Underlayment help (spacing/screw patter)

[Wookiejack]

Hello, I own a 1,200 sq ft home (built 1954) in southern CA. I have a raised foundation with diagonal subfloor planks (maybe 5/8) that set on 2X8 joists that sit 16 on center.
I plan on running 3/4 tongue & groove plywood over that as underlayment for the luxury vinyl planks (with the cork backing attached). The I will screw them down with #8 1 3/4 subfloor screws. The problem is I'm hung up on the spacing and screw pattern.
I've seen videos where I saw the guys just pound the t&g together, yet I hear you have to leave a 1/8 gap between all the boards, as well as a 1/8 - 1/4 inch gap where the boards meet the walls. As for the pattern, I hear it's 3/8 from the edges (which seems too close to me), and every 3 inches all the way around the board, then every 6 inches in the field (the middle).
I also saw info that said if I'm using vinyl, to use a fan pattern over the typical grid pattern (to prevent imperfections from showing through. I'm also wondering if it's cool to use wood filler on the inset screws to make a flatter/smoother surface.
Thanks in advance.

 

[Incognito]

I've only used 3/4" tongue and groove ply as SUBFLOOR, not underlayment. We'd screw that down 2-4" staggered along the seams and 6-8" across the field. I never used any kind of "fan" pattern.

Underlayment I've used has never had tongue and groove, has never been more than 1/2" and it's best to staple or nail instead of screws in the same schedule as above. I would follow manufacturer's instruction on spacing between sheets and around the perimeter.

I always patch the seams and fasteners

 

[Wookiejack]

The subfloor isn't in the best of condition, I wanted something to strengthen it, while giving me a smooth surface. A/C quality meant at least another $10 a sheet. Regular plywood (CDX) had way to many voids and knot holes/divots to give me a smooth surface, while the t&g gave me side that was void filled and sanded. The t&g feature just happened to be a byproduct of the type of wood I was looking for, and 3/4 was the only size available to me.
But I am unfamiliar with t&g and was wondering push them all the way into each other, or leave a 1/8 space.
(Above was a copy/paste: others have wondered as well)
Is there a certain product you have used to patch the seams/fasteners, or was it just wood filler.
And if you patched the seams (spaces between boards), wouldn't that cancel out the reason for the space?

 

[highup]

Hello, I own a 1,200 sq ft home (built 1954) in southern CA. I have a raised foundation with diagonal subfloor planks (maybe 5/8) that set on 2X8 joists that sit 16 on center.
I plan on running 3/4 tongue & groove plywood over that as underlayment for the luxury vinyl planks (with the cork backing attached). The I will screw them down with #8 1 3/4 subfloor screws. The problem is I'm hung up on the spacing and screw pattern.
I've seen videos where I saw the guys just pound the t&g together, yet I hear you have to leave a 1/8 gap between all the boards, as well as a 1/8 - 1/4 inch gap where the boards meet the walls. As for the pattern, I hear it's 3/8 from the edges (which seems too close to me), and every 3 inches all the way around the board, then every 6 inches in the field (the middle).
I also saw info that said if I'm using vinyl, to use a fan pattern over the typical grid pattern (to prevent imperfections from showing through. I'm also wondering if it's cool to use wood filler on the inset screws to make a flatter/smoother surface.
Thanks in advance.

If you are really talking about a T&G "underlayment", I don't have a clue. Never heard of such an animal. T&G 3/4 inch is made for subfloors, not underlayment............. 3/4 inch T&G is structural, not for an underlayment. It's not usually a very pretty surface, but it may be cheaper than 3/4" AC............ for finished vinyl flooring of any type, never try to go cheap on the choice of underlayment.
If you have a subfloor of 5/8" thick ship lap running at a diagonal over the joists, I'm thinking that another layer of 1/2" to 5/8" AC or CPTS plywood would work well as an underlayment for you if you hand selected the sheets to eliminate imperfections.
Acclimate the sheets of plywood in the room for a couple of days with the heat set at living conditions, then fit and nail the sheets one at a time, fully nailing or screwing down each sheet as you go. Butt the edges lightly before nailing, never force them together tightly.

The fan pattern you mention for fastening is a visual trick to eliminate the appearance of any fasteners showing through. Random nailing patterns hide better because the human brain sees these imperfections much more easily if they are all in a straight line.