jacking up sagging rafters in an old attic?


Old 06-07-05, 08:40 AM
hapless newbie
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jacking up sagging rafters in an old attic?

There's been a bit of discussion on sagging rafters in the archives, but nothing on the specific problem I have, so here goes (I hope this is an attic, rather than a roofing question):

I'm working on an unfinished attic in a 75yr old brick house. After removing old batts of insulation from between the rafters I'm a bit troubled by the sagging in the roof deck. The home inspector noticed the sagging before we bought the place, noting that it was common due to age in these sorts of houses (fair enough, and he could't really see what I later found after tearing out the insulation). He did note that it would need addressing in the future, so I'd like to get to it sooner rather than later.

It's a simple hipped roof, and in the middle of the one face the rafters sag probably about 2"-4", and one has a clear stress fracture right near the roof deck (this is what the inspector couldn't see without digging under old mouldy fiberglass). The rafters are 2x4" spaced 24"oc, and the roof sheathing is 1x12" planks. From joist to ridge board the rise is roughly 9' over about 11', so the rafters are about 15' in length. No collar ties or kneewall.

My plan: reinforce the floor joists, then add some new 2x6" rafters to reinforce the existing structure, collar ties, skylights, a small gabled dormer over the stairs, and an outer kneewall with shelving, and storage space back to an inner kneewall (inuslation: I'll be doing high-density foam insulation right to the sheathing and into the soffits, i.e. a "hot roof", completely sealed with no venting).

My question: should I put the new rafters in place and then slowly (i.e. over several weeks) jack up the roof with jack posts (pushing on a temporary top plate across the rafters at the deepest point of the sagging)? Or is this a perilous strategy, especially with a roof that has presumably been slowly sagging over some 70 years?

I'm trying to find a structural engineer to help me out on this, obviously, but I thought I'd ask to see if anyone has tried jacking up an old roof with any success, and has any stories to share with a hapless newbie!
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Old 06-07-05, 09:01 AM
hapless newbie
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found online an engineering company that did what I have planned, but "Custom fabricated adjustable steel tie rods and fastened rods to bottom portion of rafters, running rods from one end of the width of the roof to the other to inhibit the bottoms of the rafters from pulling away from the walls of the house." (http://www.greatlakesbuildersinc.com/pages/6/)

the outward pressure at the base of the rafters had occured to me but I guess I hadn't really thought through how significant the outward force was likely to be.

... not sure why I hadn't found this link before I posted. uggh. Still, would like to hear from anyone who has done this, or had it done by qualified professionals. Is this something a do it yourself'er could jury rig, in anyone's professional judgement, or is that just courting disaster? (I'm guessing "disaster").
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