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I'm using dimensional lumber ( whitewood ) from Home Depot.

https://www.homedepot.com/b/Lumber-Composites-Dimensional-Lumber/N-5yc1vZc3tc

It is just a 4 foot by 8 foot rectangle with a center vertical piece and a center horizontal piece.

I am hoping to put a 1/4" plywood on top to make a small structure to hold 500 to 1000 lbs.

If 2 by 4s are not strong enough I could used 2 by 6 or 2 by 8.

I'm trying to keep the design very simple.

There will be a caster/wheel at each corner were the weight is transferred to.

enter image description here

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    Hi and welcome here to DIY. You should specify if the 2x4s will be flat or vertical to the plywood. They are much stronger on the vertical orientation. Be aware that use of 1.4" plywood is inadvisable in this application. At a span between supports of 2 feet and 4 feet it will bend almost to breaking if you were to stand on it. This calls for plywood of at least 5/8" in my estimation. – Michael Karas Oct 8 '19 at 16:10
  • The 2 by 4s will be flat to the plywood. I'm sure you meant 1/4" plywood correct? I can up it to 1/2" or so. Is there a chart online I can look at that shows it breaking at spans from 2 to 4 feet. Thanks. – foo Oct 8 '19 at 16:24
  • For a real word application a 200lb person needs to be able to walk on the platform. I aimed for 500 lbs - 1000 lbs to account for dynamic movement of the person. – foo Oct 8 '19 at 16:29
  • What will it be used for ? Will the weight of whatever is put on it be evenly distributed from edge to edge or will it be in the middle? Flat 2x4's even with plywood will flex with enough weight. – Alaska Man Oct 8 '19 at 16:57
  • Flexing is a good thing as it will not snap. I wish someone could provide a reference for what 2 x 4s can hold. The plywood on top will help distribute the load evenly to the 2 x 4s and to the casters at the corners. – foo Oct 8 '19 at 17:54
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The 2x4's are also the problem. You want to support this from casters only on the corners. That means you need an 8 foot span, as well as a 4 foot cross-span, and you need to keep all this rigid so it doesn't spread and lose strength.

Half of the structure is the wood to carry the weight. The other half is to keep the first half from twisting or spreading when the weight is applied.

2 by anything isn't going to work lain flat. You'll need to stand these tall and I would go with 2x6 myself.

I would create two box structures on either side of the frame running the long way. How do I describe this?

Start with two 2x6's that are 7'9" long, set on edge and parallel to each other, and about 8" gap between them. Got it?

Under load, these beams will want to twist, spread or lay over. We don't want that. So we put stuff between them. Spacers made out of more 2x6 material 8" long. In the middle we set those vertically, making a tall "H", except we need more than one cross-spacer. Near the ends, different deal. We lay the spacers horizontally - and this is what we attach the casters to. We can position these spacers up or down as needed to set the desired height for the platform. Got it? We build two of these.

Now we get two 4' 2x6's. These will become the end caps. Lay the box structures parallel so their outsides are 4' apart, and attach them to the end caps.

That leaves a big, thin 2 foot by 8 foot rectangle in the middle with no support whatsoever. Another 2x6 joist would suffice there, or even a 2x4 if weight is a worry.

Now the framing is done.

The top plywood is very important. It's not just a hole filler. It also provides structural stiffness to keep the whole thing from skewing or spreading. So 1/4" will certainly not cut it. Given that we now have joist support every 12" or so, 1/2" will probably do.

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  • I'm not disagreeing with your answer but can you provide any reference online as to how you came up with these numbers. I will defenitely move to 2 by 6 but using them standing up requires more technical aptitude then I have. They have to lay flat on the outside perimeter as the caster requires a 4 inch by 4 inch mounting point. – foo Oct 8 '19 at 17:57
  • @foo well, there's a problem. 2x4's are only 3-1/2" wide. Hence my advice about creating "pads" between the sistered 2x6 beams specifically for the caster. My answer was based on an old fashioned technique called erring on the side of caution, which risks overbuilding. If design margins are tight and you must engineer to minima, then you'll need actual engineering skills by possession or hire. This is much to complex to google. – Harper - Reinstate Monica Oct 8 '19 at 18:22
  • New question posted based on this question's answers ... diy.stackexchange.com/questions/176059/… – foo Oct 8 '19 at 19:06
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The 2x4s are not the problem. The plywood is the problem. I would go for a minimum 1/2" plywood, possibly even 3/4". That will make it a lot heavier - but also a lot sturdier. Since you are putting it on casters, the weight of the plywood should not be a big deal but also make sure the casters are rated for the total weight (1,000 lbs+).

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    This may be true, but we don't know what the load is. If the object has a flat bottom of its own, the plywood's strength is mostly irrelevant. – isherwood Oct 8 '19 at 16:17
  • Casters will hold 1200lbs. Do you suggest the plywood would break at 1/4". If so can you reference a chart online. Thanks. – foo Oct 8 '19 at 16:26
  • For a real word application a 200lb person needs to be able to walk on the platform. I aimed for 500 lbs - 1000 lbs to account for dynamic movement of the person. – foo Oct 8 '19 at 16:28
  • @foo In the future, include information like that in the question. Yes, 200 lb. person is a bigger deal than 200 lb. static, but also in some ways quite different (some better, some worse) from 500 lb.-1000lb. static. I don't have any reference charts (I did a quick search earlier) but my gut tells me don't have someone walk on 1/4" plywood like that, it may not break but it will bend and is looking for trouble.... – manassehkatz-Moving 2 Codidact Oct 8 '19 at 16:44
  • New question posted based on this question's answers ... diy.stackexchange.com/questions/176059/… – foo Oct 8 '19 at 19:06
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http://www.pacificwoodlaminates.com/img/PDFs/APA/APA_LoadSpanTables.pdf Table 2 Face grain PARALLEL to supports

At 1000#/32sf = 31.25 psf (pounds/square foot) Therefore, you will require at least 5/8 (Bending = 37)

Note: Bending means how much it can support if you don't care how much it will sag. L/360 - L/180 are measure of stiffness used if you were going to use it as flooring vs. roofing.

@isherwood is correct in saying if the load is structural enough to be able to transmit the weight directly to the 2x4s without bearing on the plywood (e.g. a cabinet), then all the plywood is doing is holding the 2x4s square.

However, I would still use 5/8" just in case anybody stepped on this thing.

BTW going to 2x6s will not help in this situation.

Also I believe you will need six casters. one at each end of the bottom 2x4s. 8' is a long way for three 2x4s to span without sagging a lot.

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I would use two 4x4s in the long dimension, or maybe 2x6s if you can stand the height. The 4x4s have more width and will be easier to attach casters to.

Definitely not 1/4" plywood, it has no structural strength at all. It is used for cabinet backs and similar applications. You want at least 5/8".

Unless your floor is really smooth, there will be a lot of stress between the lower and upper structural members when you roll it and hit a bump. They need to be connected very well.

enter image description here

Edit: calculation for one 2x6 flat. I wouldn't want much more than 1 inch of deflection, you can decide how much is acceptable to you. I am an Electrical Engineer, not a structural engineer, never risk your life based on what I say. My qualifications (besides general DIY): I got an "A" in Statics 40 years ago. I strongly recommend measuring the deflection yourself as others have said in the other question.

enter image description here

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