Table Warping

You can skip the back story and go to the last paragraph for my question.

Last week (on a Friday) a student donated a top that wasn't in use. I removed the frame leaving just the board/top. And played a couple hours of table tennis after school with students. When everyone was done I placed the two halves upright against a wall for the weekend. Over the weekend I purchased another used table, but kept frame on that goes around the underside boarder, which gives support each half of the table. 

I returned to the school on Monday to find that the two piece I used on Friday had warped significantly. All 4 corners on both tables were bending downward to a very obvious degree. They weren't stored in the best conditions, so it wasn't too surprising. I laid the two halves flat on the floor and put heavy objects on top to reduce some of the warping. The table I picked up over the weekend was checked a couple days after I put it into storage and noticed that even though there is a metal frame around the boarder, it was still starting to bend. The newer table was stored in much better condition. Both tables were leaning up against a wall, and the only space I have for them is to lean up against the wall; I can't lay them flat forever. 

My question is... does anyone recommend a conversion top that can lean up against walls or be stacked next to each other without warping at the corners? My friend had a pool table top that wasn't always on top of the pool table and instead was leaned against the wall when not in use, and I never recall it warping. It has been years since they sold their pool table and table top, and even longer since his family purchased it, so they don't know the brand. Any suggestions would help. Thanks for reading and I'm looking forward to replies/suggestions.

The first table top is empty underneath (no frame), was leaned up against a wall, warped. I understand the frame along the edge is important.

Second table top has the frame around the bottom edge, leaned up against a wall, it warped

I had a friend with a conversion top, I don't remember a frame around the underside of it, it was placed up against a wall for long periods when we were in the pool phase vs the table tennis phase and the top never warped.

Are there some conversion tops with "anti-warping technology" because even the metal frame on the bottom of my second table isn't preventing warping. Or are conversion tops made differently? I remember my friends was completely painted while the tables I have are painted on the top and exposed on the bottom, which could allow for moister to enter.  

In case you do not have luck in finding a non-warping conversion type top I have a couple of suggestions for storage of the regular tops.

1. Do not let the bottom edge touch the floor.  Get a couple of 2x4's and put them up against the wall so that the tops will sit on them and not on the floor.  Mopping, natural condensation, spills, etc can put water on the floor that will hasten the warping.  Getting them up even a little off the floor can help.

2. Come up with some kind of restraint system (bungie, rope, stops on the 2x4's) so that the tops are standing vertically (not leaning) against the wall.  Any force on the edges in a bending direction will lead to warping.  If the weight is straight down (in line with the plane of the top) there is no bending force.

Hope you have great luck with your school project.  TT really needs to get into more schools.

Mark

is it a block wall, outside wall, damp wall

 frame and storage are the key, I have many tops leaning against walls, boxs without warping

maybe you have some compilary action so as Mjamja said keep off the floor and also something to stop table from leaning on the wall, most tables will be mdf and will absorb moister