Twin-size loft bed - Woodworking Talk

MBfardo · 02-06-2012, 09:39 PM

This is furniture, not a house, but I would really rather not get sued because my client's son fell and broke his neck - so I'm asking here, where material strengths might be better understood.

I am trying to get an idea whether the strength of the platform in my design is over- or under-built: It consists of a sheet of 1/2" pine plywood over four evenly-spaced 2x4s on edge (pinned so they can't twist under weight), just under 13" apart, the outer two being screwed to 2x6s, which keep the mattress in place and provide extra strength at the edges. Boards are standard Lowe's framing lumber, not hardwood.

From my own experience, this seems more than strong enough, but I'd really appreciate your input. Thank you!

cabinetman · 02-07-2012, 08:45 AM

Where are you planning to install this?

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Jim_Rogers · 02-07-2012, 09:23 AM

It is hard to say whether or not if it's over built or under built until you tell us of the load being placed on the platform.
Is the client's son 100 lbs or less or over 200 lbs?
It could make a difference.

MBfardo · 02-07-2012, 07:27 PM

cabinetman: It will be in a bedroom...

Jim_Rogers: I'd like to build the bed to support anyone up to 300 lb as a safeguard. That, and I thought it might be rude to ask about her son's weight.

Their current metal loftbed (a Walmart purchase, I think) isn't holding up very well. Of course, that could just be because it was the cheapest thing they could find.

I suppose a better question to ask is, how much weight is this design likely to support?

The rails are mortised into the posts and bolted, and the two middle 2x4s rest on a 2x6, also mortised and bolted.

Would 2x6s and a 2x8 be a better bet, or just more 2x4s, or is the whole thing more than strong enough already? Being a loft bed, it won't have to support a standing person - only someone sitting or lying down.

The ladder (at a 15-degree angle) has 2x6 legs with 2x4 rungs, turned on their sides and leveled with the floor, notched 1/2" into the 2x6s. The 2x6 legs are 16" apart (O.C.), for a 14 1/2" span.

Jim_Rogers · 02-08-2012, 08:56 AM

Ok, so in review you've told us the load you would like it to hold, 300 lbs. And you've told us the spacing of the 2x4's at 13" oc or so, and that it will be standard 2x4 stock which most likely is SPF (spruce, pine, fir). And you have told us that these 2x4's will be attached to a headboard and foot-board, so they are spanning a distance between them which would be the length of the mattress and box spring. We should know what this span is in order to figure the unsupported distance. What is the distance between the headboard and foot-board?

Jim Rogers

cabinetman · 02-08-2012, 09:48 AM

Using dimensional lumber can be overbuilding. My bunk beds and platform beds are all done with 3/4" plywood. With sides captivating a 3/4" substrate, will support adults with no problem with just using a mattress.

Posts can be 3/4" ply boxed out and finished however needed. That design is more stable than using dimensional lumber. No splitting, cracking, wood movement, twisting, or warping, to worry about.

When installing, just attach to the wall to minimize movement.
Some bunk bed pics:
Example 1. 3/4" Red Oak plywood with solid wood edging.
Example 2. Laminate over 3/4" plywood.

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MBfardo · 02-08-2012, 08:20 PM

Jim_Rogers:

A standard twin-sized mattress is 39"x75", and the posts will be roughly that distance apart, give or take an inch or two. So let's say 6 1/2' with spacing 12.5" O.C.

cabinetman:

Not to be snoodish, but I'd really rather stick to solid wood as much as possible if it needs to look that way when finished.

I don't mind over-building, especially if I can do it for a reasonable price - something that feels and looks solid gives confidence.

I should mention that, as far as stability goes, there wouldn't be a problem with the frame flexing - both ends and the sides have diagonal 2x4 mortised bracing.

Very nice beds, by the way!

MBfardo · 02-08-2012, 08:24 PM

Update: I posed the question to a structural engineer and he laughed, then walked me through the calculations. A 400lb person would have to hang from one of the middle 2x4s to cause it to fail. This is with 'SPF', standard framing lumber. I'll post a render of the final design, and if my client gives the go-ahead I'll post images of the finished bed.

Jim_Rogers · 02-09-2012, 05:34 AM

Well, ok that saves me from running the numbers through my beam sizing calculator.

I guess you now have your answer.

Good luck with your project.

RandyL · 02-20-2012, 08:55 AM

So, the bed will be supported against a wall on 2 sides and then have a post supporting the one corner???? Still trying to understand the set up that your going with. If its fully supported then you have no problem. If you want to see what a 2x4 can support, simply support it on both ends and stand on the middle of it. You"ll get a good idea fairly quickly on the deflection of the wood. As far as supporting load is concerned, you could probably have a small party on that bed and be ok.

MBfardo · 02-23-2012, 05:32 PM

The whole thing would have been free-standing, symmetrical in X and Y axes minus the ladder. I scared my client away when I expressed my uncertainty about the strength of the design. That was before I talked to the engineer... I wish I had BEFORE talking to my client!

I may still build the bed if I ever find another contract for one, and then I'll post the final design and images here.