When replacing a tar-and-pebble flat roof with TPO, how much does TPO thickness matter?

TPO or thermoplastic polyolefin is a membrane material often used on flat roofs or low slope roofs. It usually comes in sheets and is fastened to the roof deck mechanically or is self adhering -- like a big roll of duct tape. It sounds like you have already figured out that there are numerous TPO options.

One big decision is the TPO thickness. The 45 mil is generally the standard, but 60 mil is also common. Heavier thicknesses, like 80 mil, are generally reserved for more specialized applications because such a thickness can become cost prohibitive for residential projects.

The TPO thickness typically corresponds to durability. At the core of the membrane are layers of fiber and polymers, and it is this core that actually protects your roof. Above and below the core, on the roof deck side and the side you walk on, extra material is added to increase durability. Over time this added layer will break down, or chalk, and eventually the core becomes exposed. Extreme weather and walking on the surface speeds up the wear-out timeline.

So if you have found TPO products that you like, with little price difference between them, then it makes sense to go with the thicker membrane. You can achieve greater longevity, and the core should also be more robust.

Most TPO manufacturers make the membrane in different colors. Since you are in Albuquerque, you might want to consider a lighter colored product, which can help reduce the solar load on your house and might help save on energy costs. A good roofing contractor can advise you on finding the right fit for your roof.