How does a room-in-the-roof truss work?

how does an attic trust roof work an attic truss is a type of truss as you can see here that gives you space within the roof space to have a room and usually this is a habitable space and the truss is usually installed in new builds where the plan originally is to have this extra room within the roof as you can see here there is quite a steep angle to the roof in the first place to allow for this now a attic truss or room in the roof truss here you can see that they're spaced around about 600 millimeters maximum and they sit basically on the wall plate here and on the other side here so this truss is supported on two outer walls and it takes no support from any internal load-bearing wall so at the first floor you're able to have stud walls that are non-load-bearing so these are basically just craned up onto the roof and then they're spread out and the bracing here that you can see here in the dark brown color is what's put in initially to space the trusses uh to keep them from moving basically in the wind and then also the bracing gives it a permanent stability so you can see here that we need to have longitudinal bracing here and here above the collar tie in the ridge and then here above the stud wall and within the eave space we've also got diagonal bracing here so if we just look at this on plan there's the roof so we've got diagonal bracing going all the way here and here so this gives the roof stability if the wind is blowing on this gable end then given that this wall is braced back and strapped back to the roof structure then that's not going to move because of the longitudinal bracing which spreads the load across all of these trusses but more importantly the diagonal bracing which stops all of these from collapsing in a pack of cards type effect so the main reason why this truss is shaped as it is is because first of all there's no diagonals in there quite obviously because we want to use this space as a room but the issue here on this particular building that i've used as an example is from wall to wall here is about six meters which is too far for a timber to span so we couldn't simply just put timbers across here and then have a traditional cut rafter roof so what a attic truss does is it provides these studs here at the sensible spacing so here we have about 4.3 meters which is acceptable for a normal floor joist so this bottom cord here i've grown in as 200 millimeters acting as a normal floor joist and it's also acting as part of the truss as we'll see in a minute these upright sections here which are basically hangers to stop this floor joist from bending too much also creates a wall on which to put our timber or plasterboard so let's just have a look here at the when we put the walls in so you can see here that that forms the the walls of the room and this is the area within the eaves anyway where you wouldn't be able to make much use of the space because even if you're sitting you can't get into it so this area here this triangle is not usable anyway and even if you've got a normal loft and you use it for storage you wouldn't put anything normally in that area anyway so it's a very clever system and it is based on sound structural principles so here we've got the truss within a analysis package this is a quick frame by quicksoft and i'll leave a link to this software in the notes below it's very easy very simple to use and i recommend it for any kind of 2d analysis so i'll just show you what section sizes i've got here so i've got generally 50 by 200 c 24 and then i've got 50 by 150. so what i'm going to do is just show you what this looks like when a load is applied first of all if we look at the deflected shape we can see here that the floor's sagging and basically you can see by the curvature here that the upright or the the wall section is doing what it needs to do let me just show you the the axial load so it is acting as a truss but because it's not fully triangulated there's also going to be bending you can see here that all the rafters are in compression apart from this top part here so basically red is in compression green is in tension so the collar tie here is actually one of the highest loaded parts of the whole structure it's in compression so we've got the bottom cord in tension the tie in compression and then we've got these walls are actually acting as hangers so they are working quite hard here we've got around five kilonewtons of tension in those sections that become the wall stood work well what does that mean in practice well if we look at the bending here we can see the the timber is bending from point to point here so if we've got the floor joist which is supported off these hangers the floor joist is is bending like this but also it's hogging over the top of the joist so what that basically means in practice is that the floor joist or the bottom cord is actually working very efficiently as is the rafter here which is has got a maximum bending at the wall section and a similar kind of hogging over the the tie and the tie itself has got little or no bending in it at all so unlike a fully triangulated truss which would be only in tension and compression your attic truss is in bending as well as being in tension and compression and that's why your attic truss is much more expensive and you've got much larger section sizes so just looking here at the geometry this one here is 50 by 200 which is quite a large section size for a truss rafter so if you've got a triangulated truss i think truss then you might have half that size so coming back to our attic model here we can see that the floor is formed here by the bottom cord and that's going to just accept standard chipboard flooring obviously we're going to need a hole in here somewhere for a staircase and i'll show you just how that would happen so if we want the staircase to come up here then basically we would remove that we're now creating if you just look underneath we're now creating an area where we could put a staircase so let's just open up an area here so we've opened up an area for our staircase but the edge is still supported by that truss but of course because now you've got double the load on this one we've got to double that one up and so for the truss designer it's a simple procedure just to show where the double trusses are and that one as well and it's really not nothing nothing more complicated than that now given that this is still going to be loaded so on the outside we've still got the tiles here they would need to be a common rafter laid across here in in the middle in order to pick up the roofing and then we'd have some structural cross members here to support that that rafter and of course the same goes if we want wider windows then you would double up the the trusses each side and put timber across so let me just show you that you would put some timber framing in here in order to pick up the the window and obviously we would turn these around to be perpendicular that that type of thing so all we need to do now here is add the plasterboard but we'll see that there is a slight problem here so it's absolutely fine with the ceiling so that's nailed or screwed straight onto the timbers but if you notice here when we add the plasterboard then the bracing's in the way so what we need to do is button that out so let's just move this one out so we've now pulled the plasterboard out to the outside of the bracing and if we just hide that what would need to do now is batten out in here with timber battens so that we can basically lift the plasterboard proud and away from the bracing so once the plasterboard's put on it's going to be held off the bracing by this button now if you don't want to do any of that if you don't want to have all of this bracing or at least get rid of most of it what the codes do allow you to do is put sarking on the roof now it's got to be approved sarking let me just draw this in first so we're going to put sarkin nail it straight to the outside edge of the trusses so let's make this 18 millimeters and let's make it exterior plywood so we could do that yes that's not going to be a single sheet it's going to be lapping if we look here at the nhbc manual then here in underlaying sarking we can see that the acceptable materials for use as underlaying sarkic we have traditional tongue and groove timber but also we can have osb type 3 chipboard type p5 which i think is the water resistant or exterior grade plywood so that might be an option but bear in mind if you put the sarking on then when you come to put your battens on you can't just put them in normally in that direction you've got to first put counter buttons in that direction and then put your normal battens in this direction so either way you're going to have to put extra battens in anyway so i hope that's been useful that's basically attic trusses what they are how they work and some of the structural considerations and where you need to brace it if you want to get hold of this drawing i'm going to output this as a labeled up drawing for you to download please go to the link below and you'll be able to download that straight away if you've got any questions or comments please leave them below the video i'm always very interested to hear particularly what your experience might be if you've installed a roof similar to this

Channel: Robin de Jongh

Views: 50,454

Keywords: attic truss, room in the roof truss, how an attic truss works, attic truss roof, attic truss loft, loft conversion structure only, attic truss structure, room in the roof structure, attic truss construction, attic truss bracing, attic truss deflection, attic truss moments, attic truss sizes, attic truss sizing, build it with rob, loft conversion truss, timber loft conversion, loft room structure, room-in-the-roof truss, room-on-the-roof, room-in-the-roof, roof framing

Id: HWShk_6ZITQ

Length: 14min 4sec (844 seconds)

Published: Mon Mar 14 2022