| Q: My question is: What size material should be used for roof rafters on a shed roof built 16 inches on center, doubling or sistering the rafters, for a 21-foot span? One problem you might face is finding lumber more than 20 feet in length. If possible, move your supporting wall to accommodate a 20-foot rafter. Checking the 2003 International Residential Code book, I find that both 2-by-12-inch lumber and some grades and species of 2-by-10s will span 21 feet or more. The species are Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir in grades of No. 2 or better. The lumber dealer will be able to identify the species and grade of what they sell. But in all cases you can expect the lumber to deflect or bow due to the weight applied and the weight of winds, rain and snow. Installing solid blocking in between each rafter at midspan will help reduce some of the deflection. Attaching a color tie or ceiling joist to every other rafter will prevent the outside wall from rotating and will add strength to the structure. The color ties or joists can be spliced by sistering
... read the whole article |