In the present paper the results of experiments on study of high-speed deflagrations in flat layer of hydrogen-air mixtures unconfined from below are presented. The experiments were performed in two different rectangular channels: small-scale with mixture volume up to 0.4 m3 and large-scale with volume up to 5.5 m3. The main goal of the experiments was to examine the possibility of the layer geometries to maintain high-speed deflagration and detonation. With the aim to study a range of combustion regimes the experiments were performed varying degree of channel obstruction, hydrogen concentration and thickness of the layer. Depending on the experimental conditions all major combustion regimes were observed: slow flame, fast - "choked" flame and steady-state detonation. It was found that minimum layer thickness in the range of 8 to 15 detonation cell widths is required for sustainable detonations.