5) You don't need an extra layer of plywood to do the wires, SLC, Ditra, and porcelain tile.

I can't speak to the impact the washer & dryer nearby might have on that, though.

Thanks Weedyacres for your reply. I am still thinking about this floor. I have been seriously thinking about getting a Miele W/D and have heard that the spin cycle causes a lot of shaking. I read in the installation manual (on line) that they recommend a VERY thick ply layed down on the wooden joist system and bolted through the joist. Now I may have some of the details wrong on this but the bottom line was that this floor has to be very ridged. It is also recommended that the set sit in a corner where there is less flexion of joist. I have a corner for the set where both walls are load bearing with block walls underneath in the basement. I have now flipped the plan for a 3rd rendition. I actually like this one the best. Anyway I am going to call the Miele tech department tomorrow to ask about this issue. Again thanks for your response.

Sorry I don't know all the details of joist spacing and bracing or the proper installation with in floor heating. But I did switch to a stacked full size LG washer and dryer and tiled my floor at the same time. I have the current standard joist width with plywood subfloor and replaced the underlayment with OSB (due to water damage) and put Ditra over top and then slate/quartzite 18x18 tile. It has been installed for a few years not and I have absolutely no cracks in tile or grout. Honestly, the washer and dryer don't move that much when operating. The washer is directly on the floor and the dryer is much lighter on top. Every now and then things get unbalanced and they will vibrate, but they actually seem to vibrate less than my old (think 1970s) washer. They back up to a load bearing wall, but although they're in a corner, the side wall is not load bearing. Ditra is an isolation membrane that accommodates some motion. I know there's also Ditra XL, though I think that's typically used for very large format tile.

Thanks pricklypear, I ordered my set of Miele w3033 on Monday. I have redrawn my plans to put the set in a corner as the installation suggests. This corner has 2 load bearing walls. Ain't I lucky :) The Miele installation also suggests that you install a 1&3/16" thickness of plywood. I can do that but now how does not tile :/ I guess tile a short step. I like the Ditra idea too. Or I could not tile that corner and fasten the ply down and paint it and trim it out with wood to match the built in cabinets I will have in this area. I called tech support before I purchased, and he went by the book. I would like to tile the site, but he didn't have an answer for that. I might post over on the john bridge forum for help on this floor and tile situation.

I'm still thinking on this.

1) You can brace or block the joists per standard construction, but what might be easier is to strap the bottom edges of the joists. Typical strapping is done with a 1x3, but a lot of times I'll simply use a 2x4.

Run a 2x4 across the bottoms of the joists, perpendicular to the joists. Add a dab of PL adhesive and use a couple of screws at each fastening point. You have specific needs here, so spacing the 2x4s as you need will work very well.

What it does is if a single joist wants to deflect downward, the strapping transfers that load to the adjacent joists as well. So it's good as a floor stiffener. For a washer/dryer set up, it also dampens the entire floor platform in terms of the possibility of the spin cycle causing the floor to reverberate.

The strapping serves the same purpose as cross-bracing or solid blocking between the joists. It's code-approved, though code only requires it for 2x12 joists, with the strapping being made of 1x3, and the strapping spaced every 8'. The strapping is more effective long-term than bracing or blocking, due to fitment, installation, and shrinkage issues.

2) Do the whole floor. There are only restrictions on maximum spacing (every 8'). So if you make it every 2', or 3', or 4', no worries. Strapping is also easier because you're not wrestling with any of the utilities that take up space between the joists.

3) Your joists span 8-1/2'. For simplicity, why not do run three pieces of strapping? One at 1/4 span, one at half-span, the other at 3/4 span. Just a little over 2' spacing.

4) 3/4" ply over the diagonal boards is plenty. It does not have to be exterior grade.

5) Your floor sandwich can be made up of:

-Tile
-Ditra
-Floor heat wires encased in SLC (no wires under the washer/dryer area through, see comments later)
-Plywood underlayment covered with SLC primer
-Diagonal plank subfloor
-joists

I usually advocate a small platform under the washer/dryer stack. It's more for damping than load spreading. I've done sandwiches of plywood/cement_board/plywood, or plywood/MLV/plywood. MLV is mass-loaded vinyl. Screw through the top layer of ply, through the middle material, and into the bottom layer of plywood to secure it together. Or just use construction adhesive.

I drill a hole through the middle of the platform sandwich to accommodate the flood pan's drain line. I usually put a square of rug pad on the floor. The inexpensive, maybe 1/8" thick, rubbery mesh or open weave type of rug pad. It acts as a bit of a damper, but also holds the platform in place. I then set the platform on the rug pad. Then put the flood pan over the platform with the drain plumbed through the hole in the platform. Then set the washer in place and the dryer on top of that.

I recommend you tile the entire floor, but again, no heating wires under the location of the washer. You'll be putting the sandwich platform on top of the tile in that area, and the insulative value of that platform can shorten the life span of the heating element.

Pricklypear, the Ditra-XL is simply thicker than regular Ditra. It really wasn't designed for a specific tile size. It's sole purpose is to allow the thickness of "Ditra plus tile" to match up with the thickness of a typical hardwood floor.

THANK YOU MONGO!
This is very helpful and just what I needed to see. The strapping will be easy peasy, thank you. The platform sounds easy too.

1) So the platform doesn't need to be fastened to the joist underneath?

2) Regarding the flood drain system. My plumber told me he doesn't like plumbing for that because the trap dries out and sewer gases get in the house. So how does one plumb for this without needing to pour water down the trap every few weeks. I can imagine just running a hose from the drain hole, underneath in my unfinished basement, over to the floor drain :) But surely there is a more sophisticated solution. Do you run a PVC pipe to the drain in a straight run, not hooked up to the sewer line? I would like your opinion and solution ideas. Then I will discuss this further with my plumber because I do not want water damage. He is an amicable guy.

3) Where do you get MLV? Is it preferable over cement board? How thick MLV would I use, and likewise with cement board?

You're welcome!

1) I never have. And pretty much every house I've built has had the laundry upstairs. I've never heard of a platform shifting or moving. Mine hasn't and my front load washer has been in for...I don't recall. Eight years? 10 years? I have an LG.

2) Remember, this is a FLOOD drain. Only if the washer leaks or a hose fails. Unless your code requires it, you don't necessarily need it plumbed to septic/sewage with a trap. You can simply stub the pipe right down into your basement and let it hang in free air. If it leaks, yes your basement floor will get wet. Or you can run it elsewhere, or to a trap and into the waste system. If you do need a trap, consider a trap primer, or one of those "duck bill" thingamajigs.

Your plumber will be able to advise you on a proper solution for your local code.

I do recommend one of two things:

Either install a manual ON/OFF valve like this Symmons Valve (or like any of the Watts' valves), and exercise the valve when you start and finish the laundry. At a minimum I install one along the lines of this:

But any single-throw lever type of thing will do. What you want is a valve that is easy to actuate, because then they tend to get actuated.

Or for added protection install a water sensing auto-shutoff like the Watts or the Floodstop valves. The washing machine plugs into the Watts valve, when the washer cycle is done it automatically closes the hot and cold water supply valves. There is also a remote sensor that you slide under your machine. If the sensor detects water from a leak or blown hose, it auto shuts off the hot and cold supplies.

If you really want to upgrade, look at Floodchek hoses:

Forgot about 3) MLV.

MLV is usually used for sound proofing. I use it in home theaters, etc. The material I use is 1/4" thick, I figured it'd be good as a damping material, so I've put it under a few washing machines.

I don't notice any real difference between it and cement board. Cement board has mass.

My platforms usually end up 2-3" tall. A base of 3/4" ply, then multiple layers on top. You can do 2 or 3 or 4 layers of half-inch cement board in the middle, topped with a layer of 1/2" ply. Or alternate ply/CB/ply/CB/ply, etc. The 3/4" ply on the bottom is thick enough to catch screw threads. Or omit the screws and use construction adhesive.

I've never used drywall (gypsum board) but some have used layers of that under washing machines too.

If the edges will show I'll band it with wood to make it all purdy.

Hey Mongo, today is the day I build my platform. I am going to use 3/4" Strud-I floor pieces that I have left over. I will put a section of stall mat in between and maybe at the bottom where it might conform to the tile floor. What do you think of this plan?

I am planning to screw and glue the layers together. I re-read this post and see that you suggested cement board. I could use this too. The reason for the stall mat idea was that I have seen several references to this as a vibration control, over on the laundry forum. I like the plywood addition because it would seem to make the whole sandwich ridged. the platform will also raise the set up a bit which would be very nice.

Any drawbacks that you can see?

Mass is good, but I'd pass on the Durock NexGen if that's what you're going to use for the cement board. The NexGen formulation can be a little soft versus the older true cement boards. You can see how the edges of your own boards are a little raggedy.

The stall mat can be a good touch. I have a price out sheet on my corkboard of things I need to price and buy, stall mat is one of them. I remember writing it down, but now I can't remember why I wanted it!

LOL, may be you wanted it for your horse?

Anti fatigue mat?

Put under a front load washer?

I can get a 3/4" 4'x6' stall mat at our farm stores for $40 ($39.99). They are recycled rubber.

Up date on the platform. I ended up not using the one I made as it had a wobble in it. It was this way any way I laid it. So W/D went on the floor and guess what? No problems :) Below deck there was a 2x4 strap across 2 joist and I never got around to doing any more. My expert DH, thinks the floor is just fine and the washer at top spin is solid. I love the machines.

Still waiting for the cabinets, hopefully next week. The right side of the washer set will have a tower. The mid section will have a door. through that door I will have modified removable shelving of some sort so I can access the water, electrical cords, and the dryer vent. I left the planning of this to the cabinet people, using my layout of the utilities as shown. The shower plumbing access on the right wall will be accessible too, but after discussion with the carpenter, we will not have a port to get into the access, instead we will take off the cabinet.