Is the 1/2 mega roll firm enough to use over fairly soft carpet for a treadmill? I bought a mat from local fitness store, think it was 1/4 or 3/8. It bends quite a bit when I walk on it on my carpet, and I'm not that heavy. Have about 400 lb commercial treadmill on it and its barely better then just being on the carpet alone.

Our 1/2" Mega Rubber Rolls perform best over commercial grade low pile carpet. Since your wanting to use this roll as a mat, the weight of the treadmill should keep the roll solid underneath the treadmill. There may be some bending due to the carpet being a high pile carpet, but there shouldn't be much. This roll should perform better than the thinner mats you have been using. Other products that you could consider that perform better over high pile carpets are our 1" Monster Tiles and our 1" Sports Play Tiles.