Flat Roof Rafts And Joists

Roofs come in various sizes and shapes such as shed hip flat gable gambrel to name a few.

Flat roof rafts and joists.

Joist is a term used in construction for load bearing planks that run horizontally or nearly to horizontally to ground. Boarding to be fastened on top of flat roof joists with a depth to width ratio greater than 4 the allowable clear spans have been calculated in accordance with bs 5268 2 2002 structural use of timber part 2. Code of practice for permissible stress design materials and workmanship and bs 5268 7 2 1989 structural use of timber section 7 2. Span tables for flat roof joists to bs 5268 7 2 imposed loads of 0 75 kn m for maintenance and snow load are applicable where there is no permanent access no fixed ladder or staircase and most areas where the altitude does not exceed 100 metres refer to bs 6399 3.

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. Overwhelmingly popular roof rafters shown on the left image above were used in most houses in the past. Maximum clear span of flat roof joists in metres for sc3 timber spacing between joists size of roof joist 400mm 450mm 600mm 47 x 97 1 84 1 81 1 74 47 x 120 2 43 2 39 2 27 47 x 145 3 08 3 02 2 87 47 x 170 3 75 3 67 3 42. Ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice.

In traditional timber they are nominally 2 inches thick and typically 8 to 10 inches wide. The rafters in a flat roof serve as ceiling joists for the space below. However with advanced technology new construction techniques have started using trusses shown on the right image above. Rafters form the foundation for attachment of the roof boards onto which the final roof covering is fixed.

If your project had a rafter length of 8 or less a safe and strong method is to use 2x8 framed 16 on center. Also available for the android os. Rafters are the building components that run diagonally from the wall plate to the apex of a roof structure. It is always better to have a large rafter rather than be concerned with a sagging roof.

Floor joist span use this table to determine the maximum lengths of floor joists based on species of lumber joist spacing and joist size. If you have a low sloped roof or flat roof of less than 2 in 12 these evenly spaced planks will be called roofing joists. Roof rafter span use this table to determine the maximum lengths of roof rafters based on species and grade of lumber spacing dimensions load and slope.