find a single plywood board big enough for the back panel. I want to ask if it is ok to use 2 plywood boards? Would it give the structural integrity that I am looking for, to avoid the cabinet from wobbling? I only have a circular saw so the boards might not join perfectly. The seam would also be visible from the front of the cabinet, which is not visually appealing. If there are any other solutions to solve the issue, please let me know.

Some details about cabinet below:

Material for box and shelves: 19 mm Chipboard (3/4 in)

Dimensions: 150 x 85 x 50 cm (H x L x W)

Shelf joining method: Cleats

Answer 1

If you do not have and cannot find sheet goods big enough to cut a single sheet for your backing panel, then yes, use as many pieces as necessary.

My wife has a flat pack 9-cubbie cabinet similar to what you're building (hers is a simple 3x3 construction) and there were only 5 pieces of cardboard provided for the backing. We installed one on each corner cubbie and one on the middle cubbie and there is no racking whatsoever. Of course, things fall out the back of the other 4, but that's a different issue.

If you want to make your backing from multiple pieces and don't want the seams to show (which is reasonable), just cut each piece to ensure the seams fall behind shelves or uprights. I'd probably do 3 horizontal pieces, but you could do 6, one for each shelf space if you wanted.

Four screws or nails for each backing piece (two on the top, two on the bottom) should be more than sufficient to provide the rigidity you're after, more if you feel like it. Of course, gluing would work equally well.

Answer 2

There are two reasons you want to add a backing panel to this construction:

1. To keep things from falling out the back
2. To stiffen the construction

The benefits of the first are obvious. The second is to manage racking forces that any square construction will have a problem with. The idea is to provide a sort of "truss" for the rest of the build to resist it wanting to go out of square while in use (or over time).

To do this job, you don't need a continuous piece across the back. If we wanted to ignore (1) above, we could do the typical Ikea solution, and just provide a few inches of material as backing across the top, from corner to corner. And another one or two strips across the back down the length of the construction. This will be just enough to stiffen the construction to help keep it square (and, in the case of Ikea furniture, reduce the overall material cost).

In this case, by connecting some or all of the edges to a backing panel we are creating a number of triangles that will resist both tension and compression, helping keep things square. In order to do this, that backing does not need to be continuous or even solidly connected. Because it is the mechanical connection between the corners of the cupboard, through the backing, to the other corners that gives it its strength.