So I have very limited space for my comics. I currently have one long box and 5 short boxes, all pretty much filled. The time is coming for another short box. All five short boxes are stacked on top of the long box. I feel like they are getting heavy and could possibly collapse. Or even worse, slowly bend all the comics on the bottom efore I even notice. Most everything is bagged and boarded but that is no defense against gravity and pressure.   
 
All my Silver Surfer comics are in the long box. I would be devastated if they were to be harmed.  
 
So the question is: How high can you stack them before something gives? Anyone have any experience in this matter? Surely some of you have far more comics than this. 

I think if  you wanted to be safe, I would stack the boxes crosswise at each level - like Jenga game. That way the weight would be distributed a bit better. I don't know if you could do that with a mix of short and long boxes. I only have 4 long boxes and will probably need to get another long box soon. I'm stacking mine 2x2 right now, but I had the same question about it.

" I've got about 6,000 comics.  All I use are the long boxes.  I keep them stacked 4 high in the same place for years.  Never had an issue with bending.  I think you're probably good, but I wouldn't go much higher. "

I gave up the long boxes, because they're so freakin' heavy and unwieldy, when they get full.  I have close to 100 short boxes and magazne boxes.  I usually stop at 4 high, but it really depends on where you stack them.  I have a closet in an unused bedroom where I can store boxes 5 to 7 across (I forget), and 5 high, because they're all the way across the closet - there's nowhere for them to lean.  Out in the middle of the room, I stop at 3 or 4 high, because they lean a little more, sitting on carpet with no wall to support them.  I've noticed the box tops crush in just a tiny bit when stacked four or five high, but I've never had a problem with them falling through or bending any of my comics.

I've sealed myself in my room with mine

faced the same problem when I rearranged all my longboxes too.  
I switched to a drawer based system so I could simplify access and do the stacking. With the cheaper housings I've found about 5 high to be the breaking point. The bottom boxes start to bow out from the weight. With some light boards in between each row for distribution I might be able to get 6 high but it'll make more sense to add another column than another row at that point. It looks something like this