3/18/2024 0 Comments Tying roof rafter to wallsHe also suggested installing rafter ties which I am planning on doing. That should hopefully prevent any further spreading out. ![]() The structural engineer suggested laying another 2X4 over the joists in the affected area. I'd like to repair the crack in the ceiling. I had the old roof torn off last year and now there is just one layer of shingles so the excess weight should not a problem any more. The area above the crack does not have a 2X4 brace immediately over the attic joists. My roof had three layers of shingles bearing down and we believe this excess weight has over time caused the wall to be pushed out a fraction of an inch which has caused cracks in the ceiling. 2X4s are laid flat and nailed across the attic rafters to provide bracing against the outward force of the roof pressing down. Most roofs use attic joist that run parallel to the rafters but in my roof the attic joists are perpendicular to the rafters. After three visits from two different structural engineers I think the cause of the cracks has been identified. It's about five feet long, surrounded by hairline cracks and is about a foot from the the wall is lined up with the wall. ![]() Solutions to rafter thrust should also be designed by a Licensed Professional Engineer.Over the past few years a long crack has developed in my living room ceiling. Damages from rafter thrust, as well as costs to address the condition, can be extensive and should be fully assessed by a Licensed Professional Engineer prior to purchasing a house. ![]() Rafter thrust can be a serious concern for a homeowner, but is always a serious concern for a home buyer doing due diligence on a property. Collar ties can assist with the rafter thrust, but their effectiveness is limited the higher they are on the rafters. This could be installation of an attic floor, or installation of wood or metal ties at selected intervals. Solutions to rafter thrust generally entail tying the tops of the walls together across the space. Rafter thrust can be a slow process or it can occur rapidly. This can occur due to design errors, field changes that the design professional was not aware of, or modifications to the structure after its construction is completed. Issues arise when a roof is functioning as a cathedral ceiling although it is not designed to be one. A properly sized ridge beam can allow a properly functioning vaulted or cathedral ceiling to exist with no issues. When the top of the rafters are supported on a beam, as opposed to leaning on each other, the thrust at the base of the rafters is eliminated and the force at the base of the rafters becomes vertical only. In this case, the beam will be an actual structural beam that is designed to support the top of the roof. The proper way to frame a roof such as this is to make the ridge beam what is called a structural ridge. When this is the case, the wall can still resist the vertical loads, but there is no tie to resist the outward thrust. When a cathedral or vaulted ceiling is present, the attic floor joists are not present. Most existing light frame construction is constructed in this manner, and almost all housing stock from the 50’s through the 80’s (typical Cape Cod, split level, bi-level, center hall colonial construction) is constructed in this manner. The outward force is resisted by the bottom piece of the triangle, in this case the attic floor joists. The downward force is resisted by the wall below. ![]() Since the rafters are sloped, the downward forces of the self-weight of the roof, and any loads or snow on the roof, will push the bottom ends of the rafters both down and out. The ridge piece, which is at the peak of the roof, is usually a single 2x member, which acts a nailer, and is not performing any other structural function other than to attach the roof rafters to each other. In this case, each sloped side of the roof and the attic floor members form a triangle. There are generally two conceptual designs for typical light framed residential roof construction. This action can happen independent of, or in conjunction with, sagging of the rafters or sagging of the ridge. This can result in a bending outward at the top of the wall, as well as a dropping of the roof ridge. Connections can also pull apart. Rafter thrust occurs when the rafter tails (the bottom of the roof members of a sloped roof where the roof meets the wall) thrust outward and push the top of the wall outward.
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