![]() If you are incorporating soffits into your design I agree with PEG688 sometimes smaller porch truss's are more economical and can save time. Pay attention to the connection detail at the "long end" - imagine your big brother on the end of the playground seesaw.įor uniform loads here are some reasonable lengths for cantilevered rafters with snow loads below 45lbs sqft. Cantilevering should not exceed 1/3 to 2/3 ratios. Most building codes have no prescriptive tables for this application however there are some general rules of thumb that you can follow. Realize you are cantilevering the rafters. Remember to plumb and level wall before adding porch rafters ![]() Treat wall as a shear wall – some sheeting might be necessary Studs should be spaced the same as truss layout for single bottom plate Draw to full scale before building – use purchased new porch roof sheeting for drawing board rather than lines on new kitchen floor vinyl. The stud for the knee wall will have 2 angles – No need for runner as your bottom plate is your runner. Knee walls will work and is quite common. Only one plate is used in the image, however it could be at any height – which would further make a large "continuous runner" redundant.Īs for slope anything less than a 3/12 can get spendy as the roofing tar paper requirements change at this pitch. Stacking the rafters on top of the truss plane is a good practice but you can use code maximum offsets for a single plate – usually 9" The slope of the new porch roof in the image is steep for illustration purposes. This edge should be nailed 6" OC to the old roof to help anchor your new roof this is particularly true if your area sees any snow. The only thing touching the old roof should be the new porch plywood. ![]() The rafter seat cut can taper to zero without support as long as the start of the seat cut is supported. This prevents the rafter from rolling under load, if drilled and screened it can be used as a vent and also this keeps the birds out. It should also be noted that it is a very good idea to put some blocking at start of the seat cut. It should be noted that the drawn width of the "continuous runner" only needs to be as wide as the wall plate. If you look at the PDF document (Johns link 1 post up) in lower right corner - you can see that the porch rafter seat cut that sets on the "continuous runner" tapers to zero and although the wall top plate is not shown the taper will extend past the inside edge of the wall– This is especially true if your porch rafter is wider than a 2圆. Plate line is the vertical "plane" of the top wall "plate". This should work, I'd felt paper up and under as well, there might be so little old roofing left that you might just redo the whole side of the roof, that's depends on color match and how fussy you are about that sort of stuff. Shingle up to and under your existing shingles. Sheet the roof as normal, venting the new space might be a problem depending on how you finish off the porch ceiling, but that's another issue. This would put the real load of the porch shed roof on a truss designed for the load and the old roof would only be picking a small part of a new load. Then I'd sister along side the 1/2 truss to die into the old roof, With a 2圆. If it is a truss roof I'd cut back the existing eave line so I could put a half truss on the existing wall plates, cutting out the existing pressure blocking and place the new 1/2 truss, full bearing on the top plate and along side the existing truss. You say the existing cabin has truss's, if that is right I'd worry about adding addtional roof loads to truss's not disigned with that in mind. Steve In West Virgina ? Any snow load to concider? Maybe the others have some more ideas- what I have told you is just a suggestion based on the problem as I see it - you have to determine if it actually may be appropriate for what you are doing. A bit tricky to do and keep from leaking but I have done it before. This would mean removing shingles where necessary -the new shingles would have to be carefully worked under the old or a flashing slipped under the old shingles and over the new. Your new roof sheathing would have to join the old- new roof joists would have to meet on top the existing roof joists or trusses with appropriate bracing modifications. The ends of your new joists can't be supported by the roof alone -I think I would brace down to the top of the stud wall top plate. If getting permits you may need the help of an engineer or architect. If you have trusses on the existing roof they may have to be modified as they probably won't be made to take a load where the new roof joins in. ![]() The old roof under the joists won't be doing anything then so you can brace as necessary. ![]()
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