In a scissor truss the lower angled members have to tie in to the higher angle member on the opposite side directly. Snowy’s example one will also fail, but it was the inspiration. Examples 4,5,6 & second/horizontals example are weak, but they’re covered in my second example, you could use that. The shorter the short side of the triangle, the weaker it Oaks examples 2&3 will fail, dangerous. Right angles are very week if not part of a triangle. The strength is in the triangle, whether it’s one or a combination of several. Here ya go, obviously I did this very quickly. If there are no sags in the roof it should work." Some real old ones have no ridge boards at all, remember that wood shingles on nailers common then are a a pretty light roof covering. Many older houses in this area have ridge boards, mostly T&G 1/2" x 6" (sheathing boards), regardless of the roof pitch (pre-date the modern codes). Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams." Hip and valley rafters shall be supported at the ridge by a brace to a bearing partition or be designed to carry and distribute the specific load at that point. At all valleys and hips there shall be a valley or hip rafter not less than 2-inch (51 mm) nominal thickness and not less in depth than the cut end of the rafter. Ridge board shall be at least 1-inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. Rafters shall be framed to ridge board or to each other with a gusset plate as a tie. If no sagging, and the rafters line up opposite each other, should work. "A 3/12 slope is the lowest acceptable in the code for a ridge board. I think it should be a beam, not a board for that little of a slope." "Saw a 3/12 pitch roof today with a ridge board instead of a beam. Here's a couple of photos I found online of some raised ceilings.ĭid a quick search to see if there were any discussions online about roof pitches below 3-12 without a ridge beam. Ridge straps can be used in lieu of collar ties to resist uplift. Unlike ceiling joists, collar ties can be made of less-substantial material (1x4 minimum) and can be spaced up to 4 feet apart. Collar ties, which are designed primarily to resist wind uplift, are required in the upper third of the attic space. Also section R802.4.4 states that a roof with a pitch less than 3:12 requires a structural ridge.Īs a final note, do not confuse rafter ties with collar ties. With a properly engineered structural ridge, rafter ties can be eliminated completely. Section R802.5.2 also states, “Where the ceiling joists are installed above the bottom third of the rafter height, the ridge shall be installed as a beam.” In other words, to raise the ceiling joists more than one-third of the ridge height, a structural ridge would be required. That table provides the number of 16d nails at each connection based on rafter slope, rafter spacing, and snow load. For example, in a roof structure where the ridge height is 9 feet above the top of the support walls, the maximum height that a rafter tie can be raised is 3 feet (3/9 = 1/3).Īdditionally, be sure to adhere to the specific fastening requirements in the 2018 IRC Table R802.5.2 for the rafter-to-rafter-tie (ceiling joist) connections. HC is determined by a simple formula in which that height is a function of the ridge height (HR): The ratio of HC/HR cannot exceed 1/3 (see illustration). Collar ties (designed primarily to resist wind uplift) must be located in the upper third of HR.įigure R802.4.5 of the 2018 IRC states that a rafter tie can be raised a maximum distance of “HC” above the top of rafter support walls. In a roof with a non-structural ridge, rafter ties (which resist the outward thrust of the rafters) can be raised a maximum distance (HC) that is no more than one-third the distance between the top of the supporting wall plates and the top of the ridge (HR).
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