The effect of surfaces on the extent of high pressure vertical and horizontal unignited jets is studiedusing CFD numerical simulations performed with FLACS Hydrogen, and Phoenics. For a constantflow rate release of hydrogen from a 284 bar storage unit through a 8.5 mm orifice located 1 meterfrom the ground, the maximum extent of the flammable cloud is determined as a function of time andcompared to a free vertical hydrogen jet under identical release conditions. The results are comparedto methane numerical simulations and to the predictions of the Birch correlations for the size of theflammable cloud. We find that the maximum extent of the flammable clouds of free jets obtainedusing CFD numerical simulations for both hydrogen and methane are in agreement with the Birchpredictions. For hydrogen horizontal free jets there is strong buoyancy effect observed towards the endof the flammable cloud thus noticeably reducing its centreline extent. For methane horizontal free jetsthis effect is not observed. For methane, the presence of the ground results in a pronounced increase inthe extent of the flammable cloud compared to a free jet. The effects of a surface on vertical jets arealso studied.