Single storey industrial plant, civil building and shelters from rain in stadium and gymnasium both have roof system. They are spatial structures. This paper intends to calculate the internal load of a simple truss when external load is applied between truss nodes. Quadrangular pyramids supporting system was also discussed. For the roof trusses, if the node is rigid connection, they can not be simplified as hinges connection. If the node of quadrangular pyramids is rigid connection, the author of this paper speculated that if the supporting bar does not occur bending deflection, the calculation that simplified the rigid connection as hinge connection is approximately right which needs future experimental validation. The buckling analysis of the top chord for roof trusses and the bars in quadrangular pyramids supporting system are both important.