I was reading an old thread that CX asked Dave Gobis about testing of floor boards as a sub floor. I have 1/2 or 3/4 (need to measure tonight) floor pannels and just layed down 3/4 plywood over it. I want to use 1/8 ditra and install granite on this. The x factor I was told by Bill is the floor boards. What do they "count as" ?? In other words, do I have what is considered at least 1/ 1/4 subfloor now (before I lay the Ditra?) Forgive me if you see me posting in a couple forums some of you may visit. I am learning so much from you all and am trying to make the best possible choice based for success taking into consideration height limitations. Thanks, Mike

I also want to say thanks to CX for bringing up the floor boards point. It is true, many of us Do-It-Yourselfers have this as the first subfloor. Many of these houses (like mine) were built in the 50s and it was standard to lay this down followed by 3/4 oak floors. I have refinished my oak floors through out the house and love them. However, my foyer is much more practical and elegant with granite (or so I am envisioning). Anyway, thanks to all for the help. I am tired of hearing "ummm" from Home Depot Idiots. Mike

The Boards you have are probobly laid on the diagonal across the floor joists, Correct? This is called shiplap subflooring.it is generally 3/4" minimum up to 1" depending on Era and locale.With 3/4" Plywood over this you should have a very sound floor but keep in mind, Span ratings are critical.For a stone installation you need a floor twice as stiff as ceramic tile.L/720 for stone L/360 for ceramic. Our deflectometer will help you to get an idea where your floor lies.Try to determine what type of floor joist you have and the shape they are in, measure your longest span and plug in those figures.The deflecto icon is up top on the right.

Mike-- I still say that what you have, with ditra over the top will do just fine. It'll reduce your height by eliminating the need for CBU over the plywood by about 1/4", and protect the granite by isolating it from the subfloor at the same time.

Bill, I agree, but we still need to check the joist span. Mike, The floor will certainly be stiff enough between the joists, but the deflection of the joists themselves is also a consideration. That's what the deflectolator is for. ;)

Bill: I actually just ordered the Ditra 5 from home depot. I found a cust rep on the ditra site who was very helpfull. For those not in the know, Home Depot has a national contract to deliver Ditra TO YOUR HOUSE free of charge. I paid $92.98 for 54 sq feet. I needed about 48 sq feet. The Depot guy tried to deny any involvement in Ditra untill I told him to call the rep and gave him the number. he then returned my call and apologized. the Ditra will be at my house a week from tomorrwo. I plan on tiling the next Saturday. The 40$ of wonderboard is going back this weekend. I seem to be very good at buying that stuff ans returning it...its the second time I did this. Thanks again, Bill for your help. Tile Guy/John: As far as the deflecto factor, the calculator seems very straight forward. I will get needed info tonight and report back. ONE other question: The spacing joints. I read about this after I put my 3/4 ply down. Turns out I have the needed 1/8-1/4 space between most of the interior ply joints and all of the exterior edges and plan to fill with caulk. EXCEPT one run. I have a 4 foot 8 inch wide hall. That 8 inch 8 foot strip I had to cut is fit tight aginst the other ply with 1/4 space between wall and ply. Do I need to remove and cut the length of the 8" x 4' piece to allow 1/8 between the full size 4'x8' ply and the 8" by 4' ply or am I ok because I have 1/4 inch between the 8" x 4' and the wall? Sorry that was such a mouthful. You guys have been incredibly helpfull. Thanks again. Mike

Mike, Do you have the HomeDepot part number for the Ditra? Jerry

Mike-- In that one spot that you didn't leave a joint between the sheets of plywood, what you might do is take a skilsaw and cut a relief joint right in the wood at the joint as it sits. That should be fine. Just don't hit any screws/ nails!

Bill: Funny you mentioned that. I was considering doing just that. Then I said well it like d I could do it on my table saw clean and not worry about the cutting floor underneath, screws etc. I guess I can't go wrong if I set my saw for exactly 3/4 inch. Bottom line is I do need to make the 1/8 joint Bill???? Jerry: I ordered Ditra 5. It comes in two order qtys- 54 sq foot and I think 300+ sq foot. I did not get a part number. It is special order. They Home Depot does not stock in this part of the country (NY). I was told a couple of areas mid west do keep in stock. Delivery is 6 business days. There are cust service reps listed on the Ditra site that are very helpfull. Mike

I measured my dflection using the calculator. It came out to be L/436. I measured joist length from header to header. It would be much higher if I new if I had Doug Fir. One other factor I could not add: All the joists are all supported by metal bands at center. Also they are not ALL at 16 inch center spacing. There are four joists, thus 3 spacing qtys to report. One side is at 16 inch, the middle is at 16 inch and the other side is at 13inch. Apprarently there is a shorter distance as forced hot air duct was set there once. Bottom line is I think that 13 inch will dramatically decrease deflection...Yes? Thanks, Mike

Right, and the deflectolater will figure that for you, but it will be around L/580 or so. The metal bands are a modern day version of bridging, and are important, so don't remove them. L/436 ain't gonna cut it for stone, and having douglas fir or southern yellow pine won't make up enough, so don't sweat it. Plan on adding sisters to each joist, or even better, adding a support beam down the middle of the room. The sisters, if full size, will double the stiffness of the floor, or L/870. The beam will quadruple the stiffness, or L/(more than 1200, I'm to tired to do the math!) If you add the sisters, be sure to replace the bridging or install new. Still want the granite? :D Bob

Bob: Thanks for the advice Bob. I have to admit I find it amazing how much support one needs for natural stone. That floor feels like it is going nowhere. That is why we ask the pros though ...yes? Couple questions: 1)Adding sisters means doubling up the length of the joists and anchoring them to each other ...say with lag bolts? 2) What do you mean by running a beam down the middle? Do you mean adding another joist centered between the space of the two middle joists, running it the length of the room and resting on header on both sides? Lastly, does the shiplap subflooring have to be secured to the added joists? If it does I will have to take out mt 3/4 ply...this is many many...many many screws. If it does, is there another way to sure up the floor with our doing this? Please advice. ...and sure...I still want Granite. I don't mind extra work as long as my results are what I'm looking for. As a matter of fact doing it right the first time is half the fun. Thanks, Mike

Bob: Too add: I saw this post by Frank earlier: "Thanks Bob it helps a lot. Every joist does have added support, the joists that are not supporting the tub and the shower have at least a 3/8" thick by 6"wide steel plate running the entire length of the joist." Could this method be beneficial to me? I think I need to educate myself a little more on methods and installation of supports...

Mike-- for one joint across the hall, I wouldn't worry about making it more than the width of the wood blade.

Jerry, All: The Special Order SKU at HD for all Schluter products is 344-388. This will bring up the catalog, along with the HD special order pricing.

Sistered joists should be glued and screwed together. You need a lot of screws, say every two rows 6" apart on 6" centers , so lag bolts would be overkill. Don't worry about nailing the planks to these sisters. Frank had already placed the steel plates on, and they worked. Wood will be just fine, easier to aquire, and easier to install. If you added a beam, it would be perpendicular to the joists, and support the joist from below. The beam would need posts every 6 feet or so. This option works best in an unfinished basement or crawlspace. Check out the artical in the Liberry about Cures for Bouncy Floors. Bob

I checked out the cure for bouncy floors article. The plywood stress skin is an interesting alternative. I may use that along with sister joists. I think I have a handle on what needs to be done to further stabalize the joists now. In addition, I am set on the process up top utilizing the Ditra and thin set. Thanks again to all who contributed. I'll let you know how things turn out... Mike