Grout lines: 3/8 vs 5/16 and I'm stuck in the middle

I was reading an old thread from like 2018 where a guy in Chicago argued 3/8 inch lines are the only way to go for patios because of drainage. Then my buddy from the union hall swears by 5/16 for anything residential, says the bigger lines look sloppy. I just finished a walkway for a lady in the suburbs and went with 3/8 because the stones were kinda rough cut. But now I'm second guessing myself after seeing a job last week where the 5/16 lines looked so clean and tight. Does the size of the stone really matter that much or is it more about what the customer expects? Anyone else go back and forth on this?

3 comments

3 Comments

sandrat241mo ago

Did you catch that article from Fine Homebuilding where they tested water runoff on different joint sizes? Apparently anything under 3/8 lets water sit and cause issues with freeze-thaw in colder climates.

tyler_wilson1mo ago

Actually Fine Homebuilding did a follow-up on that article (I think it was in their winter 2024 issue) where they admitted the 3/8 inch rule isn't as strict for residential patios with proper base prep. @sandrat24, even their tests showed 5/16 joints drain just fine if you angle the base right, which a lot of guys forget about. Stones being rough cut is a solid reason to go bigger though, you want that fill to actually stay in place.

alice_palmer201mo ago

Yeah that Fine Homebuilding followup was interesting, @tyler_wilson is right that base prep matters way more than most people want to admit. I did a job last fall where we used 5/16 on some irregular flagstone and it worked fine because I spent extra time on the crushed stone base and gravel pack underneath. The bigger lines thing is totally about the stone shape though, rough cut needs that extra room for the polymeric sand to actually lock in.

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