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Flat to Gable roof (A truss)

Flat to Gable roof (A truss)

Flat to Gable roof (A truss)

Owner wants to replace his existing flat roof with a gable one.  I have never done this before, therefore, like to ask what kind of calculation and details might be needed for  permit to construct? Appreciate any feed back.  Thanks in advance.

RE: Flat to Gable roof (A truss)

Check your new vertical load (will you be removing some existing roof dead load).  Check your bearing conditions.  Check new lateral load on building.

RE: Flat to Gable roof (A truss)

Details would vary depending upon whether this is a wood or steel frame building. There are companies out there that have retro components for changing the slope on metal buildings.

RE: Flat to Gable roof (A truss)

You will need a framing plan showing the joist layout.  You will need a detail at the top plate showing your overhang and joist seat condition. Then I would show a cross section through the building with insulation, plywood, roof ties, vents, shingles, etc. called out.  If you are going to have a special return or dormer detail show that too. Then you might want an elevation to two to illustrate the siding or finish condition. Round it all out with some general notes and viola you are done.  As far as Calculations go, I wouldn't get too extravagant. This sounds like residential, which means just size your joist and beam if you don't use roof ties. Maybe show the load path to footing also.  With an assumed bearing capacity of 2000 psf you should be able to handle most gable conditions up to two stories (sans concentrated loads)  Your window headers should be fine but  if your joist direction changes, be sure to take a look at those.   You will need to show hogs per your local required code (UBC, IBC, SBC whichever).  Probably need to show a venting calculation also.   With your soffit and ridge vent areas.  Bam! Your done and several $$ richer :).  

All in all for something like this i would expect maybe 2 - D size sheets of drawings and notes and a couple of pages of calcs.


RE: Flat to Gable roof (A truss)

Bah, I just noticed your post say "Truss". If the roof will be trussed then you supply truss manufacturer with plan.  They take care of the rest. Shop drawings, calcs, etc.  You need to verify that the loads from the trusses are okay for your framing condition.  Posts under girder trusses, etc.  I like to have at least a 2x4 under each truss end if they are at 24" o/c but you won't have that luxury. Maybe add another 2x4 to the top plate to help distribute the loads. It really depends how large the truss span is.  To submit for permit you would need a plan saying "Trusses by others" and elevations. A section will show the truss MFR. how to design their end condtions.

Bam ! Your done.

RE: Flat to Gable roof (A truss)

Thanks Simon for your valuable input.  Here is my situation: the existing wall consists of 2x4 studs, the room size is 21 feet x 23 feet.  I am thinking of using regular rafters (2x6 @ 24" oc) and a ridge (2-2x6 put together) including collar beam plus 2-gable frame at both ends (on existing walls). The owner does not want to tear down the existing flat roof (made of 21'x2"x10" ceiling joists on 2-2x4 plates over the stud walls), rather he wants to use the atic as an extra storage space.  Do I need a king post to support the roof? If so, can I place the king post on the existing ceiling joist?  Will the existing wall be okay to support the additional roof load?  How do calculate how much load the king post will carry? Thanks   

RE: Flat to Gable roof (A truss)

First thing I noticed is that you want to use a double 2x6 for your ridge. Some inspection departments require the depth of the ridge beam be greater than or equal to the cut depth of your joist.  Normally, a 2x12 will do, depends on your roof slope and is a geometry problem. You will not need a king post , and if you space your roof rafters on same spacing as the 2x10's (remember a 2x10 is nominal dimensions, detail this as 1 1/2" x 9 1/4" actual dimension) then you will have very good lateral support at the top plate, which will control the deflection of the walls due to rafter horizontal loads... Adding collar beams near ridge will allow for a ceiling if desired.  The new framing normally would not be an issue because it is just the weight of lumber, shingles, etc.  The issue you need to be concerned with is the storage weight.  Typically, storage can be a large dead load.  If he is putting old clothes and x-mas ornaments don't worry, if he is putting stacks of book and magazines, this could be an issue.  You will need to analyze the 2x10's to be able to carry the additional dead load.  I don't have span tables handy, but 21' for 2x10's sounds like they are maxed out.  You will get additional deflection from storage loading and may need to add some additional ceiling joists. Plus, they would not be able to use the space as a bonus room or apartment without some concern for deflection of the ceiling.  Hope this helps.  Sounds like you are on the right track !

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