Builders/Architects, can I bounce a home addition question off you?

We are wanting to bump out the back of our house to create a large dining room / sitting area. 

What I can't visualize is how to tie into the existing roofline.    

Picture of house here

We would like to extend out 14'  (roughly where rocks are) and the width is 21' (Between the red line)

Trusses on the addition would come out perpendicular the existing roof, with the peak at the chimney.

What would need to be done to pull the top roof over to the trusses of the future addition?

We get heavy lake effect snow here as well, so flat roofs are not an option.

Less clicking for the lazy.

That's no problem.

What they'll do is peel all the shingles off the rear of that roof, replace any bad sheathing, and frame the tie-in right over the existing roof.  The ridge of the new extension will continue over the existing roof until it dies into the sloped plane of the back side.

This will create a 'valley' up either side that gradually rises to a point as the planes of the new overframed roof are absorbed by the slope of the old.  Keeping the new ridge below the elevation of the existing for a simple tie-in is probably what will determine the slope of the new, but there are ways around that if you prefer the slopes to match.

The chimney complicates things just a little.  You may need to keep a lower slope so that the valley on that side clears it.  That's something that will have to be laid out in the field.  For shingles you typically want to go no shallower than 4/12 pitch (which looks about like what you have) but if they used self-adhered underlayment / ice shield over the whole thing you can go down to 3/12 if necessary.  

Since the 'live load' (weight of snow, people, etc) is much greater than the 'dead load' (the weight of the structure and materials themselves), you're really not adding much load to the existing roof.  You're only adding the dead load of the tapered portion of the new roof that overlays the existing, and that's minimal.

With a little measuring, the truss manufacturer can make the overframing set, which would be even lighter than stick framing, and would distribute the load perpendicularly across the existing framing.  But either way will be simple and solid.

The peeling back the shingles I understand,  I'm more worried about the deadish space that will develop between the new peak coming out and the existing hip roof to the left.

In reply to Grtechguy :

The valley will run a diagonal line from about the ends of the lower gutter to about the chimney.  It will slope away from the hip roof the whole way.  The flashing and siding where the existing gable roof joins the side wall of the second story will remain as is.  I don't think it will look funky.  You can also hip the back wall of the extension to match, which will reduce the apparent bulk of the extension even further (much like the hipped roof at left appears much smaller than the gable at right.

What Duke wrote.   

Really no problem if being done by a good framer.

You'll need to put better flashing under the siding on that one upstairs wall that's facing the new roof.

usually we open a hole in the old roof where it would be under the new roof so that one can traverse through that whole attic for repairs, wiring etc.

you don't think it would be a snow trap?

SWMBO has been bugging me about adding a "3 seasons" room on the back of our ranch, so this thread is relevant to my interests.

It probably would be.  Is there going to be a step down into the addition? If so, could you extend the upper roof pitch down over the back of the existing roof and lower the peak of the addition to meet it to keep the pitch even both ways over the new space. That way you could just fill in that gap with a vertical wall covered with siding to match existing.

Probably wouldn't work that exact way without a step down. The other option would be to just extend the upper roof to meet the lower addition wherever it ends up and fill that area up with a wall.

The best option will be obvious to an experienced framer once they can throw a measuring tape on things.

The kitchen is extended on my house just as you are shown.

That part of the roof is not a snow trap, as much as it is a rain trap. It did need some significant sealing when I had the roof re-done the year we bought it.  If you can have the extension NOT slope INTO the upstairs wall, you'll probably be golden.

In reply to jgrewe :

We are not wanting a step down.

In reply to Grtechguy :

Yes, it may trap some snow, but shouldn't a problem unless you are located in the great white north and get feet of snow at a time..

Pull the siding off and run the self-adhering waterproofing underlayment from the roof sheathing straight up the wall to the soffit.  That gives you a continuous, self-healing membrane to protect that area.  When you reinstall the siding over it it will seal itself around the nails.

Your valley will extend all the way to the intersection of the back wall and the new side wall of the exstension, rather than terminating somewhere along that little tapered wall you're worried about.  This means that the rainwater runoff will be the same as it is now, rather than sloping into the sidewall as mentioned above.  With a diverter at the end you can direct rainwater from the valley away from the sidewall and into the gutter of the extension roof.

Duke said:

In reply to Grtechguy :

... shouldn't a problem unless you are located in the great white north and get feet of snow at a time..

he is.  he does.

In reply to AngryCorvair (Forum Supporter) :

I still don't see it as a problem if he does the membrane flashing as described.

Existing on the left, new on the right. You can end the addition with a gable or hip roof.

Honestly, the only way I would do that is to take off the lower roof entirely, extend the walls there up to match the rest of the house, re-do the upper roof and extend it over everything, and then add your addition out the back.  If you used scissor trusses over the raised portion you might even get a nice room up there.

Shadeux (Forum Supporter) said:

Existing on the left, new on the right. You can end the addition with a gable or hip roof.

This looks a little funky but is very doable and the cricket (the little triangle with the chimney at the upper right apex) will stop snow from collecting where you are worried.  That slope will probably be shallow enough that you'd need to use EPDM or modified bitumen roofing rather than shingles.

In reply to Duke :

Grace Ice & Water Shield or similar should be used on the cricket. And, yes, I screwed up the chimney part on the new roof. The new ridge would extend to a peak, not get truncated like I show it.