What is the purpose of the 8" horizontal spike? Only 2" is going into the next plank, not much holding power there. The double spline is doing the alignment, so that is not it. I would see a GRK doing a much better job. 4" nails going through 3" material also does not offer much holding power, 1"??

When using screws to pull something into place I often wind the screw through the board so it sticks out the far side, this is at about a 45 degree angle, with the plank held slightly off the rafter or surface it is going to be held to, this protruding part of the screw will enter into the approximate area and suck the plank home, bending it to where it need to be. It works most of the time. Something that does not work as well with a spike or nail.

One aspect of the Timberloc screws Brad mentions which I don't like it the washer under the hex head is not very big and can sink too far into the wood making contact with the driver socket a bugger to deal with at times. GRKs have a larger washered head and the T-30 bit fits into a hole and this is never an issue. You would be wanting to use the 1/4" or 5/16" lag screws from GRK.

I also think the 8" spike is a little much. These are planed to about 2.25 x 5.25 so there is a good chunk of nail/screw biting.
I was thinking #10 x 4" R4 GRK'S.

Another question to ask is what are the fasteners going into, a soft, soft wood or oak?

A couple of years ago I built a plank on frame using wide pine, 2-1/5" full thickness, wall plank house. These planks had a single spline and pegs along the edges. The frame was oak and pine, I used 5" GRK lag screws. It went well, I think installing 3x6 on a roof deck would be easier, you shouldn't have any problem.

Ray,

check the cost between screws (especially GRK) and spikes -- not even close.

You can get the material predrilled for the tying spikes. I don't have a source for the material or any practical experience, as I've so far gotten out of it everytime I've looked at a job like this.

I've heard nightmare stories from guys I've worked with about warped and twisted 3x decking, so make sure you are getting good stuff.

I would rather supply and lay 2 layers of 2x decking -- is that an option?

your scaring me Gabel. It might have been an option but it's all ordered and paid for. Next time I'll look into that. It's KD and pre-stained with Sansin so I'm hopeful it's straight and true.
GRK's arrived yesterday. Crew's lined up. I'll let you guys know how it turns out.

I'm sure it will be fine. I'll look forward to hearing how it goes.

For a 2nd floor studio loft we used 3 x 6 dbl T&G, solid decking (c.2 5/8" x 5 1/2" face). It was pre-drilled through on the horiz for the 8" Pole Barn Spikes,Ring Shank.
4" PBS, RS for edge nailing to the joists/beams. Nailing was straight forward, a good workout. Stay loose. Get some fat nail sets - those L shaped Japanese ones are good or a punch w a 1/4" diam tip.
Because of the 8 ft OC beams under the floor we needed to use the solid 3 x 6 w 2 tongues & grooves on each edge. 8" spikes connect and stiffen board to board to board. This met load and deflection code for 2nd fl residential (for 8' span of the decking).

I've also used 3 x 6 single T&G, laminated (3 stacked 1"x 6" pcs glued, w middle pc pushed sideways c. 3/4" to create the T&G.) Using Doug Fir, this decking from Potlach spanned joists/beams 5' OC for 40#/sf load and 1/360 deflection for code for 2FL Res bedrooms. This material is end-matched so joints don't have to land on joists. Significantly less expensive than the solid. Shop around.

I got these products in DFir from Billerica MA, straight and dry. Be prepared w some short cut-off pcs using the groove edge and your mallet or lump hammer to persuade and snug up an occasional piece before edge nailing.

I've also bought 2 x 6 solid T&G in Spruce, Pine & D Fir locally in NY and CT. On all these remember the appearance face is down, shows to the room below as finished ceiling. No dirty hands or shoes allowed on the material while it's stacked inside or well covered outside. If a pc needs some touch-up cleaning or sanding do it on horses before installing it. Have some cedar shingle tips on hand for cutting shims, as needed.

"I would rather supply and lay 2 layers of 2x decking -- is that an option?"
If you wanna do that you should install one layer perpendicular to the joists/rafters and one layer diag. Nail the 2nd layer thru the first into the framing members. You'll have a lot more waste this way, as the 2x decking will have to be cut to land on framing, it's not end-matched.

Laid 2200 sq/ft in 4 days between Cristmas and New Years. Took Mark Gillis's advise and had someone drill out the pre-drilled holes as they don't go all the way thru for the spikes. That saved alot of pounding. The space between tongues is wide enough that you can drive the spikes home with a hammer and not smash the tongues. Those 8" spikes are necessary because you can't pry the 3x6 to marry-up with it's neighbour, too damn beefy. The spikes draw it together nice. Used 4" GRK's everywhere else. Butt joints were cut square at a ever-so-slight angle so the visible joint was tight with a 1/8" gap on the top side. All in all it went well but I wouldn't want to lay 3x6 again any time soon.

I'm just glad not to be a piece of 3x6 flooring that Ray Gibbs is looking to put down.

That 3x6 is way to expensive to go medieval on Joel. Remember our talk about peace and harmony?