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Sagging / Snapping Trusses

Jacob Russell's picture

I've got a problem I'm looking for a bit of help with -

My house (100 years old) has 2x4 rafters supporting the roof  that are entirely unsupported in the middle.  They run a 14' angled span from their base to the peak, where they meet in a butt joint.  Aside from a few cross and vertical supports added by previous owners of my home, there is no interior support for those rafters.

Because of that, a large number of those rafters have flexed fairly significantly over time.  A smaller number (half a dozen) located near the middle of the roof have snapped or cracked.  I'm planning on sistering 2x6's to the snapped rafters full length so the roof is supported entirely by the 2x6's at those locations.  I'm also planning on adding vertical supports every third or fourth rafter to the horizontal rafters below them, directly above interior walls for added support, although I expect the double 2x4 (true size, they're old) and better supported horizontal rafters to handle the load better.

All of that I'm in good shape about, my question however is twofold as follows:

Should I attempt to correct the flex of the roof at all when sistering / adding vertical supports?

If yes to the first question, what method should I use to straighten the rafters while I put the sister boards / vertical supports in place?

I appreciate the help!

Minor point:  What you have (post #207243, reply #1 of 4)

Minor point:  What you have is not commonly referred to as a "truss" (though technically any structural component with triangular sections is).  The diagonal pieces are "rafters" and the horizontal pieces, if at the ceiling level, are "joists". 

Some info about the composition of the roof would be helpful in further discussion.  Presumably there is "skip" sheathing (you can see large gaps through the boards from below), topped with some sort of shingles.  Worst case you have wood shingles topped by a couple of layers of asphalt (in which case a total reroofing is probably in order).  Or you may have one or two layers of shingles on the skip sheathing, or, if you're lucky, plywood (with or without the skip sheathing) and a single layer of asphalt shingles.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Your roof is not uncommon at (post #207243, reply #2 of 4)

Your roof is not uncommon at all. Usually the 2 X 4's are larger than the ones we use today but are still too small for the span. Adding supports may opr may not help as you may find the joists are also 2 X 4" s and not large neough to carry the load. We repair roofs like yours by removing the shingles, usually more than one layer, then pull up the sheathing, usually 1x material, in the middle to give us better access to the attic space. We cut 2 X 6's or 2 X 8's depending on the span and install them next to each old rafter. We cut the old rafter in half and push the roof up to a straight line. Typically we'll have 3 or 4 rafters cut at the same time so we can move the roof.up. We can usually do half the roof one day and the other half the next including dry-in.

If you don't take the sag out you're going to find it close to impossible to get the new rafters in.

Florida Licensed Building Contractor, 40 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

+1 (post #207243, reply #3 of 4)

In addition to what Florida said, I would build a 2x6 L accross the joist beneath the center of the rafter span.  Nail one 2x6 flattothe joist like a plate and the other standind on edge , nailed to the side of the plate. (we always called that arrangement a "hogs troff".)  In new construction that L helps keep the joist from sagging, but in your case it can serve as a lifting base that will help spread the load as you jack the rafters up.  When you have the roof back in plane the L beomes the plate to set the vertical braces on  with the upright side giving you a good horizontal surface to nail through, rather than just toe nailing.

use a jig? (post #207243, reply #4 of 4)

I'm not an expert, but I was once surprised to be able to pull a snapped rafter back into line just by screwing the pieces together.  It had a longitudinal crack in the middle of the span, and a lag screw across the crack was all it took.  I sistered it after that.

Sagged wood might be harder, but a jig sistered along the rafter with a lag on each end would probably be strong enough to push the middle straight.  A simple L-shaped beam that catches the bottom of the rafter, with lags to pull it up flush under each end, would be a cheap experiment.