Hydro-thermo-mechanical tests on joints in the Carnmenellis granite from Cornwall, southwest England, were carried out using a geothermal rock test facility. Both natural joints and artificial fractures were tested at effective normal stresses up to 40 MPa, differential pore pressures to 6 MPa and temperatures to 200°C. Experimental effective normal stress-joint closure and effective normal stress-joint permeability data were fitted by a range of deformation and hydraulic models. Difficulties were encountered in finding single values of material and geometrical constants that applied to the complete range of data and samples. The joint condition factor (JCF) is introduced as a factor to account for deviation from the ideal conditions assumed in the smooth parallel plate theory. The experimental results show that the value of JCF depends on a combination of joint surface geometrical characteristics. Initial apertures increased with a rise in sample temperature depending on the joint surface geometrical properties. An empirical logarithmic relation between initial aperture, temperature and joint roughness coefficient (JRC) is proposed.