Upon intricate work, it is best to use the mathematical formulas, as well as the steel square, to insure accuracy, as one proves the other. The length of any straight rafter, of any pitch, anywhere in the roof, may be found by applying the above method, though in certain instances there are better methods of finding it. will give S these figures are rarely used in marking angles, as their principal use is in ascertaining the length of the rafters, though they are frequently used when irregular roofs are being framed. If the square is laid upon the rafter with A on the tongue, and R on the blade, To. The length of any rafter may be found upon the steel square thus:. The length of any kind of rafter of any pitch may be found mathematically by using the following formula : T = pitch line of ½ of the thickness of the ridge.Ĭ = constant of rise of roof - 6, 8, or 12. The following is a key to the formulas for finding the length of a common rafter:. If there is a lookout upon the rafter, the stick must be long enough for it to be added, though upon an open cornice roof the lookouts are usually sawed to some design, and nailed upon the sides of the rafters. If a ridge is used, one half of its thickness must be taken off parallel with the plumb cut at the top end of the rafter, as will be described later in this topic. (A.) The length of a common rafter is not from the apex of the roof to the eaves, but from the apex of the roof to a point directly over the plate, upon the top edge of the rafter, as in the figure. Because of this we put a 50mm square block in between the common rafters when building the hipped roof above.The Common Rafter. Let's assume the ridge and all roof members are 50mm thick, so 50mm would be removed from the length of the span before calculating the common rafter lengths. So to use the same method for common rafters on each side of this hipped roof, the center of the above roof would need a square block in between all the rafters for the common rafters to be fixed to. That means each pair of common rafters is too short by half the thickness of the ridge. You'll know from the page about calculating common rafter lengths that we subtract the thickness of the ridge from the span so that the ridge will fit in between them. Every common rafter is the same length (brown) and each hip rafter is the same length too (red).Īlso, the measurement shown above in green from the corner of the roof to the center of each common rafter is equal for each as well - 1 metre. The green arrows above show every side is equal, we'll assume 2 metres for this example. The hips are in red, common rafters yellow and for hips to work on a square roof like above each member is equal. If the wall plates are all square/of equal lengths then the hipped rafters would form a pyramid shape like the picture above (normally a roof is rectangle and there are more yellow common rafters). Hip rafters are the diagonal rafters that span from the ridge at the top down to the corners of the roof. I'll show more detailed pictures of each section as we go, but above is a basic picture of a hip rafter roof from above. If you know how, it's possible to cut all or most of the roof members safely on the deck before taking them up and installing them all together. I'll start at the beginning with the easy bits before tackling the more complicated stuff and that way hopefully I'll make it really clear and easy to understand exactly how to set out, measure, cut and install hip and valley roof rafters. Hipped roof construction can seem quite complicated to grasp at first but i'm going to break it down and explain each area in detail.
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