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Concrete slab pour on existing upper floors

I'm doing an entire house renovation, and swapping from oil forced hot air, to radiant. Total square footage is around 3200. Part of the house is a barn from around 1900, the other half was built(poorly, lots of unlevel floors) around 1940.

I've done heat loss calcs and have learned as much as I can about what size system more or less I need. My main concern is the weight of a 1.5 inch slab poured on one of the larger rooms. I know the simple answer is to get an engineer in there to poke around and give me what needs to be done. I've been scouring the internet for hints as to how feasible this is and there isn't a lot out there.

Basically I'm trying to find out how reasonable or not, my desire to pour slabs on the existing upper two floors is. I understand that this is a very specific question with quite a lot of variables, but I'm hoping someone can point me in the right direction. My current thought process is that I can fix the level issues 1/4 inch here and there, while simultaneously adding a perfect mass for radiant. Any thoughts or links to similar projects are welcomed. Thank you.


  • hot_rod
    hot_rod Member Posts: 14,785
    I think something like 12- 15 pounds per square foot for a 1-1/2” thin pour.
    Find joist span tables online to check deflection. Excess weight will cause an annoying bounce in the floors, tile would be out of the question.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ewang
    ewang Member Posts: 73
    Ever look into staple up radiant with aluminum plates?  Would provide the same type of heat, maximize ceiling height and would probably be less costly. 
  • ParAvion
    ParAvion Member Posts: 4
    I have looked into staple up. I agree it would be obviously a huge gain in weight savings, the heat wouldn't be as good. Also I am really trying to to a two birds one stone type of deal with the floor un-evenness issue.

    My current thought is to do the pour in all but the one large span room and do staple up if need be. That room is a bedroom.

    Span calculators absolutely help, and I feel silly for having forgotten to research this in depth.

    I'm just surprised with the popularity of radiant, that there aren't more examples of people doing retrofits like this out there.
  • Jamie Hall
    Jamie Hall Member Posts: 15,833
    Check your deflection as well as load capacity. For a concrete pour, deflection under code live load must be less than 1 in 360. Also remember when checking span load capacity (usually they are looking only at moment, not stiffness) to account for the full dead weight of the floor -- including the concrete -- plus whatever code requires for the occupancy.

    If you can't gain access to inspect each joist -- and its hangers or ledgers -- individually, be very very conservative in your assumptions as to what wood was used and what the actual dimensions are.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 6,562
    Concrete weighs 150# per cubic foot. That works out to 18.75 lbs @ 1.5". Very few older buildings can tolerate that much additional dead load. Not only will the joists deflect over time, The extra 60,000 lbs will stress the beams, walls and foundation.
    I would lean toward and underfloor low mass installation.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ParAvion
    ParAvion Member Posts: 4
    I should mention that all the floors throughout, are already removed down to the sub floor. I do have access to to underfloor in most locations but really was leaning towards the slab install for the previously mentioned reasons. I will be able to get accurate measurements of the joists in all locations prior to any decision with which way to proceed.

    With cursory looks towards the span tables, it seems like there should be no real issue with the overall load, except possibly the one larger room. I appreciate all the feedback thus far.
  • PerryHolzman
    PerryHolzman Member Posts: 234
    edited December 2020
    Common house structures would not support a concrete slab on top of the support rafters or the total weight on the sidewalls.

    I think you would need at least 2x6 (and perhaps 2x8) sidewall & Interior walls studs spaced about every 8-12" to even consider it (and that may not be enough). The floor joists would likely need to be at least double 2/10's again on about a 12" center.

    The weight of the concrete would likely collapse your existing house - although it just might stand until a major storm and the wind would deliver the final stray that collapses the house.

    I'd not want a concrete floor to collapse on me and my family.

    I think you better just go with staple up; or is there an above sub-floor similar option.

    Alternately; build a new house that is designed for the weight of concrete floors (and would likely use steel I-beams for floor rafters as are used in industrial buildings that have elevated concrete decks).

  • hot_rod
    hot_rod Member Posts: 14,785
    If it is just floor leveling you can pour gyp down to about a feather edge. The original intent of gypcrete was noise, fire and floor leveling.
    And you could still add radiant below.
    This may get you the leveling you want without too much additional framing load.

    You are a bit limited in floor coverings with gyp, tile for example in wet locations like bathrooms can be a challenge.

    Those gyp pours will find every small opening, so be sure you putty all the cracks, even nail holes. Basis sparkle works fine to seal cracks, most gyp installers have the procedure down.

    Do you have a gyp applicator nearby? May want to price it out, it is a bit of setup work for a small pour, could be cost prohibitive?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • pecmsg
    pecmsg Member Posts: 1,977
    Expose the floor joists, Sister up to level.

    NO need for all that extra weight!
  • ParAvion
    ParAvion Member Posts: 4
    So, I've decided to compromise. On the main floor, which has the most leveling issues I'll do a slab. Having the high mass on the main living space will also be more advantageous than if I had poured one on the second floor, which consists of the bedrooms. On the second floor I'll do staple up. This will avoid the weight issues altogether as this floor has essentially two foundations and no large spans.

    I appreciate the feedback, even though there wasn't any precise information, it was eye opening enough to get me to be cautious with adding weight to the upper floor.

    Any suggestions on what to do in the basement? It is a slab, and I have concerns with radiant on a slab without losing lots of room height due to insulation. I really would like to avoid baseboard/radiators but I understand it may be the best/only option.
  • hot_rod
    hot_rod Member Posts: 14,785
    When you say "a slab" are you thinking a concrete pour or gyp? Concrete needs to be at least 1-1/4" if you use a pea gravel mix. A gyp pour could be a bit less. I'd still suggest you look closely at the span tables and deflection number @Jamie Hall mentioned. I hate a bouncy floor, you will too :)
    Radiant ceilings are a good option when height is an issue.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • hot_rod
    hot_rod Member Posts: 14,785
    I've done a few span deck radiant, that metal sure likes to conduct heat the wrong direction :) And into the metal bar joists that it sits on in some cases. Spray foam$$ is about the only good way to insulate under a metal ribbed floor .

    Structural or engineered lumber is fine, as long as you do the calculations.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • WMno57
    WMno57 Member Posts: 194
    ParAvion said:

    On the main floor, which has the most leveling issues I'll do a slab.

    When concrete is dry it will push down. When it is wet, it will push down and OUT.

    The sills and studs at the bottom of the wall may need additional bracing until the concrete sets.

    I've seen damage to block walls that were not braced to hold back the force of a four inch pour. It doesn't happen every time. Depends on the condition of the wall.
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