How a purlin and cut-rafter roof works

I want to show you how a traditional cut rafter roof works and these kind of roofs are common in the UK well most of the properties built before trust rafter roofs became common in the last few decades so what we have here is we have outer walls in blue not necessarily going to be a cavity wall as shown here but sometimes it is and we have an internal wall we've got the ceiling here we've got the rafters there and what we can see here in this section is a purlin and a ceiling binder and at the top The Ridge board so what's happening here well the roof is forming a triangle so when we have the weight of the tiles and wind and snow on the roof on the Rafters we have a reaction at the top trying to push the rafters left and right which is resisted at the wall plate by the ceiling joist which goes into tension so we have a triangulation of forces as we would in most roof structures what's unique to this kind of structure is that the rafters are very slim so that what they do is they put a purlin at Mid span to break the span so this rafter effectively is just supported here and here and it's half the span that it would be otherwise without this purling now the purlin is supported on the wall so let's just get some color in here you can see here that the purlin is supported off a continuation of the wall which is load-bearing below we can see that What's Happening Here is in this this type of property the purlin is supported on this wall built off a first floor low bearing wall which is built off a ground floor low bearing wall and the same thing really happens here now with the ceiling joist because this is again a very slim member supported off the load bearing wall here and at the wall plate here and then the span is halved by using a member we call not a purlin this time although it's very similar that's called a ceiling binder and again it's supported on this low-bearing wall so that's basically how a traditional roof works and if the ceiling joists are supported on an eternal wall as they are here then those are nailed together very securely because remember that these ceiling joists or intention resisting the roof spread here at the wall plate and another feature of this particular kind of roof is that at the wall plate the rafter is nailed both to the wall plate and to the feeling joist and the ceiling joist is also nailed to the wall plate so we get a very secure connection here because this is resisting the rafter pushing outwards at the wall plate here so if we just remove the ceiling joist we can see that the rafter is usually bird mouth cut over the wall plate giving it a very secure connection and then it's nailed in or some kind of angle bracket is is used to nail it securely in place the Ridge board is not the same as a ridge beam so it's not a load-bearing member what that does is it transfers the equal and opposite reaction from the top of your rafter to the adjacent rafter and basically it's there to keep everything in line to transfer that load from one to the other and to allow you to have a Timber to to nail to so the purlins and the ceiling binders are bedded into the gable walls here and here and sometimes what we also have is a prop so we can see this particular one is quite long we might have a prop here going down from this purlin to the wall below so that prop might be sitting on that load-bearing wall and then usually within the ceiling in order for this not to again to move out to push out we would probably have another Timber strut lying in here to resist the lateral load now what some of you will be used to seeing is having these purlins on a not on an angle so the reason that this is on an angle is because the rafter is triangulated so the only force that the purlin needs to resist here is the deflection perpendicular to the rafter and that's why we angle it like this if you had it upright then it would be more likely to bow now if you didn't have the ceiling tie so here we've taken it away and we're showing it without then it would be better to have it upright

Channel: Robin de Jongh

Views: 7,989

Keywords: how a roof structure works, roof framing, common rafters, roof framing 101, roof framing basics, how to cut roof rafters, roof structure, traditional roof construction, traditional roof design, cut rafter roof, roof purlins, ceiling binders, ceiling joists, roof support, how to support a roof, how to support a roof structure, how to build a roof, how a roof works, roof construction, timber roof, purlin, purlin roof

Id: mX2bWCsf7qI

Length: 5min 48sec (348 seconds)

Published: Wed Dec 21 2022