Void forms are used to support rebar and workers as well as form the bottom of a reinforced concrete slab so that concrete can be poured on top of the void forms.  The void forms are not removed but they are designed to effectively create an under-floor space.  There are different types of void forms but the vast majority of them are a degradable type of material such as wax-coated corrugated carton material with an internal cellular configuration so that they are strong enough when they are dry but they weaken by degrading over time (e.g. a few months) when exposed to the moisture in the subgrade.  Typically, a vapor barrier, such as a 15 mil sheet of plastic, is installed on top of the void forms.  

The sequence of construction is different from a slab-on-crawlspace.  Consequently, before the Mudskipper System was invented, there was no way to install plumbing before a slab-on-voidform in a manner that would actually isolate the plumbing.  Historically, plumbing was first buried under the void forms but that led to many cases of damaged plumbing and litigation because the plumbing was not accessible.  In the last decade, some approaches have been tried but they have only created non-isoalated void systems.  Non-isolated void systems create a voidspace but they retain soil and these soil-retention systems are connected to the plumbing after the slab is poured, allowing expansive soil forces to be imparted vertically upward, vertically downward, and laterally onto the plumbing, which can cause damage.  The Mudskipper System does not retain soil, but instead utlizes a two-stage temporary support system which is both compatible with the sequence of void form construction and isolates the plumbing so that the plumbing complies with Section 305.2 of all published editions of the IPC and UPC as well as complies with the new requirements in the 2024 IPC for isolation of plumbing.

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