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Howard, judging by the picture, there was an explosion, as the whole window, being the weaker part of the structure, blew out, and also the attic roof was blown through, whereas the bricks below the window are completely intact. An explosion of "air" always sends pressure outwards and upwards, and will always break the weakest part of the structure in order to expand, which can result in the stronger parts remaing intact or only partially damaged. Hoping sincerely that no-one was injured, I can say that the damage is relatively restricted, as the weaker parts ie window and roof tiles, took the brunt and gave way.
Air explosions, such as gas, do not carry with them any components as a shell would, hence the absense of greater damage. obviously, we may assume that the windows were closed, so the gas could not escape slowly, and the preassure built up. very fortunate that the explosion did not take part on the lower floor, as this could have caused the house to receive much worse damage. As it is, the explosion found at least half of its outlet in the attic tiles!
