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Community and Q&A

Insulated Raft Slab with Interior Floor Trusses for Plumbing Chase

Amy Link | Posted in General Questions on

Hi there, I am considering a raft slab build (high water table/seismic area) but am not crazy about the whole plumbing under slab idea…is it possible to use floor trusses to allow plumbing chases? I have not priced it out, certainly see I am doubling the floor materials. Does this make sense?
Thank you for your thoughts.

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  1. Expert Member
    Malcolm Taylor | | #1


    Yes you could, although before you go that far I'd look at the alternatives.

    Assuming that you concern is with waterlines (drains are all located underground anyway), you could:

    - Sleeve all the lines. Running them in 3" PVC with sweeps protects them, but also allows for easy replacement if necessary.

    - Only run your main supply into the house underground, then run all another waterlines though interior walls, floors, or close to the ceiling under your insulation in the attic.

    Whenever someone suggests adding a service cavity, I always suggest drawing out what will actually be run in it and where. Too often they end up with very little in them.

  2. Amy Link | | #2

    Thank you, Malcolm...I have heard of the sleeve approach, and will definitely request that detail. I plan to do all wall runs for water inside, with the closest bth/kitchen in copper, and the one long run to a guest bath/laundry area in pex. I may do a secondary line from the well as well :) for an easy switch. It is the underground drain lines I am wondering about mostly...can I double pipe them, and/or trench them outside the pad footprint? to bring them to the septic? Both toilets are very close to exterior walls. Can I combine drains for more simple runs?
    Thank you!

    1. Expert Member
      Malcolm Taylor | | #3


      It depends on your code. Ours only allows one drain to exit the house. Buried drains under slabs are used on the overwhelming majority of both commercial and residential construction, including in high seismic areas.

      Drains all get combined as soon as possible after they exit the fixture they serve. They should be sleeved as they pass through the slab, but can't be sleeved along their entire run. With a bit of care you won't have many under your slab. To be honest it's not something I''ve ever heard of anyone worrying about.

      1. Amy Link | | #4

        Well then, that's no doubt a good thing? Thank you again, Malcolm.

        1. Expert Member
          Malcolm Taylor | | #5


      2. Trevor Lambert | | #10

        What happens if they aren't sleeved? They aren't in my house.

  3. Expert Member
    Michael Maines | | #6

    Amy, I know several architects who have done just that: Elizabeth DiSalvo, Steve Baczek and Nathaniel May. Elizabeth and Steve discussed their approaches on an early episode of the BS + Beer Show: (Nat May contributed in the chat box but that doesn't make it to the YouTube version.) In contrast to Malcolm, many--I'd say most--of my clients for whom we consider slab construction are concerned about future access, and many decide to go with a crawl space or a full basement largely because I can't convince them that pipes breaking under a slab is very rare.

    1. Expert Member
      Malcolm Taylor | | #7


      Perhaps the difference out here is that everyone is exposed to that type of construction in the institutional and commercial buildings they use daily. A trip to Costco or other Big Box store, their school or office, will all likely yield visible plumbing descending under slabs, so it isn't seen as a variant from normal practice.

      I remember seeing the drawing posted of the truss-joists immediately over the slab. It avoids having t0 cut the concrete to make changes or repairs, but it doesn't give access the way a crawlspace does, as you still have to demolish your wood floor system to get at the pipes.

      There is definitely a downside in terms or accessibility and ease of making changes using a slab as compared to exposed plumbing in basements or crawlspaces, but I just don't hear the concerns your clients raise about possible breakages.

      1. Expert Member
        Michael Maines | | #8

        Malcolm, it's not a well-founded concern, and I agree that you would still have to go through some effort with the design proposed if there was a leak. We have plenty of big-box stores here with slab floors, and a growing number of homes, including many high-performance homes, built on slabs. But most New Englanders seem to expect a basement or at least a conditioned crawl space. It's always a relief when I don't have to try to talk clients out of those spaces, because I have not had a lot of success in doing so.

        1. Expert Member
          Malcolm Taylor | | #13


          Another advantage of building out here in La la-land on the West Coast.

  4. Amy Link | | #9

    Interesting, both Michael and Malcolm. I admit, I am trying to solve all problems that may arise...which in some respects is an impossible feat. At this point I am considering running the drainpipes outside of the slab I am also going to do a curtain drain, what do you think of the audacious idea of joining those two acts?
    Thank you for responding to my thoughts.

    1. Expert Member
      Malcolm Taylor | | #11


      The first thing to do is check if it's even a possibility under your plumbing code.

      1. Amy Link | | #12

        Yes, it would require 2 separate exit drains, which is not always supported by code, I have just heard. Will look into that.
        Thank you for the reminder.

  5. Mark Nagel | | #14

    I've gone back and forth on this very topic (for a similar proposed build).

    At this point in time I am leaning (back) toward running my potable water lines under-slab. BUT! Only SOME. And those that will run under slab will be chased into the (under-slab) insulation (between layers) to help maintain respective temps. These will be my main trunk runs which will come up to manifolds to then feed the final leg to each target device (most everything will be straight runs down a wall- no weaving walls, just going through a handful of studs at the most). The manifolds will be accessible via wall panels (I'm thinking one will end up in a closet; another under a bathroom sink).

    My design has a couple of chases, one being for ERV intake and exhaust and will be mostly contained within my mechanical room, and the other one being the main feeder down the [near] center of my building. A dropped ceiling will provide for branching out from the chase: run ERV ducting to target areas; run electrical wiring and ceiling lights; and running my PEX lines for dropping down walls to panel radiators.

    I was going to run the potable water lines UP in the main chase but am backtracking because I feel that it's better to go more directly, to have the shortest amount of piping as necessary for potable water. My jitters of running stuff under-slab can be calmed to a degree by running those lines n PE tubing; the thought being that I'd be able to replace the working line if needed (the reality may prove otherwise; but, it's most likely that I'd never have to find out- PE is pretty cheap, and you need something to protect the piping as it goes through the concrete anyway- the PE will provide for that).

    I was going to run potable water lines UP in chases that will be for feeding electrical wiring and ERV tubes. In addition to a main chase running most of the length of the building I'll be utilizing a dropped ceiling (which will provide lateral branching). I haven't decided where I'll run my PEX lines for my radiant panel heating: if I run in the chase and dropped ceiling I'll have more flexibility on where I can locate the panels; running mostly UP will reduce energy losses, but lengthen run lengths.

    As far as drains and waste water goes, keep in mind that you have to have proper slope. Any significant total length can add up in drop, which might require having a deeper chase than you might realize. At 1/4" drop per foot of run (assuming 1/4" - code varies, some is 1/8", so be sure to check!) if over 20' would mean you'd have to have 5" of space just for the drop- you'll also have to factor in the OD of the pipe (if 4" pipe then that would be 9"). NOTE: this is just rough info; code and a real professional (or professional source) ought to be consulted to be sure- you don't want to have a HUGE inspection fail. It's all a scary thought to think of burying stuff under concrete, but if done correctly there's little reason to think twice about it: yes, it's pretty much not changeable after the build, which is why you really want to be sure of your floor plan/layout and know that these things won't be part of any meaningful future renovation project (w/o huge costs that is).

    All said, I think that it's an interesting idea (one that I hadn't thought of or run across!). Think about thermal issues, the building as well as the water that would be going through those various pipes. I'd also consider how that floor height would impact building ingress/egress: another step needed?

  6. Amy Link | | #15

    Such a well thought out response, Mark! Thank you! I am having a powwow with my plumber this weekend, before I finalize my plans, and you have greatly helped with your thoughts. My floor plan is solid, as it is a smaller build....I DO plan to stub in an upstairs drain, for the next lucky owners :) As I am contemplating keeping my ceiling open to the joists, I am only planning on wall runs at this point. I am trying to decide on the Zehnder HRV or the Lunos, precisely because of the venting tubes...I believe, I would have to enclose the ceiling downstairs if I went with Zehnder. Best of luck with your build...aren't we fortunate? I love projects, and this one is a Mother (respectfully).

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