Diamond sheets were grown on p-type Si(100) substrates varying the deposition pressure from 20 to 140 Torr at 1163 K by a hot filament chemical vapour deposition (HFCVD) process. A mixture of 0.8% methane in balance hydrogen was used as a precursor gas. Diamond sheets deposited at low pressures are translucent with very low concentration of hydrogen and non-diamond impurities, The amount of non-diamond impurities and the hydrogen content increase in the sheets significantly with the growth pressure. A systematic variation in the value of the room temperature DC electrical conductivity (sigma(300)) of diamond sheets was observed. Highly pure sheets show very high values of sigma(300). It is suggested that the paramagnetic defects intrinsic to CVD diamond might be one of the controlling parameters of sigma(300) Of the sheets.