Its probably just going to delay it. If the grow is long enough, and the roots are not kept too wet, they WILL go to the water source eventually. Last time, when they were down low, it took maybe 6 weeks or so to start covering up the nozzles. It was so bad I was forced to move them to the top of the chamber. The roots still tried to get to the new locations, but I won the race that time.
The trick is regulating how wet the roots stay. If they stay very wet, droplets form on the tips of the shoots. The root then thinks its found the source and trys to grow further into that drop of water and will follow that drop all the way to the floor very quickly - they cab grow several inches in just a few days in some cases. On the other hand, if kept just right, they will form fuzzy hairs and can grow any direction at will - out into space for several inches with no support at all. Its pretty amazing.
I think the foam sheets would work well for the top of a flood chamber unless its too large. You could always add some plastic legs/supports near the net pots if needed.
Just be sure to get the solid/extruded foam and not the expanded foam. The polyurethane foams are more toxic if burned, and not as strong, I prefer the polystyrene types like this pink Owens Corning foam. It also comes in blue which is less dense and not quite as strong.
This foam absorbs very little water over time and should hold up well. I havent had to really clean mine yet other than a quick wipe down, and this is the third grow with this piece of foam. I see no signs of mold or nasty crap building up, so Im happy. Roots that stick to the lid while growing easily wipe off once they dry up a bit. I had a huge mass of roots grow from one wet spot on the lid where a nozzle was keeping it wet. Grew to be quite a mass before the roots fell off and then they just hung in space.
HDPE board would be an excellent choice, but its $$ and heavy and requires real tools to cut and machine. The insulation board can be cut with a utility knife.
You can do it with or without the root mats, but the mats do seem to help the roots spread out. Ive seen old videos of guys doing large flood tables that way. The put a layer of fabric batting in the bottom, then start seeds/clones in 4" rock wool cubes, cover the whole table with panda film with small cutouts for the cubes. Then just run water from the top of the table through a manifold that drips the water all across the top of the table. It drains out the bottom back into the rez.
With a solid foam lid, you could do without the rock wool cubes and use net pots instead. You could easily do NFT or ebb/flood.
The other option for NFT is larger PVC pipes. Ive seen it done in 4" up to 10" pipes.
Edit: I actually dont think you really need the fabric in that situation. Looking back at my notes from when I tried it, the roots mostly grew UNDER the fabric and didnt really make use of the fabric at all. They went straight to the floor - where the water was. Roots always go for the water source.
I think it would work fine to just cover the table with foam sheet, and stick net pots in there and either flood/drain or NFT. As long as the water flow is spread out, the roots will spread out. If the water all goes to one side, so will the roots, etc etc.
you can always grow nft using hydroton not rockwool, always preferred this as it stopped many associated problems with the rockwool style and my cubes used to always be too wet and cause rot problems especially with stemrot so simply held my cutting in the middle of a netpot and filled it in with hydroton with the bottom 1/2 inch of the stem coming all the way through and use a bubble cloner then simply gently rest the resulting rooted clone on my nft tray dropping them thru the cordex i used with holes cut out as a cover for my nft tray, kept the netpots in place and obviously i tied them from above as well to support them and this then suspends the netpot just above the tray with the roots in contact with the channels, just a suggestion as to mediumless in nft
lol this is what im talking about and exactly how i used to grow, have used dutch gutters, nft trays of all shapes and sizes, pvc pipes, verti, bubblers, rcdwc, only thing i have never tried is aeroponics cos seemed like a headache too far lol
you need whats called a spreader mat, laid out evenly across the whole tray before placing anything on there then let the nutrients flow over the mats for a little bit to thorougly wet the surface and do as described above
Roots have an amazing ability to find the water. With that table, they will fill up those channels first. That will force the water to flow over the other areas, and the roots will follow and fill in everywhere.
That said, a tray with less dramatic channels would probably be better.
Thats exactly what we were talking about above. Almost any non-organic fabric should work, but like I said, in my tests, the roots just stayed under the fabric for the most part and filled in the channels, then spread out to find the best water flow.
Adding some type of fabric will not hurt anything and will probably help to some degree, but I still dont think its a critical component⌠Just dont use cotton fabric - it can rot. Use a polyester, nylon, etc. It does need to be a loose, random weave. Batting fabric is what Ive seen mentioned and its what I used. Any craft or fabric store or WalMart will have it.
it is quite critical in nft as it ensures there are no dry patches or pooled water and makes sure nutrients are wicked evenly across the root surface. also i wouldnt use just any cloth as you will run into potential mould or rot problems from being constantly wet so again i personally wouldnt reinvent the wheel and just buy made for purpose equipment as you are much less likely to run into unforseen effects and will achieve better results and for the sake of literally a few quid i personally wouldnt cut corners, i do agree in theory it is simply material to do a specific job that can be mimicked easily with another suitable material
nicely said 
