Is 5/8" plywood over 1/2" board subfloor adequate for tile?

Question

Hi there hopefully I’m posting in correct spot I’ll try and get right to the point. House built in 1987 I’ve been here 10 years and redoing the 2nd floor bathroom that was vinyl flooring.

When demoing the vinyl came up with the 1/4 underlayment attached for the most part while some was still adhered to the 1/2 subfloor underneath. I was able to scrap the remaining 1/4” off the 1/2” subfloor.

1/2 subfloor doesn’t appear to be tongue and groove. It’s nailed into joists 16” OC. 1/2 is in ok shape and would be a nightmare to rip up and properly secure back down it’s a small bathroom with a lot of corners and walls build on top of the subfloor.

Here’s my thoughts going to 1 1/2” deck screw the 1/2” subfloor into joists every 6 inches and seams. Do 17/32 or 5/8 on top of that not sure if it should be glued to 1/2 and screwed into joists, or just screwed to the 1/2” itself?Then Ditra and 1/4” tile. Thoughts? Thanks in advance guys

isherwood · Answer

Your plan is reasonable, but I'd make a few suggestions...

Most tile and mortar manufacturers want 1-1/4" minimum subfloor. I'm not sure if the Ditra helps with that, but you probably want to go with 3/4" plywood. 
1-1/2" screws are likely to strip in that aged wood. I'd use 2" or 2-1/2". They need to sink flush or below.
You don't need expensive coated deck screws. Plain gold construction screws will do. Don't use black oxide drywall screws. They're too brittle.
It's not so important that you screw every 6 inches as much as it's important that every board is down securely. Screw about 3/4" in from each edge of each board. 
Consider a small-bead zigzag of construction adhesive under the plywood, perpendicular to the boards below. This will help lock in any movement (which makes noise). A screw pattern of about 12" on each joist should be plenty for the plywood, then.

Is 5/8" plywood over 1/2" board subfloor adequate for tile?

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