Engineering an Open Attic with Site Built LVL Trusses

[Music] oh man look at this attic this is awesome we're coming to you from my house this is our real rebuild project and I gotta say this attic is incredible it looks like we've stepped back in time nothing like most new construction here in the United States we got a great episode for you today I've got my engineer with me we're gonna show you how we design this cool space let's get going [Music] alright guys we're back with my engineer wit Smith with I really appreciate you taking time to come out and visit with me on the job say it now I got to tell you I've got a lot of projects with you and I'm a little biased here but this is my favorite reframing my favorite attic I've ever built I love this design now what's typical that wit for addicts for roof framing in America these days well what we see most often are either roof trusses because they're efficient and they're a cost-effective solution so we could have trust this roof or another very common method would be to use interior load-bearing walls and actually brace rafters so we could use a smaller size rafter and brace that down to those interior walls with 2x4 tracing something called a strong back now this house wave prior to me deciding to rebuild it was gonna be a remodel and I had to buy four trusses up here from the 70s those are getting built all over America today I had to buy four was going everywhere in the Attic I was trying to figure out how to get my HVAC system up here and when we decided to rebuild the project I said to you I want the most wide span attic the most amount of storage the most amount of space I can get in the Attic I think this is amazing talk to me about this design and what's happening from an engineering perspective up here we went through a couple of options to determine what might be the best layout for this attic so one option that we always consider would be to design a structural Ridge the stands exterior wall to our interior load-bearing wall and then the Raptors would span from the exterior wall and be supported by the ribs and that would allow us to have an open floor plan in this case because this was a pre long span we looked at another option and that is to essentially analyze your rafters and your ceiling joists as we would a truss so we designed what is essentially a three-piece trust where we connect the top cords to the bottom cord and all three members act like a truss and allowance to have an open floor plan yeah now this looks like a 1920s or 1930s house in terms of the rafters in the openness but it's very different now talking about the individual members we've got lvl rafters going on right how's that different from let's say using solid sawn lumber here well it once again as we discussed earlier another big we get excellent dimensional stability here in addition to that we get strength so that gives us we can have we've reduced depth have more head hiding here an important consideration whenever we use this three-piece trust scenario is the connections the the top cord has to be connected to the bottom cord with enough fasteners to be able to resist that compressive force that then becomes a tension force and at ceiling joists so using engineered wood actually is a good option because we get higher strength out of those connections that's a very critical part of this yeah so the ceiling joists which are underneath our feet right now then we decked over top of that and is this this Advantech 3/4 decking give us any structural benefit as well well it does your floor decking can actually help serve as that tension tie that we need to keep these walls from separating what I like to do is design because the ceiling joists are going to be lapped over a wall so I'll go ahead and design that lap connects and have enough strength to resist that tension force okay and then we'll rely on this decking just as extra you know we would call that redundancy and design and redundancy is a good thing we were going to have the floor decking anyway so we try to take advantage of that you know when we built this one if you watched our other videos guys I call this monopoly framing where our rafters don't poke through and sit on the wall with a bird's mouth are actually clipped at the end of the wall so that my Hueber zip decking on the roof transitions right to my wall framing and I could get a very airtight shell did that hurt you structurally but not having those rafters poked out over the walls not necessarily we can still have rafter tails or extensions if they're properly connected and what we focus on the most at these conditions is that we have a good connection between the rafter and the wall so that's our continuous load path that we're trying to achieve so what that means is and there's a product and several many products out there actually to connect the rafter to the wall other than just a toenail so that's what that's another important connection we'll be looking at at the top plate down below us to make sure not only do we have that transfer but also if it connects and down to our Keith wall that will extend down to the foundation be connected connected to the foundation yeah let's go downstairs in a second I'll share that but before we go talking about these two by fours that are tying these two rafters together are these doing a lot of load are these carrying are these kind of making the roof not spread apart well we that's something we run into a lot so these are called collar ties and almost every framer I know is now installed if nothing else they do it out of habit just because it's the way almost every frame job has been done there's a lot of people believe that collar ties help prevent spreading similar to what our ceiling joists are doing for us but in fact they really don't help prevent spreading much at all because they're so high they're very inefficient they essentially have no resistance to spreading I mean they can't take any load now what they are good for particularly in a low sloped roof and a high wind reaching where you can get uplift on your roof the roof lacks similar to an airplane wing if wind passes over it it'll try to lift the roof up yeah these help prevent the Raptors from separating from the ribs so that's an important even that Ridge kind of tight so to speak correct that's right now we don't have any strong backs or any knee walls up here how are you able to eliminate those or are those necessary in this system they're not necessary here because we designed it as a truss so we designed the rafters to span all the way with this connection so had we not done that that's why you might see a knee wall a strong back some of those other connectors and because that ridge is not structural - we don't have additional posts coming down from that ridge - right that's right and we've got other options so while we're talking about these types of systems another thing you see these frequently if we had a let's say we didn't have to see we wanted a vaulted room you can actually have a bent steel beam in here that so you take the beam you cut it you reweld it to the profile of the roof and that can support your Ridge intermittent points that's pretty smart well I love this attic man this is gonna be incredible let's go downstairs and show these guys how you tied everything together all right okay so we're on the second floor now we were standing right above us on top of that decking now what do we need to know about tying all this together all right these ceiling joists are what we were using is the bottom cord of our trussed up there what we were calling on trust so there's those rockers try to separate as they get loaded they put a horizontal force on the top of the wall the ceiling jewelry so it resists that horizontal force so the joists will have a tension load in them and it's important that where we splice the joists that we have a and it's properly designed to transfer that it's a tension load and the member but it's a shear load in the splice so they want to come out so we're resisting that force that's correct part that's correct so there's a great new line it's a relatively new product line out there among several the manufacturers where they have what is a replacement for a lag screw and so with this power lines rise back so here we have specs power lags these have similar values to lag screws but they're a smaller diameter they don't require pre drilling and they're easily installed with an impact driver that's pretty nice well big fans of these yep we're using them a lot and talk to me about the outside speaking of those because we actually kind of did a little extra on this outside wall didn't we well I like it those instructional engineer I like seeing a little redundancy in the project so what we have here is we have our power lag we're using this as a trustee they make other other product lines out there that are similar to the truss through and what they're designed to do is be a replacement for a traditional hurricane time we like hurricane ties we use them a lot but these can be a little bit more discreet they don't protrude out from the wall and conflict with your drywall installation and actually install faster because this will have 10 nails where this is only one yes so in this case we're going through that double top plate into the rafter with a long power lag and then we've got a shorter power lag that's connecting the ceiling joist or ceiling rafter no ceiling goes with the rafter together right so there's a both inch and 3/4 members we've got 3-inch screws right and that we can't lose sight so that's a very important connection in this structural system because that's what connects this compressive member the top chord of the trust to our tension member whose bottom chord of our trust yeah so in this case we've kind of done a redundancy though on the uplift capacity because we we didn't necessarily need to use these funny story for you though whit designed us all to use the spax power lags and my guys are so used to installing these but before I can tell them they don't bother we don't need those they were already installed I think that happens a lot on job sites we like it engineers like and we come out and we see double duty but it's double duty and of course the spectres see that a lot now instantly enough one other side note with the guys that fasten master let me borrow this tool which is fairly new to the marketplace and tell me about this system this is a trust group so this is another product similar what we're talking about and it is this is a preset angle so we can actually come in and install the trust screw and screw that right up and up into the top plate that's right and what's nice about these is they have a very small head so once again it's a very discreet connector some people don't like the look of the exposed metal connector so the manufacturers know that so they're constantly coming up with new products to help help structural engineers with a you know slightly more concealed connection yeah that's a cool connection unfortunately I did find out on this job though is we kind of demoed it that with the engineered lumber that lvl a little harder to get that screw started than with traditional lumber I've used that on a house with traditional to buy SPF framing and it was no big deal just ran right in there no problem but on this is a little bit harder so this spax power lags were nice because those just sunk right up no pre drilling nothing anything I missed on this roof framing that you wanted to mention no but I do have one more thing I'd like to mention about floor decking something we talked about it earlier about a quiet floor I use the term glued and screwed and that's something you hear a lot of what I should emphasize is all the structural design tables that we use for floor diaphragms roof diaphragms they're actually based on the installation of nails and I think earlier conversation we had we're talking about screws and structural applications is that particular screws aren't meant to be structural screws such as deck screws they're brittle and they don't have the ductile behavior we need out of structural connectors so in this case as we were discussing we did screw it but that was that was extra we had the nails for the structural diaphragm strength that we asked for and then we have a screws in here extra assurance against yeah I use the fasten master system as well they have a really cool system with a gun that collates and makes it super easy to screw it off but normally we're gluing with the Advantech glue which is an all polyurethane glue and then we're nailing but I did the screws extra sweet protection I think that's a good good solution one last thing that I did want to mention on this attic guys if you haven't seen my other videos this is a fully insulated roof deck in fact we've basically made a it's panel out of the roof system but it's a non vented attic meaning there's no vents either at the ridge or at the rafter tails on this job my house is fully insulated with exterior insulation from Atlas poly I so so upstairs in the Attic all I'm gonna have to do is come back when it's time to insulate and I'm gonna use Rockwell bats up in those rafters and then that's done which means all that attic space that I was in just a minute ago with wit that's all part of my air conditioned envelope fully air sealed on the outside insulated at the roof deck which means that all that space up there that's some great storage space it's gonna meet an amazing attic huge thanks to wit for coming out with me today on this series of videos about the engineering on my house I'll put a link to wits firm if you're interested in hiring him for your custom house here in Austin Texas because if you're not currently a subscriber hit that subscribe button we've got new content every Tuesday and every Friday follow me on Twitter Instagram otherwise we'll see you next time on the build show [Music] [Music]

Channel: Matt Risinger

Views: 782,309

Keywords: house engineering, build show, whit smith engineer, smith engineering, matt risinger, real rebuild, real remodel

Id: 0scGE11HKi4

Length: 12min 58sec (778 seconds)

Published: Wed May 06 2020