Two situations call for the installation of a membrane: extreme moisture, or a substrate that may shift or crack.

If a floor is likely to become very wet, water can eventually seep through the grout and even through concrete backerboard, damaging the wooden flooring beneath it. A waterproofing membrane avoids the problem and protects the home's structure.

If a floor substrate is likely to crack or shift slightly over the years, an isolation membrane can prevent cracks from transferring through to the tile or grout.

Membrane materials: Older installations often used roofing felt as a membrane; today, polyethylene is more common for most applications.

For floors that will be exposed to standing water for prolonged periods, use a professional-quality waterproofing membrane. Many types are composed of two layers, one fibrous and the other made of modified bitumen.

To get maximum protection against cracks, use an isolation membrane composed of a layer of polymer and a reinforced fiber sheet.

Trowel-applied and sheet membranes: Any membrane must firmly bond with the substrate, using either paste or mortar. 

A "trowel-applied" membrane comes in two parts, the membrane and the adhesive or paste. One type has a temporary paper backing. Paste is also used with a type of membrane, used for cracks only, that consists of a narrow roll of mesh tape. Spread the paste with a trowel, roller, or brush, roll the mesh onto the paste, and apply another coat of paste. When the paste dries, you're ready to tile.

A "sheet membrane" is applied in much the same way, except that it is set in thinset mortar rather than adhesive. Trowel the thinset onto the floor using a notched trowel, and roll the sheet membrane onto the thinset. Allow it to set; then apply another layer of thinset and set the tiles.