exposing roof trusses to open up ceiling?
17 years ago
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Disadvantages of roof trusses?
Comments (21)brickeyee Your point about RH of winter air seems well taken. What was missing from the Carson Dunlop explanation, I think, is that attic air in a home is often more humid than air outside the attic due to leakage from the conditioned to unconditioned space. Here's how another P.E. describes truss uplift: "A problem that can occur with this system causes cracks in wallboard over time. In recent years, attic insulation usage has increased, which ends up covering the bottom chord of the truss. This tends to keep the bottom chords warm and dry while the top chords absorb moisture and expand. The top cord expansion causes the top chords to lift, pulling up the bottom chord as shown in Figure 1, hence the term truss uplift. " Charles C. Roberts, Jr., Ph. D., P.E. My untrained guess would be that the lengthwise shrinkage of grain does not relate to the length of the wood but to which portion of the tree the wood was cut from. Here's a longer explanation: "Truss uplift is caused by wood's natural response to moisture exposure. This response occurs when humidity changes differentially in two areas (an attic floor and attic ceiling, for example). A truss's top chords expand as their moisture content increases. Problems resulting from truss uplift are most common with flat-bottomed trusses in homes during winter when a truss's bottom chords, often covered by insulation, are kept warm and dry while its top chords are exposed to moisture from condensation. During winter months, warm, humid air emanates from buildings' interior use and occupants. The uplift movement occurs because, as the truss's upper components resize themselves under the differential moisture conditions, internal tensile stress occurs in the truss's web members, which then pulls up the bottom chords (and the attached ceiling drywall), resulting in a separation along the wall-to-ceiling juncture of the drywall. To put this into perspective, consider a truss with top chords made of two members, each measuring 30 feet in length. The total movement anticipated from winter swelling caused by the truss's moisture content increasing is a 0.72-inch expansion per side, or a 1-inch total rise of ridge above the bottom chords. And once a crack forms inside a home, additional moisture generated by occupants and their activities can escape into the attic, travel to the cold attic ceiling surface, condense on the truss's top chords and cause further truss uplift. " Lucas J. Hamilton manager of building science applications, CertainTeed Corp.'s Insulation Group For interest, I will forward this thread with a request for comments to Carson Dunlop. lzerarc As I said, different places, different practices. I've changed only reluctantly. Seeing those spindly stuck-together 2x2s gives me a visceral reaction and I have to rationalize using roof trusses. However, my engineer says they're much better done now than ever and with wall lengths less than 50 feet uplift is rare. None of this applies to warm climate homes....See Moreexposed trusses in kitchen and LR?
Comments (18)Here's one example of trusses exposed and clad in a ranch house. Note there was no rebuild of the trusses to gain any pitch to the ceiling a la scissor rebuilds discussed in this thread. Just left open and insulated up top. I also like the use of skylights spanning multiple truss bays. (No association whatsoever to this project.) Gallery: http://planforward.photoshelter.com/gallery/Adrienne-Craftsman/G0000L341r.kL4zQ/C0000343iOOnp1Bg Construction Photos/Blog: http://planforward.typepad.com/ Here is a link that might be useful: Exposed & clad truss kitchen...See MoreCost to build a roof with exposed ceiling joists/collars?
Comments (11)Well, it all depends on your preference for appearance and utility. Do you want to simply use the normal construction grade rafters and blocking? Do you want to look up and see the bottom and raw edges of the roof sheathing above the rafters? Do you care about seeing the roofing nails penetrate the roof sheathing? How will you route electrical power, lighting, signal and HVAC? When you omit the ceiling joists how will you offset the outward thrust of the rafter bottoms on their supporting walls? And how will you support the ridge beam which supports the upper ends of the rafter? If none of these things matter, then simply deduct the cost of hanging the ceiling gypboard and proceed to the bank with your savings. On the other hand, if you have to add materials and labor for any or all of these things, add by a measure of 1X-5X, depending on complexity....See MoreHow to hide Metal Brackets on Exposed Trusses
Comments (34)People frequently say they are working with Pros when they are DIYing and are not. Because of the deserved heat level associated with not hiring a pro when they needed to. An electrical engineer, or a roofer is not a structural construction specialist, despite engineers thinking they know enough to remuddle the world. And then, they go on to describe situations that no construction Pro would ever allow to happen. Which betrays their incompetency to do the job. Or the total incompetency of the supposed pro involved. An architect would have handled the aesthetic and structural questions raised by the thought of the project. Trusses are not stick framed rafters, and cannot be thought of as stick framing. It’s an engineered whole system that must be linked together to work together for proper support. Those “drywall supports” linking the bottom chords are structural in a truss system that is meant to be covered with drywall. They lend required lateral bracing to increase rigidity to the support system. Trusses meant to be exposed have to be originally designed to have the strength and rigidity to be an exposed structure. There is no such thing as “just” removing the drywall and braces and having that be a viable structure long term. And that is just one of the telltales that indicate no professional involvement. Or, incompetent professional involvement....See MoreRelated Professionals
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