I find it difficult to imagine a situation where it would be necessary to make a Tzuras haPesach against a building and not rely on the wall behind it. Apparently you are referring to a situation when part of the Tzuras ha'Pesach is against the building, but it also includes a passageway under the Tzuras ha'Pesach, for instance when the Tzuras ha'Pesach is made in order to contain an opening that is greater than 20 high or 10 wide or Parutz Merubah, but the Tzuras ha'Pesach includes more than just the opening and is resting on the wall in which the breach exists. Alternatively, the Tzuras ha'Pesach begins against a wall, and extends out past the wall such that its outer post is away from the wall.