Rafter Vs Joist On Flat Roof

Discover the pros and cons to multiple roof designs.

Rafter vs joist on flat roof.

This is our ultimate roof rafter guide for people designing and building an off grid cabin or tiny home. For heavy roofs or in areas with substantial snowfall or winds call your local building department for code requirements. The major functional difference between the two is that trusses are built mostly with 2x4s in place of the wider dimensional boards. The rafters in a flat roof serve as ceiling joists for the space below.

A joist is a horizontal structural member used in framing to span an open space often between beams that subsequently transfer loads to vertical members. The size of rafters for a flat roof depends on the horizontal span of the building the type and grade of timber used and the load on the roof. In traditional timber they are nominally 2 inches thick and typically 8 to 10 inches wide. Rafters form the foundation for attachment of the roof boards onto which the final roof covering is fixed.

Maximum rafter spans for a patio roof in areas with mild climates patio roofs are generally designed for loads of 30 psf pounds per square foot. Trusses and rafters have many common parts including the sloping rafter boards and a bottom joists that form the ceiling of the space below. Ceiling joists can be on both low sloped roofs and high sloped roofs and help to prevent racking and spreading of your rafters or walls which could lead to sagging. They are nailed securely to exterior wall plates and to each other.

Similarly ceiling joists are the planks of wood that create the flat surface of your attic floor or top story ceiling. Rafters are the building components that run diagonally from the wall plate to the apex of a roof structure.