Tile underlayment size
 

I know there are may people with different opinions on brands. My question is, should I use 1/4" or 1/2" underlaymet for my new construction floor? The subfloor is 3/4" T&G over 9 1/2" engineered joist. The tile is 1/4" ceramic. I am doing around 800 sq ft throughout the house. The tile will be next to carpet with 1/2" padding using a schutzer strip.

Thanks, Will

Will, by underlayment, if you mean backerboard, then use 1/4" for floors, unless you need to build height. Anyone's backerboard will do, so pick what's cheap and readily available.

How far are the I-joists spaced? Do you know if the deflection rating is L/480 or better?

Are you sure your tile is 1/4" thick? Those are usually wall tiles, so you need to verify they are floor tiles.