Tying Into Existing Roof

Milhaus

I set some trusses for an addition on my house today. I've set trusses before but not tying into and existing roof. Is there a standard way to approach this? (The new trusses are are perpendicular to existing roof)

I ran a string line across the peaks of the new trusses to the existing roof deck to figure out where the new roof will end. From there I snapped string lines down to where the truss that abuts the house intersects with the roof.

The trusses that sit on the existing roof are stepped down in size accordingly.

The problem is that the new trusses don't match up to my string lines - I made the peak at the right height, but to do that I had to fudge layout on the first truss by 1-1/2.

Seems like there should be a definite approach to this, but it IS remodeling. Maybe I'm thinking about it too much.

mark

wallmaxx

What's on your existing roof? Just plywood...or shingles and the like?

You may have to slide the trusses downhill out of their 2' layout in order to sit correctly on your existing roof all while planing correctly with the new roof..

If you popped lines for where your valley go...chances are that you may need to adjust them and re-pop. You can't pop lines in-plane and then nail the trusses to that line, unless the uphill side of the truss has a 0" heel height.

Try nailing a long, straight 2x4 along the new roof - extending over where the tie in area is at three locations...peak and a foot up from the two bottoms. Then slide the trusses one by one up the existing roof until they make contact with the 3 boards and the existing roof all at the same time. Verify that everything planes for each truss as you go.

I know it sounds kinda wonky...but tying into existing is a science unto itself.

Kgmz

You usually have to do things like this with those type of trusses. Nothing is ever perfect. Just make sure your ridge and valleys are straight, like you seem to have done by stringing it.

I never use these overlay trusses and will hand frame the overlay. Start by setting my ridge, and then run a line for the valley and then fill in the framing.

Also don't forget to figure where the plywood is going to land in the valley. If you run the trusses right up to the valley line then the plywood will extended out to far, so you need to figure what the setback will be.

dkillianjr

I usually cut and frame my roofs, but every once in a while I will use trusses. The couple times I have used trusses I stick framed the overlay part of the roof. If you are usung the premade trusses, just move them where they line up and you can add blocking,and lath catchers as nessesary. And like Kgmz said, don't forget to account for where the plywood will end.
I usually run a level from the top of the ridge board make a mark then snap two chaulk lines connecting the eaves to the ridge. Then rough cut the shingles a few inches away with a circular saw, install a bottom plate, then the rafters.


Dave

dkillianjr

Good call Wallmaxx nice tip!

Milhaus

I stripped the shingles off, just OSB for them to sit on. Factory angle on the bottom chord of the trusses to match the pitch of the existing roof. There are five trusses that sit on the existing roof. The first two that I set don't meet my string lines on the valleys, but the peaks are on the money. Sighting the truss tops it seems like I am low with these two, but again, the peaks line up so I thought it more important to hit those than the valleys. I'll tin the valleys so I should be able to hide it if it doesn't sheet perfect.

Definitely not standard framing. It's good to know I wasn't off in my thinking. I was looking for this exact way to figure it out, and from what I found out, there's nothing exact about it, unless you frame them all yourself.

Thanks -

mark

toolwiz93

Welcome to the world of truss fill-ins that don't work like you think they should. A truss company can design a whole roof which can be very complex at times but they never seem to get these fill-ins on the money. Like others have said: use a string, mark your lines, and make the truss work. You will scratch your head all day trying to figure out what isn't working the way you think it should. Anymore, I like to stick frame the fill-ins because most of the time you have to do something to make it work. It is even worse if your old roof has some bumps in it that don't allow the truss to sit tight to the roof on the ends. Then you have to bow the bottom chord so it will lay tight to the roof. Best advice is keep your peaks right and make the rest work.

stp57

Recently, I converted my parents 2 car garage into a 3 car garage. I used the string, long board & long level technics. In the end what saved me was that I used 3" deck screws to secure everything. The roofers backed out the appropriate screws & made the right adjustments & cuts & viola! They made me look like a pro framer (which I am certainly not).
Steve

wellbuilthome

On a reno the truss never hit on the valleys because most existing roofs dip in the center on the ridge and the rafter bow down in the center from ridge to plate The valleys ether bow out or bow down . Just line up the ridge and add a strip of wood to the bottom of the truss, or push the truss up the roof till it hits and nail it . I like to cut my own roofs

bee1706

This is a hard one, and the truss people don't have all the knowledge to figure these . You are way ahead to just stick build the tie in.