It is very common in office buildings and in hallways of apartment/condo buildings for the carpets to be installed using the double glue down method. This method is not often used in homes as it is much more expensive and lasts longer than most people want to look at the same colour commercial grade carpet.
Essentially the underlay is glued to the concrete floor then the carpet is glued to the underlay. But to do this properly not just any underlay is used, the correct underlay is called Duracushion or a similar kind of underlay with a different name. This cushion is made up of shredded rubber, quite dense, usually one quarter of an inch thick and quite expensive. It also is designed to last the lifetime of two carpets.
Trouble is, this underlay is often substituted with the common household underlay which just doesn’t stand up as well or stay adhered to the carpet. And because of this substitution repairs have to be made frequently to re-glue the carpet both to the underlay and the underlay to the floor causing un-necessary expense that could have been avoided by the installation of the proper underlay.
If you are considering a double glue down installation, ensure the correct underlay in installed and you will save yourself a lot of un-necessary expense in future repairs, the carpet’s appearance will retain it’s “like new look” longer and the carpet will last much longer before wear uglys out the appearance requiring replacement.