Building structure achieved of pre-cast monolithic walls and interfloor slabs, ∘ wherein the interfloor slabs comprise an expanded polystyrene block cut by means of CNC machine with a red wire to form a shaped styrofoam element which has three longitudinal channels and three transversal channels, and at the ends there is the very same transversal channel only split in two parts, ∘ wherein the longitudinal channels are conical at their lower part next to the bottom of the channel with two first lower opposite sides positioned to form an acute angle of 65° in relation to the bottom of the longitudinal channel, ∘ wherein the central part of the longitudinal channel has two vertical opposite sides from which are continued two first upper opposite sides sloped to the longitudinal channel to the top elevation and define a large surface and a smaller surface between the longitudinal channels and the transversal channels, ∘ wherein the transversal channels cross the longitudinal channels and have a bottom the height of which equals to the height of the bottom of the longitudinal channel, ∘ wherein the transversal channels between longitudinal channels are also conical at their lower part next to the bottom thereof with second lower opposite sides being defined under an acute angle of 65° in relation to the bottom of the transversal channel, ∘ wherein the central part of the transversal channel has two vertical opposite sides from which are continued two second upper opposite sides sloped to the transversal channel up to the top elevation and to the large surface and to the smaller surface between the longitudinal channels and the transversal channels, ∘ wherein the transversal channels at their endings have thin and 5 cm high vertical walls (14,15) that prevent concrete to leak out at the first stage of filling the shaped styrofoam element with concrete, ∘ wherein the shaped styrofoam element has a completely bottom flat surface and a minimal thickness of 5 cm at the thinnest part where the longitudinal channels and transversal channels are located and defines in the direction of longitudinal channels at the external sides of the shaped styrofoam element, vertical areas positioned between transversal channels. ∘ wherein the interfloor slabs comprise an expanded polystyrene block cut by means of CNC machine with a red wire to form a shaped styrofoam element which has three longitudinal channels and three transversal channels, and at the ends there is the very same transversal channel only split in two parts, ∘ wherein the longitudinal channels are conical at their lower part next to the bottom of the channel with two first lower opposite sides positioned to form an acute angle of 65° in relation to the bottom of the longitudinal channel, ∘ wherein the central part of the longitudinal channel has two vertical opposite sides from which are continued two first upper opposite sides sloped to the longitudinal channel to the top elevation and define a large surface and a smaller surface between the longitudinal channels and the transversal channels, ∘ wherein the transversal channels cross the longitudinal channels and have a bottom the height of which equals to the height of the bottom of the longitudinal channel, ∘ wherein the transversal channels between longitudinal channels are also conical at their lower part next to the bottom thereof with second lower opposite sides being defined under an acute angle of 65° in relation to the bottom of the transversal channel, ∘ wherein the central part of the transversal channel has two vertical opposite sides from which are continued two second upper opposite sides sloped to the transversal channel up to the top elevation and to the large surface and to the smaller surface between the longitudinal channels and the transversal channels, ∘ wherein the transversal channels at their endings have thin and 5 cm high vertical walls (14,15) that prevent concrete to leak out at the first stage of filling the shaped styrofoam element with concrete, ∘ wherein the shaped styrofoam element has a completely bottom flat surface and a minimal thickness of 5 cm at the thinnest part where the longitudinal channels and transversal channels are located and defines in the direction of longitudinal channels at the external sides of the shaped styrofoam element, vertical areas positioned between transversal channels.