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One problem faced when finished spaces are built atop concrete slab floors (whether slab-on-grade/slab-as-basement-floor, or a suspended reinforced concrete structural slab floor) is "where do the utilities go?" The traditional solution is to embed them in/below the slab; however, this approach causes serious maintainability challenges if a utility fails (such as a drain pipe rusting out) or becomes obsolete.

Using a drop ceiling to house the utilities appears to be an attractive option, but causes one logistical problem: drainage can't go up without a pump to motivate it. This is a serious issue from a first-cost perspective (one sewage or greywater pump per fixture) and a service/maintenance perspective (broken pump means inoperable fixture and icky service job), never mind the issue of not being able to run any water when the power goes out (or having to provide standby power to the pumps). Running all the utilities in the walls suffers from a similar problem, as well as forcing labryinthine utility routes over doors and around hallways, so it is not an option either.

This leaves the idea of an inverted crawlspace setup, where a set of non-structural floor joists are set atop the structural floor slab (with their bottom plates resting atop the slab), with a non-structural subfloor and finish floor set atop the floor joists, forming a space suitable for utility routing. This provides room for all the utilities a house or apartment needs, as:

  • vents can be run in-floor with the aid of an island vent configuration (which can simply live in the wall behind the fixture, so it doesn't need to be visible),
  • lighting can be provided using surface raceways/boxes for ceiling fixture mounting or simply by using wall sconces instead of ceiling fixtures,
  • HVAC ductwork (including dryer and range venting) can also be run in this floor space provided sufficient room is provided (or simply out the wall for dryer and/or range vents, depending on where the dryer and range are located),
  • fire sprinklering can be handled with sidefire (sidewall) type sprinkler heads,
  • and combustion venting can be handled through direct through-the-wall vents if absolutely needed (say, for a gas fireplace); this works well with sealed-combustion appliances, even, although with modern air sealing practice, keeping combustion appliances out of the conditioned/occupied space is highly recommended.

Given that the joists forming the crawlspace are going to either be wooden floor trusses, steel open web bar joists, or steel/wood composite bar joists, as wooden I-joists are much more limiting when it comes to utility routing due to web boring limits, this all works out nicely, save for one issue: how can one fit access hatches to this space for post-construction use? Note that I want to be able to use normal residential floor finishes to at least some extent, instead of being limited to commercial-style carpet square systems: at the very minimum, tile and floating wood need to be supported by this scheme, while a carpet option would be a nice-to-have as well. Furthermore, one must be able to access the entire space this way, instead of some small portion of it; having a handful of hatches is acceptable, but requiring say, one hatch per joist bay, would be considered excessive here.

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    How high is this crawlspace expected to be? If it's less than a couple feet high it's hard to imagine being able to access much of the space even if you did have one hatch per joist. And if it is high enough to actually crawl down there, won't it eat up too much of the room height? Can we add hatches on the walls of the crawlspace instead, and have purely horizontal access? Then there's no concern for floor coverings, but lots of questions about what spaces abut the room with the false floor (will they all have false floors?). – John Zwinck Dec 25 '19 at 5:26
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    This sounds similar to the false floor used in some computer rooms . instead of beams short posts are used in a grid with a grid of steel beams on top. then square floor sections on op top of that. – Jasen Dec 25 '19 at 5:50
  • @Jasen -- yeah, it's similar to a computer room raised floor, although not identical: computer rooms generally use the underfloor space as an air-handling plenum, which is not something I wish to use it for – ThreePhaseEel Dec 25 '19 at 20:17
  • @JohnZwinck horizontal access isn't workable because this crawlspace runs compartment-boundary to compartment-boundary... – ThreePhaseEel Dec 25 '19 at 20:18

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