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