HCA: Hermit Crab Association

I seen on one of these post about rubber in a tank I want to build a crab tank and I want to use rubber sheeting like they use to put on roofing n with the Sam's use silicone instead of glue my question is would that rubber be harmful to the crabs it's pretty thick ...

I'm sorry, I'm not sure what you are referring to. The only rubber I know of people using in tanks is corner guards at the top of the tank for people to zip tie stuff to. Is that what you are referring to? If so, that is safe. Make sure you use aquarium glue and let it cure for several days before putting crabs into the tank.

No it's rubber roofing material pure rubber for bottom of a project I want to do but want to make sure the material is safe ...

Anything that is safe for aquariums and ponds would be safe for hermit crabs. Some rubber roofing materials seem to be, but others are treated with or will leech chemicals. If you aren't completely sure about the brand and product you have, I would not recommend it.