The expansion rates of a pyritiferous Irish mudstone–siltstone fill material have been measured over a period of 19 months in an apparatus devised to replicate underfloor conditions. The testing, performed in a temperature-controlled environment, has shown that both fill density and depth submerged in water have significant influences on the progress of the expansion. It appears that whereas thermal expansion or contraction has an effect on expansion rates immediately after a temperature change, there is no long-term temperature effect on the rate of expansion. In addition, an examination of chemical test results for 60 houses in a housing development in the Dublin area has confirmed that pyrite content is the dominant control over the degree of expansion. A molecular–molar analysis of the pyrite chemical process equations, in addition to a knowledge of the original pyrite content and rate of oxidation, has been used to give a lower-bound estimate of the amount of heave in the laboratory experiments.