I just had another quick question. In my reading of the preparing underlayment article which I found in the liberry I read something that I should be using a screw that penetrates both subfloors every 6" but I should try my best to avoid screwing into joists.

Should I definitely try to avoid joists when screwing the 5/8 ply to my old subfloor?

I am using a 2" exterior screws, laid the ply perpendicular to joists leaving 1/8" gap on all seems and intend to use one screw every 6" avoiding joists directly. And I will be predrilling all holes as per cx's advise. Sound good?

The 2" screws will be a bit long, I'd go with the 1 5/8" so you maximize the threaded portion of the shank.

The spacing of the screws is based on 25 / 75 percent of the joist spacing. So if the spacing is 16"OC, the rows screws will be at 4" and 12".

So essentially you're saying all the screw rows will be between the joists.

You got it! Seems counter intuitive but works very well.

I believe another advantage of the 1-5/8" screws is that if you do hit a joist the screw isn't long enough to really grab it.
With diagonal planks, you won't be able to place the screws in a perfect grid. You'll miss the planks every so often. Fortunately with deck screws it's quite obvious when you do because they won't countersink.

Ok hopefully last stupid question related to this subject. The plywood sheets ( I have three of them).

When the butt up against each other should I leave 1/16" or 1/8" gap for expansion or they can butt up against each other because they are screwed?

I have left small gaps around the perimeter edge under the drywall.

Thanks again for the help!

Small gaps are required by the plywood manufacturer.

Not a stupid question at all !

Well I'm glad you didn't think that question was stupid because here comes another one. I bought deckmate brand #8 1-5/8 in. Star Flat-Head Wood Deck Screws (Home Depot).

Are these good to use? I'm having so much trouble in certain areas getting them to grab effectively to the dimensional lumber subfloor. I'm sure it's because the wood is old and it's on diagonal with gaps in between most boards.

Anyhow two more questions as they relate.
1. Is it ok to leave any holes in the plywood from the screws that don't attach we'll and I remove
2. Since ditra is going over plywood is it ok if some screws are flush with the plywood or do they all have to be recessed.

0- might wanna switch to ring shanked nails and a nail gun

1- the holes will be fine left alone

2- flush is fine.

Thanks.

I will look into the nails. Could I use those #9 durock/ wonder board corrosion protected screws? I have a bunch of those in 1-5/8. They seem to really grab better on one I tested.

Those work well too. But you have to keep an eye out for screw jacking.

The screw shaft with no threads near the head is meant to allow the screw to spin and pull down the upper layer to the lower layer.

Thanks I assume screw jacking is where the screw lifts the top layer of ply up rather then fastenings it down?

correct

Cut during layout?
 

I'm in the process of layout out all my tiles on the subfloor before I lay the ditra and mix thinset.

Do you think it's worth it to do all my cuts and literally have the entire floor cut and laid out before I put the ditra down and mix thinset?

Figured while it would take more time now it might make it faster for me when actually thin setting.

Also how large of a gap do I leave between the tile and wall, tub, saddle? 1/8 enough?

Thanks

Scott, that's entirely a dealer's choice sorta decision. Some installers prefer to do all their cuts first based upon their layout lines, others do the entire layout of full tiles first and then do all their cuts, still others cut and set (or have their helper cut) and lay the entire floor in one pass. Do whatever seems to work best for you.

Perimeter movement accommodation joint should be at least 1/4" and usually doesn't show. At tubs and similar visible obstructions I'd recommend 1/8th" or one grout joint, whichever is larger.

My opinion; worth price charged.