To repair or reinforce a roof joist, it is common practice to mount a board of equal width and thickness for part or all of its length. It is also used to extend the existing roof area by cantilevering the sister joists over the end beam or to connect the new deck construction with the old one. The term “sister joists” is the practice of superimposing two joists a certain distance along a beam to create an extended joist. This is unusual in the construction of new boards and would only be necessary for very long spans that exceed the length of the longer boards.
Sister joists are commonly used when adding to an existing platform or repairing a platform. In these cases, the overlapping joists help to join the frame by combining the rigid members onto a beam. The greater the overlap distance, the stronger the connection. It must protrude at least a couple of feet on each side of the beam.
Joists side by side should be joined by a nailing pattern similar to combining boards to form a beam. If you are adding to a platform, you must ensure that the shoes are large enough to support the added loads. Balconies are defined as small roofs built without support posts, usually built with projected joists that are repositioned in the house. This type of roof uses sister joists to project a cantilever effect.
A structural engineer should be consulted every time an unsupported balcony is constructed to ensure structural integrity of the design. If only a small section of the joist is bad, say, no more than a quarter, repair is quite simple using a method called twinning. You don't need to hold the severed joist while repairing it, unless there is additional weight on the platform. Despite some differences between floor joists and roof joists, there is no building code for sister roof joists, as this procedure is a strengthening of a current frame structure, while building coding involves directives for design and construction standards.
A traditional platform has cemented posts below ground level, has a base, which makes the platform a permanent structure, and is subject to local building codes, as are frames, joists, and sister joists within homes.