Is grout water proof or do you need an underlayment?

No, grout is porous, but you can make it and keep it waterproof by sealing it with clear silicone caulk (smeared-on by gloved finger) or grout sealer every 10-years. If you have the opportunity to get an underlayment on and haven't tiled yet, that's best. However, waterproofing is only really needed for very sloppy people that don't use welcome or bath mats. You really have to have frequent standing water to ever present a problem.
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