Wobbly bench issues

Question

I'm an absolute novice at this but furlough has made me want to try a new skill and woodworking seems like a handy thing to learn. Excuse any stupid mistakes!!

I built the frame, put the seat panels on and then added the legs. I thought with the basic design, not much could go wrong but it wobbles from the bottom left to the top right.

Please can someone explain to me in layman's terms what has happened and how I can fix it?

P.S. I know it's not on a flat surface but I can physically see that it's not flat 😬

This is the bench in question:

Answer 1

We've all been there. The wood will be expanding/contracting slightly with moisture changes in the air. As you have screwed the top tightly to the frame these small but powerful forces are twisting the frame. Cross-bracing may only get you so far. The trick is to attach the top in such a way that a small amount of movement is permitted. The key here is to make the base frame assembly a strong unit in itself, rather than relying on the screwed-on top to add strength (due to the relatively narrow pieces used in the base, and the different grain orientations, this is less likely to twist than the wider, same-grain-oriented top). Have a look under a bought wooden table; you may see either screws coming up from the frame up into the top through oversized holes or clips/blocks attached to the underside of the top that fit into oversized grooves in the frame.

Rather than tear your nice bench to pieces, your best bet is to let it adjust to the moisture where it is going to live (assuming this is inside, where humidity changes should be less pronounced than outside), trim the legs as best you can, pack the remaining gaps between floor and legs with small pieces of card/wood, and chalk it up to experience.

Answer 2

When you built this did you layout the bench frame on a flat surface?

It appears like the lumber you used for the top frame is twisted or bowed or both and if that is the case you will need to disassemble and rebuild ensuring the bench frame is square in and flat.

Barring that the only other possible way to fix this but it is not the best thing would be to add X-bracing between the legs but I think the bench is a little to out of square to be able to force it back how you want.