That means the panels are moving both forward and backward as well as side-to-side. (As an example, if the joists were slightly bowed, and the joists were installed such that they bowed up, a shrinkage in the wood would tent to flatten out the upward bow pushing the drywall panels together).īut your picture shows bulges in two directions. I would still expect tears to be more likely, but I can also see how movement could be such that the drywall comes together. I can see how undersized joist could easily cause movement along the axis of the joists. If the movement was toward each other, then there should be a bulge (sore of like when tile tents). If the movement was away from each other, then there should be a tear. It might be contributing to it, but I don't believe it is the root cause.įor starters, it would seem the mechanism for creating a bulge would be the drywall panels to be moving relative to each other. I still don't think the bulging is being caused by an undersized ceiling joist. What are your thoughts on that?īefore I say anything else, I just want to point out that I'm just a DIYer with a little bit of practical experience trying to use the combination of my experience and what I've learned on the subject of engineering and science to try to help figure out what is going on here. I have spoken to some folks and they have suggested turning my joists into a truss type system and replacing the drywall with 5/8" instead of 1/2". When I crawl in the attic above the rooms the 21 feet long joists make major squeaking noises and the 17 feet long joists make noise but not as bad as the 21 feet ones.Ĭan someone tell me what the longest 2圆 can be used without additional supports for drywall? There is nothing stored up there so its just the weight of drywall, insulation and lights. I am assuming that the joists are over-spanned. The room with the 17 feet long joists is not bulging but you can definitely see where all the seams are between the drywall. The room with the 21 feet joists has major issues with the drywall seams bulging. Both use 2圆 joists that go for that entire length between the exterior walls. One room is roughly 17 feet long and the other is roughly 21 feet long. I have two rooms with what I consider to be undersized joists. plasterboard on the underside of the rafters the effects of deflection under concentrated (point) load do not need to be considered as per guidance given in BS 5268-7.5 clause 4.3.You guys seem very knowledgeable about joist deflection so I am hoping you can help. The imposed load should be calculated in accordance with BS 6399:Part 3:1988 Code of practice for imposed roof loads, as a rule of thumb for altitudes not exceeding 100m you can use a uniformly distributed load of 0.75 kN/m² and for most other areas exceeding 100m but not exceeding 200m you can use 1 kN/m², for parts of Scotland, Pennines, North East England and heights over 200m you will need to refer to BS 6399 Part 3.Īs there are no brittle finishes e.g. Rafters are to have minimum end bearing of 35mm. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. Holes and notches cannot be drilled or cut into the rafters unless proved adequate by specialist calculations. The calculations on which these span tables are based conservatively assume that the rafters are not continuous over the purlins, but they can be continuous over the supporting purlin if required. These span tables are based on the assumption that the tiling battens fastened to the top of the rafters will provide adequate lateral restraint and also ensure that lateral loads are distributed. It is also assumed ceiling joists will be used to transmit the horizontal component of thrust occurring at eaves level to complementary rafters. These span tables only apply to roof systems with four or more rafters. These span tables do not apply to trussed rafter roofs. The dead loads shown at the top of span table above do not include the self weight of the rafters, however, the rafter self weights are included (in addition to the dead loads) in the calculations used to determine permissble clear spans. The allowable clear spans have been calculated in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship and BS 5268-7.5:1990 Structural use of timber - Section 7.5 Domestic rafters. Dead load per square metre (in kN/m²) supported by raftersĬentre-to-centre spacing of rafters (in mm)
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