![]() Common rafters transfer their loads directly to the wall plate but also can shed some of this across the roof via the network of laths that held the tiles / thatch in place. Very early (1400's) box framed common rafter roofs (sans purlin) still had principal rafters at the main cross frames and this was mainly recognition that the cross frame were also acting as wall dividers that would be infilled with wattle & daub and so needed to be reasonably substantial to secure and stabilise same. There is no doubt in my mind that you are a thinker and if not already an engineer then definately a candidate to be one !ġ) Not all principal & common rafter roofs (mainly box framed) have purlins. ![]() Primarily I am attempting to understand common rafter systems a bit better and how modern prefab trusses truly differ from their predecessors. The common rafter is where I get a little confused: do they also transfer their load directly to the top plate unless a purlin plate it used? How would they ever be strong enough without a purlin plate or internal truss elements such as in modern trusses? Common purlins transfer their load to principle rafters where the secondary members are added to resist the necessary forces. Modern trusses, then, are essentially 'common rafters' but each and every rafter is beefed up by its own devices to transfer directly to its resting spot on the top plate. What they don't do is transfer the load anywhere besides their feet (which ends up on the top plate). A quote regarding modern fabricated trusses: "The triangles formed by the beams, bars and ties allow the truss to distribute the weight it carries over a broader area." The way I see it is that these trusses reduce or eliminate the spreading force and stiffen the rafter beam itself (reduce deflection). (would it only spread from the load above its attachment to the rafters?) I get how bending stress on the rafter beams would create compression in the collar tie, but there would also be (if left unchecked) the force acting to spread the feet of the rafters, and in turn the collar tie as well, no?ģ) Last question and here is my attempt to link it to other building styles. collar tie, posts, struts) and the roof load can simply be transferred to those principle rafters via the purlins.Ģ) How is it that a collar tie is in compression (if its the only member)? It seems to me that if a triangle is apt to spread at its feet with load, it is equally likely to spread all the way up to its apex unless some other force prevents it. Do these common rafter transfer their load directly to the top plate if not for a purlin plate? Would these intermediate rafters ever have their own secondary members (collar tie/queen post, etc), or do they really have to rely more on something like a purlin plate to transfer the load to bigger timbers? Common purlins make sense to me because each bent can provide the needed resistive forces (i.e. So here are a few questions:ġ) If going the route of common rafter, it appears to me that there are still 'principle' rafters, but that the other intermediate rafters do not necessarily transfer their load to those principle rafters. ![]() I am currently trying to wrap my head around the various roofing systems (common rafter, common purlin, modern truss fabs) and how/why they all work or don't. My question pertains not though to milling but to the structural design of roofing systems. Currently I am looking into timber frame construction in books and on-line resources, hence being here. I am cutting a bit of hemlock and planning a potential future project via the chainsaw mill.
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