The Ontario Building Code | Support of Floors
(1) Material that is susceptible to changes in volume due to variations in moisture content or chemical-microbiological oxidation shall not be used as fill beneath floors-on-ground in a concentration that will damage the building to a degree that would adversely affect its stability or the performance of assemblies separating dissimilar environments.
(2) Material that is susceptible to changes in volume due to freezing shall not be used as fill beneath floors-on-ground that will be subjected to freezing temperatures.
(3) Except as provided in Sentence (4), fill beneath floors-on-ground shall be compacted.
(4) Fill beneath floors-on-ground need not be compacted where the material is clean coarse aggregate containing not more than 10% of material that will pass a 4 mm sieve.
9.16.3. drainage
In Plain Language
This clause governs fill material placed beneath floors-on-ground: it can't be prone to volume changes from moisture or chemical breakdown (in damaging amounts), can't be susceptible to frost-related volume changes if it will freeze, and generally must be compacted, except clean coarse aggregate with limited fines, which is exempt from compaction.
Common Questions
Q: Does all fill under a concrete slab need to be compacted?
Not necessarily. Clean coarse aggregate containing no more than 10% material passing a 4 mm sieve is specifically exempted from the compaction requirement in this clause.
This is a plain-language summary of the 2012/2017 Ontario Building Code text above, for general understanding only and not a substitute for the Code. Always confirm current requirements against the 2024 edition (O.Reg. 163/24) or a qualified professional before construction.