Polycarbonate roofing materials

A simple ridged roof consists of declined rafters that rest on vertical wall-plates on top of each wall. The top ends of the rafters meet at the horizontal ridge plate or ridge beam. Horizontal purlins are fixed to the rafters to support the roof covering. Heavier under purlin are used to support longer rafter spans. Tie beams or ceiling joists, are connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength.