Do yourself a favor. Take up the old carpet and put down pad. Then install the transferred carpet. If you loose lay the carpet over the old, there will be nothing to secure it to the floor. The carpet will move (even with light traffic) and the edges can curl. Then, you will have to pull the old anyway. Also, you will be supresied what is under the 20 year old carpet (and it may not be pretty).

+1. Do not lay carpet over old carpet -- even the really solid commercial stuff -- unless you want it to look bad, want to replace it in a hurry, and want to give a minor thrill to anyone who visits and who is not all that steady on their feet. Depending on what is underneath the existing carpet, you may not have to be all that careful in removing it (you want a clean surface, but carpet and pad does not insist on glassy-smooth -- just firm.

The above comments are correct. I have carpet over carpet and it "walks". I have tried under mats and it still "walks"Enough ripples to trip! I have in bedroom. The only recourse I can figure is nail it to the floor. I have tried all other alternatives.!

Thanks all. So guess I need to get out the snow shovel to unglue this stuff from the floor. :-)) Was thinking to KISS but sounds like that's not a good plan.
Again, many thanks.
Ciao H

Many years ago when living in an apartment the landlord installed new carpet over old carpet. Felt a bit mushy and most of the doors needed to be cut so they wouldn't drag on the carpet. Never did like the look and feel of that carpet.

Dom

Resurrecting this old thread to say I have carpet installed over carpet (long story, not my choice) and it's been down for 7 years with no movement at all. These are children's bedrooms so they have had lots of traffic.

The original carpet was a nearly flat loops kind of carpet, not plush or shaggy, and maybe that is why the top carpet has remained tight. It still would not be my choice, but apparently there is a way to do it that prevents the top carpet from getting wobbly.

The only 'benefit' to this installation that I can imagine is that the floors are probably warmer and more cushy than they would be if it was just a pad between the top carpet and the slab basement floor, and that was beneficial during the years the kids played on the floor.

The huge downside is that all the doors had to be cut so they would open and close and now if we ever want to do hardwood, tile, or a normal carpet install, the doors and closet doors will have to be replaced. Grr.