Slope: In construction, the slope – or angle – of the roof, is measured as a function of the increase in height (rise) over a 12-foot horizontal distance (run). A ridge board is nothing more than a solid element of wood that runs the length of the peak. Ridge board: In some cases, a ridge board may be required. This is usually only required with slopes of 1:3 (4:12 pitch) or more. Ridge beam: If the roof has a shallow slope, then the load of the roof must be supported at the peak by a beam that runs the length of the peak, and is supported by a system that eventually leads directly to the foundation of the structure, or a similar set of load-bearing elements. Rafter: This is the lumber that runs from the centre (peak) to the supporting wall. The span is NOT the distance between the supporting wall and the peak of the roof measured along the diagonal. Rafter span: this is the distance between the supporting wall and the centre of the roof measured horizontally. (Usually constructed fairly close to the wall - think "Cape Cod" style roofing.) This is often used to reduce the rafter span.ĭwarf wall: a small wall, built with a top and bottom plate (and doubled top plate if the rafters do not land directly over vertical studs) that is built to support rafters partway up the slope. DefinitionsĬollar tie: a section of wood that ties two opposite rafters together. Note that in this drawing, some of the outlined elements may not be necessary they are included purely for explanatory purposes. Illustration of some typical rafter elements. Here then, is some critical information to know about rafters. On the other hand, a carpenter or contractor can build a rafter system, and as long as it’s done right, rafter systems can meet the requirements of the National Building Code of Canada. Or, in simpler terms, a carpenter or contractor can build a truss – but it can’t be deemed Code-compliant. All of the local truss manufacturing companies do this. Here’s the critical difference: in order for a building inspector to certify a truss as Code-compliant, it has to be certified by an engineer as suitable for the snow loads in the area. The two systems have similarities – and differences. I could also argue that the structure in question is `unregulated’, so why the worry? I’ll tell you the worry! … I’m gonna be up there building the thing, and at times I will be sitting, or standing on, a single rafter!!! I’ll snoop around and find some other (independent, and free, online, for kicks and giggles) means of checking the concentrated load condition (other post).Although there are all kinds of styles of roofing styles and subystems (mansard, gambrel, gable, gable dormer, barn dormer), in the end, when it comes to most residential roofs, there are just two kinds of systems: rafters and trusses. It could be argued that the 300-lb load need not be entirely carried by a single rafter. NOTE: it does not appear that the Span Calculator took into account the 300-lb concentrated load, which, we showed earlier, `governs’ the design. 2 are relatively high grades, which is what I (we) expected with the dim lum sections. That’s what I (we) came up with earlier … the Select Structural (SS), No. I’m used to using the `2012 Version’ (the one on the left) …ĭeflection Limit: L/180 … we haven’t talked about deflection yet … I will set it to the least stringent condition … L/180 means that the calculator will select a rafter size/grade that deflects (bends, bows) not more than the length/180 under the design roof load … (L/180 = 12 ft x 12 in./ft / 180 = … in this case … 0.80 in.) I also like the AWC `stuff’ … (use it all the time). I know a bunch of the folks at the AWC, and they are some of my favorite people. The American Wood Council (AWC) has put together a free, online, span calculator. Recall that we (I) came up with 2×6 rough sawn (actual dims) stressed to 900 psi, which is probably doable in SP lumber, and likewise doable with 2×6 dimension lumber, but because the dim lum has less section, it will be stressed to 1428 psi, which is still likely doable, but using a higher `grade’ of lumber.
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