Author Topic: Tying a porch roof (shed) into existing roof  (Read 17808 times)

0 Members and 1 Guest are viewing this topic.

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Tying a porch roof (shed) into existing roof
« on: October 13, 2005, 08:23:52 AM »
Hello,

I built a 24' x 32' cabin with a roof that is 8 in 12 pitch.  It has a 36' x 8' deck on the front, and I am adding a roof to the deck.  The cabin already has shingles installed, and I am not sure how to tie into the existing roof.  My current thoughts are to use 2x6x16' rafters spanning from the porch beam to a point on the cabin roof directly over the cabin trusses.  I imagine that I will also have to add bracing under each rafter at about its midpoint.  The bracing would be nailed between the cabin trusses (through the roofing) and the rafter.  Another alternative would be to attach wide boards across the entire length of the cabin roof at the point where the roofs meet and attach the rafters to the wide boards.  I'm also not sure how to install the shingles to prevent leakage at the junction.  Any advice would be greatly appreciated.

Thanks,

Steve.

Offline Daddymem

  • Journeyman
  • *****
  • Posts: 1,235
  • Gender: Male
  • Ahmpit of the Cape, MA
    • Our First Day Cottage
Re: Tying a porch roof (shed) into existing roof
« Reply #1 on: October 13, 2005, 08:26:35 AM »
Got any pictures?  (Are you the same Steve that Pm'd me that you have a cabin with a rain screen wall?)
Où sont passées toutes nos nuits de rêve?
Aide-moi à les retrouver.
" I'm an engineer Cap'n, not a miracle worker"

http://littlehouseonthesandpit.wordpress.com/

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Re: Tying a porch roof (shed) into existing roof
« Reply #2 on: October 13, 2005, 08:28:26 AM »
I have pictures, but no means of posting them.  I'm not the same Steve.

Offline Daddymem

  • Journeyman
  • *****
  • Posts: 1,235
  • Gender: Male
  • Ahmpit of the Cape, MA
    • Our First Day Cottage
Re: Tying a porch roof (shed) into existing roof
« Reply #3 on: October 13, 2005, 08:29:02 AM »
If you have them electronically, you can use tinypic or photobucket to host them.
Où sont passées toutes nos nuits de rêve?
Aide-moi à les retrouver.
" I'm an engineer Cap'n, not a miracle worker"

http://littlehouseonthesandpit.wordpress.com/

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Re: Tying a porch roof (shed) into existing roof
« Reply #4 on: October 13, 2005, 08:31:16 AM »
I don't have a scanner.  They're just developed from film.

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Re: Tying a porch roof (shed) into existing roof
« Reply #5 on: October 14, 2005, 04:33:46 AM »
Does anyone have any ideas?  Do I need to provide more information?

Offline glenn kangiser

  • The Troglodyte
  • Administrator
  • Journeyman
  • *****
  • Posts: 24,672
  • Gender: Male
  • Central California Sierras- Home of Yosemite NP
    • Underground Cabin
Re: Tying a porch roof (shed) into existing roof
« Reply #6 on: October 14, 2005, 05:03:35 AM »
It sounds like you need more elevation than you can get by going under the eaves with a ledger lagged into the side of the studs -is that right?
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Re: Tying a porch roof (shed) into existing roof
« Reply #7 on: October 14, 2005, 05:19:54 AM »
Yes.

I definitely need to tie into the existing roof.  I've never done this, and want to get some feedback as to the best way to do it.  My rafters will probably be 2x6x16's, so I will need to brace them as well.  I would think that 2x4 or 2x6 braces at about the midpoint of each rafter would be sufficient, but again, feedback is welcome.

Thanks for your reply,

Steve.

Offline glenn kangiser

  • The Troglodyte
  • Administrator
  • Journeyman
  • *****
  • Posts: 24,672
  • Gender: Male
  • Central California Sierras- Home of Yosemite NP
    • Underground Cabin
Re: Tying a porch roof (shed) into existing roof
« Reply #8 on: October 14, 2005, 05:50:42 AM »
The old roof under the joists won't be doing anything then so you can brace as necessary.  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.  If getting permits you may need the help of an engineer or architect.  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.

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.  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.  A bit tricky to do and keep from leaking but I have done it before.

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.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Offline PEG688

  • Master Craftsman
  • Journeyman
  • *****
  • Posts: 3,515
  • Whidbey Island , Wa.
Re: Tying a porch roof (shed) into existing roof
« Reply #9 on: October 14, 2005, 05:57:41 AM »
 Steve In West Virgina ?   Any snow load to concider?  

 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 .

 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.

  Then I'd sister along side the 1/2 truss to die into the old roof ,  With a 2x6.  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.

 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 .  

 Shingle up to and under your existing shingles . 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.  

 Hope this helps , PEG
When in doubt , build it stout with something you know about .

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Re: Tying a porch roof (shed) into existing roof
« Reply #10 on: October 14, 2005, 06:21:48 AM »
Yes.  I am in West Virginia, and we do get some snow.  The cabin is built with scissor trusses.  I was hoping that the solution would not be quite so involved.  Could you explain the half truss and how far it would extend over the current cabin area?  Other than the half truss solution, is there a safe way to do this with trusses and bracing?

Thanks,

Steve.

Offline John Raabe

  • Administrator
  • Journeyman
  • *****
  • Posts: 5,332
  • Gender: Male
  • Whidbey Island, WA
    • CountryPlans.com About Us
Re: Tying a porch roof (shed) into existing roof
« Reply #11 on: October 14, 2005, 11:11:31 AM »
Steve:

Here is a good information sheet I have in my files.

I couldn't find it at the website anymore so scanned it into a PDF file and posted it in my download area.

http://www.countryplans.com/Downloads/porchdetail.PDF

Hope that helps.
« Last Edit: October 14, 2005, 11:12:43 AM by jraabe »
None of us are as smart as all of us.

Offline Ryan B

  • Junior Member
  • **
  • Posts: 46
Re: Tying a porch roof (shed) into existing roof
« Reply #12 on: October 14, 2005, 11:33:45 AM »
I would caution you from stacking rafters on trusses away from the plate line as this would defeat the truss system from reacting as the engineer/designer originally intended.
However there are a couple of common ways around this dilemma.

Position the rafter seat cut on the truss directly over the plate line.
As the rafter planes into the truss roof the bottom portion should have a gap as to not impart a load to the truss.

If the rafter is too far down the slope to give the porch a decent slope you can raise the rafter by adding 2x's to the top of the truss directly over the plate line, again paying attention to the bottom portion of the rafter for clearance.

If the rafter slope is still too flat you can build a "knee wall" again directly over the plate line. It's important to not exceed the "cantilevered" distance for that length and dimension of rafter.

Often I draw this system to full scale to determine the seat cuts and knee wall height for the rafters.

Ryan B

Offline SteveWV

  • Apprentice
  • *
  • Posts: 9
  • Gender: Male
Re: Tying a porch roof (shed) into existing roof
« Reply #13 on: October 17, 2005, 04:55:09 AM »
John and Ryan,

Thank you for your replies.  Ryan, what is the "plate line"?  Is it the location of a vertical member on the truss?  Also, when you say to leave a gap, do you mean that the end of the rafter that is over the existing roof should not touch the roof?  Is that assuming that there is bracing added to the top plate of the exterior cabin wall?  One more question, what is a sufficient pitch for a porch roof?

Thanks again,

Steve.

Offline Ryan B

  • Junior Member
  • **
  • Posts: 46
Re: Tying a porch roof (shed) into existing roof
« Reply #14 on: October 17, 2005, 07:21:26 PM »
Plate line is the vertical "plane" of the top 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 2x6.

- It should be noted that the drawn width of the "continuous runner" only needs to be as wide as the wall plate. In just about all cases a 2x4 will work. Any thing larger is a waste of material.
-It should also be noted that it is a very good idea to put some blocking at start of the seat cut. 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.
The rafter seat cut can taper to zero without support as long as the start of the seat cut is supported.

The only thing touching the old roof should be the new porch plywood. 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 slope of the new porch roof in the image is steep for illustration purposes.
-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"
-Only one plate is used in the image, however it could be at any height – which would further make a large "continuous runner" redundant.

As for slope anything less than a 3/12 can get spendy as the roofing tar paper requirements change at this pitch.


 
« Last Edit: October 17, 2005, 07:24:26 PM by RAB »

SteveWV

  • Guest
Re: Tying a porch roof (shed) into existing roof
« Reply #15 on: October 18, 2005, 10:35:32 AM »
Ryan,

Thank you for taking the time to reply.  What you draw makes sense.  In my case, the continuous runner alone will not result in a steep enough pitch.  I believe I can attach the continuous runner as you suggest, and frame a knee wall on top of it in order to support the porch rafters.  The rafters would then extend to a point that is about 4 feet up from the bottom edge of the cabin roof.  Do you think I'll need to support the ends of the rafters that extend from the knee wall to the existing roof?

Steve.

peg_688

  • Guest
Re: Tying a porch roof (shed) into existing roof
« Reply #16 on: October 18, 2005, 04:55:57 PM »
 Steve The 1/2 truss idea might serve you better , they could design the truss with a 2x6 top cord, (I doubt it would need to be larger but I'm not  eng. ) and your nailer could sit "up "the roof like you need it to.

 Will you need to pass a inspection? If so , draw up what is existing and take that drawing to either the truss supplier or the place you will place the truss order. From that the truss company will say ya or nay to the idea. In house engineering from the truss manf . should be all you need.

 That top cord might / should be run wild and  be site cut , IMHO .

If no inspection is going to take place and seeing your unsure of doing it.  You've made a wise choice in asking, I think :)

 So there are some variables that are not clear, to me .   But I'm sure the truss company that provided the org set can do the 1/2 truss idea . Good luck , PEG.  

RAB

  • Guest
Re: Tying a porch roof (shed) into existing roof
« Reply #17 on: October 19, 2005, 06:27:08 AM »
Knee walls will work and is quite common.
No need for runner as your bottom plate is your runner.

The stud for the knee wall will have 2 angles –
-      bottom angle matches old roof pitch
-      top angle matches new porch roof pitch
-      Draw to full scale before building – use purchased new porch roof sheeting for drawing board rather than lines on new kitchen floor vinyl.
-      Studs should be spaced the same as truss layout for single bottom plate
-      Treat wall as a shear wall – some sheeting might be necessary
-      Remember to plumb and level wall before adding porch rafters
         
Realize you are cantilevering the rafters.
Most building codes have no prescriptive tables for this application however there are some general rules of thumb that you can follow.
-Cantilevering should not exceed 1/3 to 2/3 ratios.
- Pay attention to the connection detail at the "long end" - imagine your big brother on the end of the playground seesaw.
For uniform loads here are some reasonable lengths for cantilevered rafters with snow loads below 45lbs sqft.
2x4 = 2.5'
2x6 = 3.5'
2x8 = 4.5'
2x10= 5.5'

If you are incorporating soffits into your design I agree with PEG688 sometimes smaller porch truss's are more economical and can save time. Truss members will have similar top cord as above table.

Depth of rafter, cantilever length, and knee wall height are all variable – so again full scale drawing in the long run is faster.

SteveWV

  • Guest
Re: Tying a porch roof (shed) into existing roof
« Reply #18 on: October 19, 2005, 07:08:50 AM »
Ryan, PEG,

Thanks for all of your advice.  You've been very helpful.

Thanks again,

Steve.

glenn-k

  • Guest
Re: Tying a porch roof (shed) into existing roof
« Reply #19 on: October 19, 2005, 10:39:11 AM »
Nice to see you here again, Ryan B. Thanks for your help.
« Last Edit: October 19, 2005, 10:39:36 AM by glenn-k »