The average house has hundreds of boards in the attic which make up the roof framing, but most of them can be categorized into a few basic types. We list below the various framing members, in order of their location within the attic (lowest to highest):
Ceiling joists – these boards are generally 2”x6” boards, up to 2”x12” and are located directly above the drywall ceiling. They will run across the ceiling of each room and be supported on each end by a wall. They are stood up on their thin edge and will usually have some sort of insulation between them. You can walk on the thin edge, but do not step in between them, because the drywall ceiling will NOT support your weight!
Attic beams – these framing elements are used to span longer distances between supporting walls, such as over a large living room. With more open floor plans the use of attic beams has increased. They may be made up of multiple boards nailed together, or several pieces of lumber glued (laminated) together to form large, strong beams. They will be oriented horizontally and may be at the same level at the ceiling joists or elevated just above them. Ceiling joists are commonly attached to attic beams, along with purlin or roof braces.
Rafters – these boards will usually be 2”x6” boards, up to 2”x12” boards and are located directly below the roof decking. They are sloped from the top of the roof down to the lowest outside edge. Like ceiling joists, they will be oriented on their thin edge. The slope of the rafters (or roof pitch) depends on the roof design and will be designated by the vertical rise vs. the horizontal distance, which is always given as 12. So, a 3-inch rise within a 12-inch horizontal distance with be designated as 3/12. Common roof pitches are 6/12 to 9/12, with some very steep roofs being 14/12 or even 18/12. Such roofs look more like walls than roofs! The horizontal distance a rafter can span is governed by the size of the rafter and is referred to as the span of the rafter. Longer spans require intermediate supports called purlins.
Hip and Valley rafters – these are specialized rafters located at the lowest part of a valley or the highest part of a hip, which other rafters are attached. Consider them as the highest or lowest parts of the roof in the corners of a hip roof. Refer to our article on hip and gable roofs to better understand a hip roof.
Purlins – these boards are horizontal and are nailed directly to the underside of the rafters. They should be the same size as the rafters they are supporting. Purlins are installed to shorten the span of the rafter, thereby allowing the roof to be framed with smaller rafters. Purlins need to be supported with braces, which run from the purlins to a support location at the ceiling level of the attic. This location can be on top of a wall, or an attic beam.
Collar braces– these boards are positioned horizontally between rafters in the upper third of the attic and resemble the horizontal, short line of the letter “A”. Collar braces are generally spaced every 4 feet, or every other rafter, and are typically nailed to the rafters. Often, collar braces are 2’x4” boards.
Ridge beams – the piece of wood at the very top of the attic. It should be slightly larger than rafters attached to it. The ridge beam will be oriented horizontally, with the thin edge at the top and bottom. Every pitched roof will have a least one ridge beam and some roofs will have several. But for each section of the roof, the piece of wood at the highest point will be the ridge beam.
Roof deck – the boards installed above the rafters. Roof deck boards on new construction generally consists of wooden plywood sheets or Oriented Strand Board (OSB) sheets. Older construction may have utilized plank board framing, which quite often consisted of tongue and groove 1”x6”, in dimension.
A final point on moving around in your attic – generally speaking, most of the framing described above can support your weight without problems. Be EXTREMELY cautious when stepping between any framing members and NEVER step on insulation without determining what is below it. There will usually be plywood platforms and/or walkways for access to air conditioning equipment or water heaters, if installed in the attic, which you should be able to walk on with confidence if it is properly installed.