Shop Products
Houzz Logo Print
enduring

Adding Joist to Existing & Intact Floor

enduring
12 years ago

Hello, I want to tile the floor of this 6.5'x9.5' bathroom. This was an add on in the 30's or 40's. The problem is that the 2x8 joists are 21" OC and not adequate to to support stone tile installation. I have added pictures of the crawl space to illustrate the current conditions. This add on has joist running perpendicular to the rest of the house.

picture #1. general area of concern:

{{gwi:1498683}}

My plan is to add 2x8 joist in between the existing joist for a total of 4 additional joist.

Picture #2. Close up of the far end resting probably 8" on the ledger (or what ever it's called).

Picture #2:

{{gwi:2069940}}

Picture #3. This is the end that meets the existing house and is kind of poorly secured in my opionion.

Picture #3:

{{gwi:2069941}}

Questions:

1) So can I add hangers to all the ends that have just toe nailed attachments to the joist at the main house as in picture #3?

2) How would it be best to add additional joist in between the existing joist?

3) How do these additional joist get fastened to the sub-floor?

BTW the bath room floor is as follows:

  1. joist
  2. diagonal 3/4" x 4"
  3. 3/4" fir tongue and grove
  4. layer of old linoleum

Any information would be very helpful. Thanks for your help.

Comments (36)

  • renovator8
    12 years ago
    last modified: 9 years ago

    What is the span of the joists? What criteria did you use to determine that the joists are not adequate?

    It is easy to reinforce existing joists and add joists with joist hangers.

  • worthy
    12 years ago
    last modified: 9 years ago

    My joist are 2x8s but really 7.75 x 1.75.

    Welcome to the wonderful world of lumber. Current 2x8s are1.5" x 7.25".

    Add the joists and at least two rows of 38mm x 38mm bridging. Also, thoroughly screw and nail down the two existing layers of t&g.

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Thanks Worthy, but what is "bridging". Is it the same as braces that go perpendicular to the joist, put in between the joist, lined up, to add rigidity?

  • worthy
    12 years ago
    last modified: 9 years ago


    Bridging Source: DaveOsborne.com

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Hey Worthy, that's what I thought you where talking about. The rest of our house has the herring-bone type bridging. Thank you so much. And the Dave Osborne link is a great resource too. I am feeling more confidant that I will get to install tile in this area.

  • worthy
    12 years ago
    last modified: 9 years ago

    The hardest part will be working in that tight crawl space. No arachnophobia! No claustrophobia!

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    I see you noticed all of those spider egg sacks! Its bad luck to kill a spider in the house I've heard.

    I got a thin 20 year old that will be doing the crawl'n. He has a history of collecting the odd spider to scare his poor older sister.

  • renovator8
    12 years ago
    last modified: 9 years ago

    No one should judge the stiffness of a floor from the joist spacing alone; the joist span is just as important.

    You didn't indicate the joist span so I will assume it is 9.5 ft. (If it is 7.5 ft you don't need to do anything to your framing)

    - A typical modern floor with a maximum design live load of 40 psf, 16" o.c. 2x8 joists, and a 9.5 ft span would have a maximum deflection of .147" (L/777). L/360 is the code limit but it doesn't consider deflection limits for tile.

    - The existing floor that you describe would have a maximum deflection of .135" (L/844). This is so high because the Moment of Inertial of the old joist lumber is 42% greater than that of the modern lumber. (I didn't bother to add the probable greater Modulus of Elasticity)

    For the existing floor defection to be equal to the modern floor design deflection, the existing spacing would have to be 27" o.c. (ignoring the sub-floor stiffness for this example).

    So, the most important issue in this situation is not the joist spacing but the sub-floor thickness which seems pretty good to me especially if you add plywood underlayment, screw it down well, and add solid blocking between the joists at 24" o.c.

    But it can't hurt to stiffen the joists beyond the recommendation of the "tile guru" especially if you plan to use large tiles. Adding 2x8's between each joist would reduce the max. deflection to .068" (L/1671).

    But I would sister the new joists to the existing joists because it would provide the same stiffness with half the number of blocking pieces. I would install metal joist hangers designed for double LVL joists to accommodate the additional width of the old joists. This solution has the advantage that the new joists would not need to be attached to the sub-floor, in fact they could be held flush to the bottom of the old joists and not touch the sub-floor (as long as they were well nailed to the existing joists) thereby avoiding potential squeaks and allowing potential differences in the straightness of the old and new lumber. You could even use 2x6's if it made the job easier.

  • pam29011
    12 years ago
    last modified: 9 years ago

    I have a small fear of spiders ... DH would call it large but I don't scream THAT loud.

    The pictures you posted make me think the same thing every time I read this thread, "Looks like a great job to hire out to someone else!" I'm glad you have an appropriately sized person who isn't afraid of spiders to do it :)

    -Pam

  • sombreuil_mongrel
    12 years ago
    last modified: 9 years ago

    If it were me I would sister the new joists directly to the old ones, and nail/screw (Spax or Timberlock screws) together firmly, put steel reinforcements at the ends (tico corner plates) then place solid bridging every 16". That way the subfloor gets more support. I believe this would be firmer and less bouncy than adding intermediate joists (for which there would be almost no nailing room with 9" on either side). I would then take out all superfluous underlayment above, to get down to the original decking, and screw all of that into the new joists and the new bridging. Then add a new layer of 5/8" 5 or 6-ply sheathing to the floor. Fir plywood's ideal.
    You would then have a tile-worthy foundation.
    Casey

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    You guys are great.

    Renovator, I will show my DH your numbers. I like the idea of sister joist. The span is probably 10' with 6" or so, of that far end setting on the ledge. The ledge looks like another 2x8". What are you telling me about the difference between old 80 year old lumber compared to modern lumber? In your paragraph #4 it sounds like you are saying that there is more flex in the old lumber. But in paragraph #5 it sounds like your saying old lumber has less flex. That is what I would assume because of the timber having a greater likely hood of being old growth and thus a tighter grain, less likely to flex.

    Thanks too Sombreuil with the sister joist input, I didn't think of how tight that in between would be with equally spaced joist. With your comment:

    "I would then take out all superfluous underlayment above, to get down to the original decking, and screw all of that into the new joists and the new bridging."

    Are you suggesting that I take out the 3/4" fir T&G that sits atop the diagonal flooring? I was going to leave it for structural support and than lay a 1/2" exterior grade ply over that. Is there a reason not to do as I had planned?

    Bridging will be done as recommended!

  • sombreuil_mongrel
    12 years ago
    last modified: 9 years ago

    Advantage to stripping away the other layers is that you get to refasten all of the old diagonal sheathing to the new framing (without having to resort to extra-long screws, etc) and then you know what you have.
    And it reduces the height so the threshold becomes less of a step-up.
    Casey

  • worthy
    12 years ago
    last modified: 9 years ago

    Each to his own. But wherever I've come across diagonal subfloor as above I've never removed it. Just screwed it down and added underlayment as necessary.

    no nailing room with 9" on either side

    I've worked in tighter spaces. Besides, today we have palm power nailers.

    For the ultimate protection against cracking, useDitra or equivalent.

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Worthy, thanks. I asked someone at Lowes today about using a palm nailer & they thought that it wouldn't work. How long of a nail can it put in? And is there any concussion force to the hand when using?

    Ditra is on my list to use for the tile job. I am not going to remove any flooring if I can help it. The floor seems tight and is in good shape my DH and I believe. I am planning on leaving the fir T&G flooring as well as the diagonal subfloor. Then the 1/2" exterior grade ply underlayment, then heating maybe & as directed, then ditra, then tile. I think that is the order Bill V. (the tile guy) reported to me on the Bath Forum.

  • renovator8
    12 years ago
    last modified: 9 years ago

    enduring
    The old lumber almost certainly has a larger Modulus of Elasticity (E) than new joists so it should be considerably stiffer but because I can't inspected the joists I ignored it.

    But it was possible to calculate the larger Moment of Inertia ("I") due to the larger joist size. The I of a new 2x8 is 47.63 and the I of the old one is 67.86 That's roughly equivalent to adding 3/4" to the bottom of a new 2x8 and it allows the joist spacing to increase from 16" to 21" with no loss of floor stiffness.

    I know I will get some disagreement to this but for this condition the purpose of the blocking is to reinforce the existing subfloor/underlayment instead of reducing deflection because the span is too short and the joists too shallow for blocking or bridging to make a substantial difference in floor stiffness. However, it can't hurt.

    The reason I make this point is to help you to put your efforts where it will make the most difference. If you replace the underlayment with 3/4" plywood, IMHO blocking would be unnecessary work and certainly not worth buying a palm nailer.

    I repeat my earlier joist reinforcement recommendation to sister them and add joists hangers sized for double LVL's where the joists butt a header. I agree with Worthy that it is not necessary to replace the diagonal sub-flooring assuming it is thicker than the 1/2" dimension you originally gave us.

    IMHO the existing floor is pretty stiff and bathrooms don't often get heavy live loads unless the tub is very large and if so it should be treated as a separate design issue. You can reinforce the joists or reinforce the flooring but I don't think you need to do both. You can also use a small tile size or the Schluter DITRA system for larger tiles.

  • brickeyee
    12 years ago
    last modified: 9 years ago

    "But I would sister the new joists to the existing joists because it would provide the same stiffness with half the number of blocking pieces."

    And put a real premium on the sub-floor stiffness between the joists.

    Not a great idea with tile.
    Flex here is just as much of an issue as the joists themselves.

  • renovator8
    12 years ago
    last modified: 9 years ago

    The clear span between sistered joists would be 17.75". A normal modern sub-floor clear span would be 14.5". With a 3/4" T&G sub-floor and 3/4" underlayment the difference in deflection could be more easily addressed in the underlayment design rather than by spacing the joists for a 9" clear span.

    Like so many design issues raised at the GW, we are trying to offer advice with too little information so everyone must make their own assumptions which leads to pointless disagreements. The thing the OP should learn from this discussion is that if you design to "rules of thumb" for all aspects of the floor system it will inevitably be over-designed in terms of time and cost. But since there is no such thing as a floor that is too strong, the OP can reinforce everything if time and money are not limited.

  • worthy
    12 years ago
    last modified: 9 years ago

    The cramped conditions give the theoretical edge to the sistering. But having frequently observed the flex in old 2x8s @ 16"o.c. over a 12' span--watching a second storey floor at eye level as 300 lb tradesmen walked back and forth--I'd add a joist or two. (But then I haven't seen the OP or dh!)

    Palm nailers can often be rented and will certainly handle the job. (See link.) A framing crew I hired frequently used them for installing hurricane hangers.

    Exterior grade ply is not needed.

    Here is a link that might be useful: Palm nailer in use

  • weedyacres
    12 years ago
    last modified: 9 years ago

    Better than the palm nailer is the Bostich metal connector nailer. It'll minimize the quality time with the spiders. Love that gun!

  • brickeyee
    12 years ago
    last modified: 9 years ago

    "Better than the palm nailer is the Bostich metal connector nailer.'

    Not for very tight spaces.

    A palm nailer can drive almost any size nail, it just takes longer to get larger nails sunk.

    There is a decent amount of 'recoil' into the hand pushing the nailer, but not as much as using an hammer.
    It is hundreds of small fast taps, and the mass of the palm nailer helps.

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Hey Worthy, I resemble that remark!, No, just kidding.

    But to the point, our floor is very rigid as it is, but I am interested in replacing the cast iron tub with a new cast iron tub, adding a stone floor, and replacing the cast iron sink with another cast iron sink set into a sink base. The toilet will be replaced too. I want the bath to last 50 years if need be. Time or money is no object on the joist situation. We have a lot of old solid 2x8 rough cut lumber we salvaged from taking down an old grainary. My 20 year old son who is a 3rd year mechanical engineering student can help with the grunt work over winter break. Between him and DH they can get it done. Of course I'll have 2 cents to contribute.

    Renovator8, I do not think this floor flexes at all. I can jump and jump and don't really feel any movement. I get the point that you make (I think) that the bridging will be to add additional support to the subfloor to carry live weight moving over the tile. My concern is the fracturing of the thinset under the tile with subtle movements as we walk on the tile. When you said -reenforcing floor or the joist but not both-, I am wanting to do the joist. I don't want to elevate the floor much.

    Brickeyee, what do you mean by: "And put a real premium on the sub-floor stiffness between the joists. Not a great idea with tile." Sort of OT, I had looked at your steel strip post from several years ago and found it interesting.

    I believe what I am trying to accomplish is to make this floor as stiff/solid/rigid as concrete. I want to provide as much support throughout as I can. I want the tile and thin set to maintain its integrity.

    It sounds like from every one that this stiffness can be achieved in a variety of ways. I am leaning towards getting a palm nailer, adding joist between the existing joist and then only bridging between if its a poor idea not to.

    So, is it advised to brige with 9" OC joist?

  • renovator8
    12 years ago
    last modified: 9 years ago

    IMHO bridging/blocking would only be effective if the joists were 2x12 or larger and 16" o.c. or greater with a much longer span (the IRC now only requires bridging/blocking for joists larger than 2x12, not that they care much about deflection). In any case, it would would only be installed at the middle of such a short span and adding a 1x3 strap across the bottom of the joists would provide an easier to install equivalent.

    Decide if you want to replace the old fir flooring first. If you replace it with 3/4" plywood the joist spacing should not matter so you can sister the joists and save some work. Don't forget the joist hangers.

    You still haven't told us the size and type of the tile.

    As for supporting the fixture loads they should be dealt with independent of the tile stiffness issue. The most important issue is not the dead load but the live load of the added water in the tub so sister a joist or two under the tub if it is a large one. Be sure to fill the tub when you caulk the rim.

  • renovator8
    12 years ago
    last modified: 9 years ago

    Enduring, to answer your question about installing bridging ... if you really think it will help, solid blocking is easier to install as a retrofit than bridging and a 1x3 strap across the bottoms of the joists would be even easier to install so I can think of no reason to buy a palm nailer unless you are looking for an excuse to own one.

  • sierraeast
    12 years ago
    last modified: 9 years ago

    Palm nailers and bridging aren't a good mix as you'll split the bridging material more times than not unless you took the time to pre drill where you fasten.

    Soild blocking at 9" centers might be overkill, but if you go that route, your blocking material wouldn't have to be the same width material as the joists, it could be narrower held flush to the bottom of the joists. You could even consider laying the blocking flat and flush to the bottom of the joists. Your substrates will keep the top of the joists intact.

  • worthy
    12 years ago
    last modified: 9 years ago

    the IRC now only requires bridging/blocking for joists larger than 2x12,

    Since when has Code been other than minimal practice?

    Under the Code section I build under (OBC 9.23.9.3.) cross-bridging, strapping, blocking is not even required for joists; toe-nailing or end-nailing is enough. And, if I'm reading it right, even a gypsum board ceiling is sufficient to meet Code.

    The palm nailer would be easier to use for securing the extra joists, though not worth the cost unless one had other uses for it.

  • renovator8
    12 years ago
    last modified: 9 years ago

    Perhaps this will clarify my earlier statements:

    The tile guru John Bridge on his website says "the maximum deflection for tile is L/360, and for natural stone is L/720".

    Since the existing floor system you described has a max deflection of L/844, it should not need to be altered or reinforced unless the existing finish flooring/underlayment is not an adequate substrate (in terms of deflection or as a tile base), or the tub is unusually large, or the wood framing has deteriorated, or the framing connections are poor, or you think John Bridge is wrong to use a simple-minded length/max. deflection rule of thumb, etc.

    But if you add joists to be safe (or to avoid replacing the underlayment), you should not need to worry about maximizing their effectiveness since whatever way you install them will result in a max deflection of about L/1670 which reduces the deflection limit favored by John Bridge from 5/32" to 1/16". Adding blocking/bridging to such an unusually stiff floor would serve no practical purpose especially if the joists have been sistered.

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Alert, Long Post:

    Thanks Renovator, Worthy, Sombreuil_mongrel, Brickeyee, Weedyacres, Sierraeast, and last but not least, Pam.

    Ok, the big picture.

    I have a plan to use slate tile in a Versailles pattern ranging in size form around 6x6" or so, up to maybe 12x16. This is an estimate. This is called the "Small Versailles". The stone varies a bit in thickness and is hefty. It is slate stone from "The Tile Shop". We have a store in the metro area of Des Moines, Iowa. I will be installing this myself.

    My goal is for the floor to tolerate:
    1) the weight and stiffness of this tile & it's underlying compound
    2) fair amount of foot traffic
    3) 5.5' cast iron tub
    4) water for tub
    5) vanity with large cast iron sink
    6) toilet.
    7) I might put tile on the wall and don't know if that extra weight matters.

    Floor as is:
    1) Floor dimensions in finished bathroom is 6.5' x 9.5'.
    1) 2x8 joist at 21"OC, about 10' or 11' in length, and about 9.5 free span. See picture above.
    2) 3/4 (or 1") x8" subfloor. And I correct my earlier mistake, it is NOT T&G.
    3) 3/4" fir T&G flooring but is covered with a layer of linoleum.

    When I am ready to remodel the room I will remove the linoleum. I had planned to lay 3/8" exterior grade plywood as an underlayment down on top of the 3/4" fir flooring, leaving all of my flooring intact.

    I will keep the bath where it is at the far end of the room (you can see the green pipe at the top of the pic). The size, either 5' or 5.5' depends on how the joist position turns out. I will move the sink to the Rt side, where the heating duct comes in. I will shorten that duct span to bring the vent closer to the front of the room (even closer to the top Rt hand corner of the pic). I will move the toilet a few feet towards the far end, about where you can now see the exiting sink plumbing on the left of the picture.

    I had plans to use self leveling compound and maybe imbed an electric floor/tile heating system ontop of the underlayment. This heating system is only for the tiles not to provide primary heat to the room. The room is ducted for forced air heat.

    Next, over the underlayment with self leveling compound (and possible heating system), lay Ditra.

    Then, as I understand it, I am ready to tile as per usual. As stated above I will be using 6x6 up to approx 12"x16" tiles randomly placed in the design.

    Bottom line is that I want a floor that will support my tile and fixtures in the BR.

    With all of the posts above it sounds like I need to do something, or nothing, to the floor joist system (except to get those hangers in there).

    {{gwi:2069942}}

    I am leaning towards adding joist inbetween the existing one in the middle.

    The spaces between the exterior joists, sitting on the sills, and the 1st free spanned joists on either side, might be a problem because of the plumbing, especially on the left where the toilet is (out of sight in this pic). So I might have to leave that area without extra support. Or maybe I could brace around the toilet to stiffen up that area. While the Rt side might be tight, I still might be able to get a joist in there.

    Anyway, that is my current rendition, taking all of the above discussions into consideration. Any feedback on this plan?

    ps, this took me FOREVER to write up!

  • renovator8
    12 years ago
    last modified: 9 years ago

    IMHO, you don't need to reinforce the joists for 6x6 slate tile but if you are going to use unusually large pieces of slate or run a hand truck across the room, you should add some joists. But, because the total floor sheathing and underlayment will be 2" thick, it doesn't matter where the joists are located and they don't need to be blocked other than at the toilet opening, under walls, and any other potential weak point in the flooring or point load condition and for that a 2x4 would suffice.

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Well Renovator, interesting you should mention hand truck. It is not unheard of to put something in that room that is heavy to get it out of the way to move something else through the house, like our range while we remodeled our kitchen.

    So, since there will be 12x16 tiles on this floor it would make sense to add joist.

    And it sounds as if you are suggesting to add blocks to the joist if there is a wall in the middle of the floor area. For example, like a stud wall in the middle of the room to support a shower wall. I don't have any walls except the 2 exterior walls and 2 interior walls that define the limits of the room. Its a simple rectangle room.

    I think I will add the extra joist as I mentioned earlier, between the center joist. I will block around the toilet. I will block under the tub.

    If other thoughts come to anyone, I am all ears.

    Thanks.

  • sierraeast
    12 years ago
    last modified: 9 years ago

    Check and make sure all your plumbing connects in the area are sane & sanitary. Nothing like a leak ruining substrates from the underside!

  • enduring
    Original Author
    12 years ago
    last modified: 9 years ago

    Thanks Sierraeast, I got the plumber on top of this issue. We are not doing any of the plumbing DIY. There are repairs to be made and we are relocating the stool and sink plumbing.

  • worthy
    12 years ago
    last modified: 9 years ago

    This has got to be the most overthought 61.75 sf at GW in a long time. The floor will be so solid with any of these suggestions that the rest of your home will feel like it's crumbling away under your every step!

  • brickeyee
    12 years ago
    last modified: 9 years ago

    "I want to tile the floor of this 6.5'x9.5' bathroom."

    "This has got to be the most overthought 61.75 sf at GW in a long time. "

    Installing tile on an already inadequate floor is an invitation to crack grout and tile.

    There are a number of ways to ring the floor up to the stiffness needed for tile, along with making the sub-floor stiff enough between the joists.

    It is a matter of cost and effort.

    A basic tile floor needs a solid inch of sub-floor with 16 inch joists at 1/360 deflection for mosaic tile.
    Larger tiles push the joist deflection to 1/440, and even larger tiles (or stone) can push the joists to 1/720.

    The subfloor may also need to be increased to ensure that deflection between the joists remains under control.

    The while floor must work together (joist deflection and sub-floor deflection between joists) to make a tile floor last.

    Since there is more than one way to arrive at the solution, cost and time considerations can come into play.

    Simply sistering the joists may leave enough deflection between the joists.

  • enduring
    Original Author
    11 years ago
    last modified: 9 years ago

    Follow up with the floor. I want to thank you all for the help with this floor. I took a while to get to this point, but believe me, I poured & poured over this thread! All of the input was so helpful. Thank you!!

    I have several more questions but you'll see those at the end of this post. First I'd like to update the progress.

    We got the joist in about a month ago. First we hung the existing joist at the near end of the room to the vertical plate. These originaly were toenailed into the plate. When we got to the additional joists they were hung on the near end but placed on the ledge at the far end.

    The joists where attached to the subfloor and the fir T&G flooring with 3" screws from the topside. The 2 existing flooring layers added up to 1.5". To do this the joists were located, measured and a line was snapped topside and pre drilled every 8" (through the flooring materials but not into the joist). Then the 3" screws were put in.

    A 3/8" exterior grade A-C plywood underlayment was installed on the top with 4" oc screw down throughout, into the fir T&G only.

    The joists on this finished floor are around 10" oc and the JB deflectolator calculates the new floor deflection at:

    For joists that are SYP or Douglas Fir, in good condition, 7.25 inches tall, 1.5 inches wide, 10 inches on center, and 9.5 feet long between supports, the deflection calculated is 0.131 inches.

    This translates to a deflection of L / 871.

    Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile or Natural stone, Congratulations!

    Here is my son installing the joist hangers on the existing joist:
    {{gwi:2060086}}

    Then a month later my DH is installing the extra joists (I think :)
    {{gwi:2060087}}

    First one up:
    {{gwi:2060088}}

    More progress:
    {{gwi:2069944}}

    Done:
    {{gwi:1444562}}

    The last area to the far left is without an extra joist. We left it open to allow the plumber better access. When he gets done we will either fit another joist parallel to the existing joists. Or, add short bridging blocks every 24" down the length to stiffen that area.

    I owe a lot of thanks and respect to this guy, my DH, shown here cheesing it up:
    {{gwi:2069945}}

    My new questions are:

    1) Do we need to add the last joist to the far left in the picture? If we don't add something this space will have a 21"oc joist spacing. This is the location of the toilet.

    2) If re-enforcement is needed in this area (#1) than which is best, bridging every 24", or put another joist in place if we can?

    3) We want to put blocks between each of the joist at the far end- is this overkill for this floor since the joist are tied in to the subfloor?

  • enduring
    Original Author
    11 years ago
    last modified: 9 years ago

    ignore the above questions...

    ...we've added the last joist, did not add any bridging, screwed the new joist to the subfloor from the topside...

    ...where done under there!

Sponsored
MAC Design + Build
Average rating: 4.3 out of 5 stars18 Reviews
Loudon County Full-Service Design/Build Firm & Kitchen Remodeler