Sagging dormer rafter reconstruction

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Old 08-29-10, 07:18 PM
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Location: Royal Oak, MI
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Sagging dormer rafter reconstruction

This old house has everything done differently for roof structure than what I read I should do if I was building from scratch. Now that I'm in to it, here's what I'm seeing:

9/12 pitch roof over the main part of the house goes up to the dormer, which is part of an addition that was done to extend the house an extra 10 feet or thereabouts. The 9/12 pitch of the main house is 2x4 framed on 24" spacing. The dormer has a shed roof with a 4/12 pitch, and is framed with 2x4 on 16" spacing. These rafters are rotted and sagging, as is the original 1x10 or whatever the width is of the original sheathing, OSB has been laid over this.

But there's more. Ceiling joists in the original part of the house are parallel with the rafter. Ceiling joists in the addition are perpindicular. None of the rafters in the house have ridge beam.

The span of the dormer rafters is a little over 12' from where they are supported on the outside wall to where they meet the rafter from the other side of the dormer at the roof ridge.

I already have sheathing torn off on one side and I plan to get the sheathing torn off on the other tommorrow. I have 2x10 boards on hand to build new rafters. I'm pretty much assuming that I'm doing this as if it was from scratch, including putting in a ridge board with temporary supports until the rafters are in place.. Of course, this won't have any other ridge board from the rest of the house to tie in to, but I'm assuming that it's fine to just put ridge board in where I'm doing repair and leave the rest be, tieing the ridge board into the existing rafters I'm not repairing.

On the bottom, I'm planning to add something to run from one side to the other as if it were a ceiling joist, but just put it across the top of the existing ceiling joists... Probably just 2x4 since it's not carrying ceiling load, just that it's there to react the spread load of the rafters. Perhaps add lag bolts at the joints or some sort of appropriate strong-ties if I can find something appropriate.

Am I going about this right?
 
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Old 09-07-10, 01:51 PM
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Well, things turned out differently now that it's done, so here's my report on what I ended up doing:

Once I got back into it after arriving with the 10 2x10 boards for rafters, and with 6 2x6 boards for added joists, I tore off additional material so I could remove the old rafters and assess the condition of the headers and determine whether they were adequate to support the larger rafters.

What I found was that the top boards of the outer wall also were rotted, under neath them were studs with no rafter, these studs had beneath them a 1x6 board over the window.

So I decided not to rely on the outerwall, and went with changing roof styles to a gable roof. I added a ridge board at the top, headers for the gable ridge, new top boards on the side stud wall, new top boards on the end wall over the window.

So after doing the framing on evenings through the week and taking last Tuesday off, I finished sheathing over the weekend and covered it with Grace Ice and Water shield on the valleys edges with the rest covered with tar paper. Before closing the last openning with sheathing, I used it to get my shingles up from the scafolding inside so I wouldn't have to haul all of it up a ladder. (Mostly I didn't get delivery because work-related travel meant while I waited for reimbursement, I didn't have the money to buy the entire 39 bundles of shingles at once, so I bought 12 then 15 then 12.)
 
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