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We had old 2x4 floor joists sitting in our bathroom that we ripped out as they were rotten and falling apart.

2x4 is ideally our limit of thickness, and I was thinking of having 4 or 5 2x4 joists going from one end of the wall to another (147cm / 58") bolted to a 2x4or 2x6 (not sure which one) bearer against all 4 walls. The bearers will be thunderbolted to the brick walls and joist hangers will be used for the 2 by 4s.

I have a very poorly drawn picture from myself to illustrate the 2 by 4 joists. We're thinking about putting the joists 30cm (12") OC.

How much weight can this take? Is this adequate for a room that is 7 ft 9 inches?

Based in the UK.

enter image description here

See a video of the floor.

https://streamable.com/2wbrxk

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    Load capacity reduces as length increases.
    – Solar Mike
    Commented Dec 8, 2020 at 13:38
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    Also, what does "thunderbolted" to the wall mean? I'm envisioning using some cables for Apple's favorite connector to hold them to the wall, but I'm pretty certain that's not what you mean. ;)
    – FreeMan
    Commented Dec 8, 2020 at 13:59
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    2x4s aren't appropriate as joists for anything larger than a stairway landing, and even then I'd use 2x6. Why is 2x4 your limit?
    – isherwood
    Commented Dec 8, 2020 at 14:35
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    In your other question (on which i provided an answer) it was suggested that the joists would be resting on the concrete slab. If not then rip 2"x 6"s down (or whatever lumber is required to get your dimensions) so that they rest on the slab. Or put sleepers down on the slab for the joist to sit on. This way the slab is doing the supporting.
    – Alaska Man
    Commented Dec 8, 2020 at 19:52
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    Is it just me or when watching the video is anyone else confused why 2x6s wouldn't fit?
    – DMoore
    Commented Dec 9, 2020 at 21:38

1 Answer 1

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When you ask for the maximum span of a 2x4 floor joist at 12” on center, there are many issues. We design from the top down and build from the bottom up.

  1. Your subfloor must be rated to span from joist to joist. If you use 12” spacing then a 1/2” thick board is acceptable, but at 16” spacing you should use 5/8” plywood and for 24” spacing then use 3/4” thick plywood. This is based on supporting 15 lbs. per square foot of Dead Load and 65 lbs. per square foot of Live Load. (This is higher than normal, but will allow you to use a tile floor without deflecting too much.)

  2. The maximum span for joists depends on species and grade. I’ll assume an average grade of Number 1 (C16). C16 is economical to use and easy to find. So, for 2x4 at 12” oc the maximum span is about 7’-6”, which is about the distance you need. (If you go to C24 it could be longer.) Remember, spans are measured from face of support to face of support.

  3. Joist hangers are also rated. Finding a hanger that will accept a true 2” wide joist will be difficult. Simpson has a hanger and is rated for 600 lbs., which is fine for your application.

  4. Using a perimeter ledger could require a 2x6, because you need 4 times the bolt diameter. A 1/2” bolt would require an edge distance of 2”. A 1/2” bolt will need to be about 12” on center to support your load.

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  • Hey Lee :), thanks for your comment! When you say face of support to face of support, what do you mean? Bit of a noob with terminology. Also, so this is technically achievable with 2 by 6 ledgers, can I have the ledgers on 2 opposite walls and not all 4? Just wondering because it would be hard to get a bearer under where the door is as there’s no much empty space from the bricks being knocked out... so not sure what to do about that. Commented Dec 8, 2020 at 18:49
  • And can this also be without concrete blocks under the ledgers? Or do you recommend I add them? Commented Dec 8, 2020 at 18:50
  • Span length means you can subtract the widths of the ledgers. Yes, 2x6 ledgers are acceptable and they can be on just 2 walls. If adequately bolted then the ledgers do not need concrete blocks.
    – Lee Sam
    Commented Dec 8, 2020 at 19:41
  • 2x4 at 12” oc (!) the maximum span is about 7’-6” - at 16" oc, I'm guessing 4' or less, but either way I wouldn't go any further just on principle.
    – Mazura
    Commented Sep 8, 2021 at 12:07

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