How to install a full length rafter sister?

[vespa]

I have a 48" skylight installed (not by me) with 2x6 rafters 12"OC running thru it. I want to cut the 3 rafters that pass thru and one rafter tie. Roof is concrete tile 4:12 and rafters are 16' long. No collar ties, strongbacks, or anything else. 1970's Ventura CA. Sagging, bowing, etc.

So the plan is to:
1. Add new ties on either side (needed anyway since walls are bowing)
2. Sister both rafters
3. Install temporary headers
4. Cut 3 rafters and 1 tie
5. Install double 2x6 headers

So my first question is, how do I install a full-length rafter? With 12" centers there's no room to angle it in place. Do I need to make the top cut short or is there some trick? Should I wedge a scrap in there afterward to carry compression to the ridge beam? Does the sister really have to be full length or can I stop short of the ridge beam? Even if I could stop 1" short it would greatly simplify things.

Second question is how do I sister a straight 2x6 to a bowed 2x6? Seems I'd have to jack up the center of the old rafter from the floor but I hate to put that much upward force on the roof, am I going to lift things and cause a bunch of cracks and leaks? If the sister is not full length I could just jack the top end of the sister and add nails along the way so the sister bends to match the rafter rather than vice-versa.

Third question is how do I connect double headers to double rafters? Do I do it all with angle brackets? It'd be easy if I installed the headers first but I'd really prefer to have the roof intact while I apply all the stress of rafter sistering. Am I being paranoid about the stress of forcing a sister in there?

Thanks for any help you guys can give!

[pushkins]

So my first question is, how do I install a full-length rafter? With 12" centers there's no room to angle it in place. Do I need to make the top cut short or is there some trick? Should I wedge a scrap in there afterward to carry compression to the ridge beam? Does the sister really have to be full length or can I stop short of the ridge beam? Even if I could stop 1" short it would greatly simplify things.

Ideally the new rafter needs to sit on the top plate and up against the ridge beam, like you said this can be difficult especially with 12" centers. A standard method (without cutting the roof sheathing to get it in) is to cut short and block the ridge beam. Make the ridge block fit neatly between the two rafters on either side (12" centers so approx. 10 1/2") this will give you a rafter approx 1 1/2" shorter than the rest. Make sure you nail the ridge block in place to the existing ridge beam.

Second question is how do I sister a straight 2x6 to a bowed 2x6? Seems I'd have to jack up the center of the old rafter from the floor but I hate to put that much upward force on the roof, am I going to lift things and cause a bunch of cracks and leaks? If the sister is not full length I could just jack the top end of the sister and add nails along the way so the sister bends to match the rafter rather than vice-versa.

A lot depends on how much the sag is, again ideally there should be no sag especially on concrete/clay tile roofs, the weight is tremendous. There are a couple of ways to address this but should be done with caution. Support the rafter in issue with braces inside the attic space, put a cut in the underside of the rafter at the sag center point, this will allow easy lifting of the rafter without stress, then add your sister and nail in place The problem here is you are lifting the roof so care must be taken with the concrete tile.
There is very little point sistering a straight piece of lumber to a bent piece thus making both bent.

Third question is how do I connect double headers to double rafters? Do I do it all with angle brackets? It'd be easy if I installed the headers first but I'd really prefer to have the roof intact while I apply all the stress of rafter sistering. Am I being paranoid about the stress of forcing a sister in there?

The connection can be made with Simpson double hangers both on the upper and lower headers.

[traveler]

There is another way if you have walls to brace off of. You can build a truss under the rafters tight as you can using your walls for the truss to sit on you may have to block up off the wall for the truss to be above the ceiling joist. Find 2 rafters that are not sagged one on each end pull a string line tight from one to the other with a 1 1/2" block under the string where you can use a gauge of the same thickness . After you have the truss in and string in place. Take some hard wedges and drive between the rafter and truss gauging by the string. You may have to notch some of the rafters and put a scab on but they will move. I've done them on a 80 foot long span. Takes a lot of time crawling in and out of the attic. good luck

[JosephtRuri]

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