Suspended Garage Slabs

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I have seen maybe 6 or 8 of these in the last 25 years. They come in many styles like solid reinforced concrete slabs, precast voided concrete panels, concrete on steel decking supported by steel beams are a few I have seen. Besides the obvious point, they need to be engineered there are a few issues inspectors need to consider. They all have a weak point which is water and chlorides entering through concrete pores, cracks, and poorly sealed joints. Another issue I have is these suspended slabs are no different than a highway bridge slab and should be inspected every two years and should have a load rating posted IMO. If you inspect a house with a suspended slab I would recommend the following:

  • Ask for engineered plans or recent inspection report from a structural engineer.
  • Looked for cracks and water intrusion under the slab.
  • Check section loss in visible steel beams, steel panels, and any support columns.
  • Look for floor slab deflection or low areas that hold water, snowmelt, etc.
  • Remind homeowners or buyers these slabs were designed with a specific load or vehicle. It’s important to know what the maximum load rating is and IMO this should be clearly posted.
  • Reinforced steel and prestressing strands in concrete slabs and precast voided concrete panels are hidden. Typically rusted rebar or strands will show up as excessive deflection under normal design loads, but could be sudden collapse if overloaded.
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SPANCRETE is very popular in my area: https://www.spancrete.com/why-spancrete

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Just to clarify Randy, a suspended slab is one with a significant portion not continuously supported by a substrate, or supported only at the perimeter?

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Thanks, Randy!

I found a lot of DOX Plank:

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Kenton, IMO a suspended garage slab is any slab designed to have a void under the slab. Most of the time the void is a habitable room. To me if its a clear span or supported on beams and columns its still a suspended slab verses a standard garage slab on grade.

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Larry, those DOX planks have allot of joints that need sealed, which would be a weak link for me.

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Yes, another SE gave me that advice early on…thanx.

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Jeez-loweez, Larry! Man, I’m hoping you can explain to me in simple terms how to use this information in inspection, because once again detailed engineering description has parted my hair.

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I used it as Randy posted in post #1. (That was the 1st DOX Plank pic that I found on Google.)

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Are you showing your new do? LOL!

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Your explanation passed low over my head! :joy:

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Kenton, suspended garage slabs from a design and inspection perspective are no different than a highway bridge. I have designed , inspected and built bridges over my 30 year structural engineering career. Like in my original post regardless of the type just look for water and chlorides entering through cracks an joints. I have inspected several thousand bridges in my career and 95% of the damage is caused by water and chlorides.

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Randy … I think your comments were right on. In KC, I see these probably 1-2 times a month.

The room under the double car garage floor slab used for extra storage, the safe room, the storm shelter, etc. I have yet to see engineered plans or an inspection report from a structural engineer NOR maximum load rating clearly posted …although both of those would be great.

The issues I see most often IF there is an issue are … .cracks or water intrusion at the slab; deflection in the I-beam; or floor slab deflection or low areas that hold water, snowmelt, etc.