Does my loft bed need diagonal supports?

Question

I'm building my son a loft bed. I'm trying to make it really sturdy because he has a tendency to break things. Each leg will be two 2x6s (brown in picture) joined with pocket holes. In the inside corner of each of those legs will be a 4x4 (red in picture) that extends up under the mattress. The 4x4s will support horizontal 2x6s (orange in picture) which will brace the slats (not pictured) for the mattress. All the way around the mattress will be three rows of 2x4s. These are green in the picture but I did not picture them going all the way around. There are two additional horizontal supports at the bottom (also green in picture) going around the back and both sides but not the front. The blue ladder on the right side will be made from 2x4s. This bed is meant to support a rambunctious 10yo boy as he grows into adulthood. I know diagonal supports do make structures more sturdy but is it necessary considering the 5 horizontal and the leg design with three boards?

Answer 1

I'm posting this as an answer because I know I will exceed the limit for a comment. I've built about 3 dozen bunk beds for a church retreat facility. They were all red oak. The side rails and the headboard/footboard rails where all mortise and tenon...that about the strongest joint out there. But unless you have a well equipped shop, not a DYI project. That said....

If you could put a shallow dado in the legs where the rollover rails go (the green boards) wide enough for the rollover rails, making it a snug fit, then fasten with bolts. That would go a long way to minimize racking rather than counting on the fasteners alone to prevent it. For appearance sake, you'll want to do that on the inside of the legs.

For the mattress surface, I used plywood screwed to cleats on the headboard/foot board rails as well as the side rails. That really ties things together and makes it very sturdy.

You are planning a purely vertical ladder. I have found those are hard to climb and if you could incline it by about 10 -15 degrees it makes it a lot easier to climb. You could do that by cutting a long wedge attached to the legs. On my ladders, I cut dados to accept the steps and screwed them in from the other side of the wedge. That way, the weight is supported by the dado in the wedge rather than the fasteners. Alternatively, you could build a detachable ladder. I'm attaching a pic of the finished product. (OK, maybe bragging a bit!, sorry), but I think it's a good simple design that maybe might give you some ideas.

Lastly, don't forget you need a rollover rail along both the long sides of the mattress.

Answer 2

Did something similar once.

Legs were four 2x4 and rails 2x8, cross pieces 2x6 and cutouts in the legs (1” deep) so that the wood supported the rails just held secure by bolts.

Used by 2 adults for “normal” night-time activities…

No racking as the legs were against the walls - it was a small room with a high ceiling.

Answer 3

Firstly, that thing is a beast and will probably last decades as-is. It's probably going to weigh over 100 pounds.

My one suggestion would most certainly be to add some shear strength via cabinet-grade plywood; 3/4" sande plywood.

This serves two purposes:

1. Shear strength; virtually eliminate any lateral movement
2. Utility: decoration, mounting things, kick board, etc...

With the plywood you could also scale back your use of 2x6 + 4x4 posts. You'll achieve massive amounts of rigidity with the plywood.

Note: there is no plywood on the right side behind the climbing rails; I merely botched your drawing.

Also, I would plan for an angled ladder with flat steps. The amount of PSI those thin ladder boards will impose will gradually get more uncomfortable as the years go on.

Answer 4

Unfortunately I am not qualified to make comments to your construction, other than to say, it is massive.

Sharing ideas:

Some time ago, I build a bed with very fancy looking legs.

I only used 2x2 for the entire structure.

I paired two 2x2 (becoming 2x4) to increase the horizontal strength to hold the mattress.

Here is a primitive drawing, as I do not have 3D drawing capability.

The red are the verticals and the gray are horizontals at different hight. So you end up with 2x2 legs with 2 inch opening in between (looks fancy).

Then the whole structure was screwed with appropriate length screws and nuts.

One day they grew up, and I just unscrewed the structure in to 2x2 pieces.