Happy B-day, Enduring! Don't discount that age, you're still in your prime :-)

As the floor goes, take it down to a 3/4 diagonal subfloor, screw down existing planks, replace bad ones, put 1/2" plywood glued and screwed perpendicular to the joist, and use the ditra membrane, be sure to follow installation specs and you will have a solid tiled floor.

Good luck

@GN Builders, thanks for the BD wishes! I was thinking like you, take it down to the subfloor and add new layer before an uncoupling membrane. Then tile. DH wants to keep the T&G but we don't know the shape its in. It will also add too much height in the end if we do that. I am the lead on this project, and all projects in the house, lol. He has been a terrific help over the years. What kind of ply? Exterior grade?

@Raimondi, thanks for posting. I call this a porch, but it is climate controlled. It is fully insulated in the walls and has a heating duct, though it is over an uninsulated craw space. No water. It houses the pantry and fridge, along with a coat closet and boots. I was thinking of porcelain and a color just like you posted; great minds think alike. I really like that tile. Thanks for the link. Would you just lay it straight with the walls. Thats what I'm thinking. I don't want to get too pretentious with the tile, lol.

The call I had out to an abatement company returned this morning. I had already decided I could do this job myself using some excellent info I found out of WA state. I believe is a gov publication. I reviewed it with the worker on the phone and he liked it. It was actually more detailed than he might have done. He gave me a few tips too.

Hey Enduring. It's been a while. So you have one more tile project in you:)

When selecting a uncoupler I strongly suggest considering Blanke Permat. Far superior IMO. Stepped away from the orange products many years ago.

https://www.blankecorp.com/products-for-tile-setting/products/floor-and-underlament-systems/blanke-permat

@enduring If you use Ditra per manufacturer specs you can use min 3/8" Exposure 1, plugged-faced plywood, or OSB.

As for Blanke, I'm not sure what underlayment is needed with that product, I just read it briefly about the product when I first learned about it at the builder's convention, and as far as I remember it had some great durability rating, and that was as far as it went since I'm not involved with many tiling jobs in general.

That said, not sure how the pricing matches up to Ditra (which is also up there), and if it's in the budget I would go per @Creative Tile Eastern CT and @Dragonfly Tile & Stone Works, Inc. suggestion (I would if it was my house) they do this day in and out and their suggestions are invaluable.

Good luck

@Creative Tile Eastern CT & @Dragonfly Tile & Stone Works, Inc. & @GN Builders, Thank you so much for the comments. I'm looking the Blanke product over on the website.

A couple of questions:

1) What is shrink when installing the product. The second video I watched below linked, the YouTuber mentioned the manufactor recommended to fill the installed Blanke product at the time of tile install, because you "lose the schrink". But he said he likes to prefill and set the tile the following day and he feels you only lose about 5% schrink anyway so he is not concerned about this.

2) The Second question is how do you calculate deflection when using this product? If I have the marginal floor that I described above.......

Deflection is marginal for ceramic tile, with the 21"o.c. 2x8 fir joist from the 100 year old house. The joist are 2x8 by about 9.5 or 10 feet long. The room is about 100sf in size. When I use the JB forum deflector calculator I get OK for ceramic with 20"oc and result of "This translates to a deflection of L / 384." With 10' long 2x8 joists. If I put in 22"oc it fails to meet the critera for ceramic with results as a thumbs down and "This translates to a deflection of L / 349.".

......How do I know how much improvement the product will give me? Will I get such improvement that I could lay stone? To match my bathroom, using a soft Brazilian black slate.

These look like a couple of good demonstrations of using Blanke product:

https://www.youtube.com/watch?v=IVQMKyBce6A

https://www.youtube.com/watch?v=oXegvHk4j5g

I should add that I ordered samples of the tile that @Raimondi linked. I hope I love them. The picture is great. I also have a price quote out for an Oxidart tile in the copper 8x8. I don't know if its too glam. I love it though, I love the nuance of the metallic and rustic-ness. Its rated for floor and I believe the PEI is 4. The DCOE is >0.42. As you can see its very variable in color. Its a porcelain. The other colorways in this line seem too industrial.

The copper. See the 8x8 on the wall behind the kitchen island:

Here is the Oxidart Iron with is too funky industrial for my use:

@enduring. The technical guru is swamped right now (making beautiful spaces with tile:) and has committee meetings tonight so might not be able to respond right away. We also can connect you directly to the tech rep for Blanke if you send me a message.

What do you folks think? I have been wondering what the deflection value of my floor will be if I use the Blanke permat. I like what I see with this product.

I have used the calculator at the JB forum. My floor with 21"o.c. and the calculations are in my opening post above. When I talked to the Blanke rep he stated that their product will get my floor to L/720 and I can use stone. The conditions of the new floor to acheive this are that I lay 5/8" ply over my old diagonal 3/4" subfloor. Then lay the Blanke permat over the primed 5/8" ply, and tile.

I don't quite understand how the deflection value is derived and how can the Blanke product actually improve it that much. I would love to use the same slate I have in another room, but I worry if I do, it will fail and be cracked due to deflection.

I would get it in writing from Blanke that they will replace it if and when it fails. :) Lots going on to consider...many times old mud rooms aren't internal to the structure...built as an intended porch usually w tt&G floor.. supported differently and utilize wide joist spacing. Meaning the deflectolator may be inaccurate as well. And not in your favor.

Any other comments about my post regarding using stone, 2 posts above this one?

If no more input regarding the topic then, @Mint tile Minneapolis, I think I'll stick with porcelain, taking what you say into account. I will use the Blanke product to keep the profile as low as possible. I'll use the recommended 5/8" ply, as well as follow the install directions from Blanke.

I have multiple tile samples now, and darn, I'm not a fan of any with the knotty pine. I need to knock the finish off the pine and see if the naked pine color will go better with the samples I have. I really do like the Daltile that was recommended above by @Raimondi. A black would work but I am not finding that in an 8x8 or 6x12 yet. Those are the 2 sizes I could go with and be happy.

Wait, I just saw the daltile comes in "Basalto"

https://www.daltile.com/products/concrete-look/quartetto/basalto

Its the tile on the wall. I don't know if this is a real installation or a computer generated one. I like the variegation in this image.

Its your home so it would be your loss and only you to blame if you move fwd as is and it may never be an issue..

ME? i have to think of every liability when i specify a compliant tile install..

Mud room and stone not really a great idea maintenance wise especially if a 4 season location as a main entrance.?

It would not be so hard to cut out all that T&G and plywood to me. Then id be able to evaluate and correct structure perhaps simple as some more joists and simpsons from above. Also id eliminate the risk of T&G movement that by now likely have rusted loose nails .......and then add a single 3/4 layer or 2- 5/8 * ply and then blanke and stone.(more ply if you need the elevation gain.) Thats how id sell it to you the homeowner in my estimate! replete w a disclaimer im meeting TCNA, IRC and all mfg instructions and standards!

Or yes porcelain is safest bet. Yet youll always have t&G under it all. There is no indusrty standard for tiling over t&G other than T&G plywood. even when additional layers are added as you have pictured.

Again I have to look at all the dramatic possibilities in my world. Porcelain........ win win.

Thanks @Mint tile Minneapolis. Excellent points.

Just to be clear, this isn't a 4 season porch, it was constructed like an addition, with fully finished interior and floor when constructed in the 30s. Its part of the house, but was added after the house was moved here. It has a full foundation. The bathroom next to this porch is the same construction, and it did very well with the remodel I did 11 years ago with the addition of joists from below, for a stone floor.

We are older so I don't plan to reinforce the joist system with this room. So I will keep with the porcelain plan.

The floor system has the diagonal wide 3/4" that I will keep, sitting on the joists. I will take off the Fir T&G flooring that is over the subfloor. This old T&G flooring had been covered with the 1/4" ply and vinyl, guessing in the 60s-70s. Once everything is off, and the diagonal boards are sound, I will put on the 5/8" ply, Blanke, and porcelain tile.

There is asbestos in the tile backing. I had multiple samples taken of all the flooring layers, and that was where it was. So there will be a prep for containment to get that vinyl with the ply up and out safely.