Sagging Garage Roof Fix — Sister Rafters, Add Ties, Reroof
The old detached garage has a sway in the ridge you can see from the street, and the walls lean out at the top. Here is how that roof gets straightened and held.
A sagging garage roof fix means shoring the roof, jacking the ridge and rafters back up, sistering new lumber to the old rafters, and adding collar ties or ceiling joists so the walls stop spreading. Then the garage is reroofed over solid decking. JMB Builders does this on older detached garages across the Twin Cities and north metro, priced per project from photos, with an engineer when the damage calls for one.
- Priced per project, framing and reroof together
- Shored, jacked, sistered and tied
- Engineer when walls or beams have failed
- Licensed MN contractor BC809005
Most sagging garage roofs are old stick-framed roofs with rafters that were small for the span, no ceiling joists or only a few, and decades of snow load. Without ties across the bottom of the rafters, the roof pushes the tops of the side walls outward, the ridge drops, and the roof takes on a swayback. Layers of shingles added over the years make it heavier.
The fix goes in order. Temporary posts and a beam shore the ridge and rafters. The roof is jacked up slowly, a little at a time, and the walls are pulled back toward plumb with a come-along where they have spread. New rafters are sistered alongside the old ones full length, and collar ties or ceiling joists go in to tie the walls together. Then the old shingles come off, bad decking is replaced, and the roof gets new underlayment and shingles.
Not every garage is worth it. When the rafters are rotted, the sill plates are gone, or the slab has heaved and cracked, rebuilding the roof or the whole garage can make more sense. When a ridge beam has failed or the walls have moved far, an engineer designs the repair. Structural work like this usually needs a building permit, and JMB pulls it.
What you supply
- Photos of the garage from each side and from the street showing the sag
- Photos inside looking up at the rafters, ridge and any ceiling joists
- The garage size and roughly how many shingle layers are on it
- A clear floor under the roof for shoring posts on the day of the work
What JMB supplies
- Temporary shoring posts and beams, and jacking of the ridge and rafters
- Wall pull-back where the tops of the walls have spread
- New rafters sistered full length and fastened to the old ones
- Collar ties or ceiling joists, and hurricane ties where the rafters bear on the walls
- Tear-off, decking repair, underlayment, drip edge and new shingles
- Building permit when the job needs one, and an engineer's design when the damage calls for it
- Debris hauled away and photos of the framing before it is covered in your job folder
How it is priced: per project
Quoted in writing from your photos within one business day. What moves the number:
- Garage size and rafter span
- How far the ridge has dropped and the walls have spread
- Number of rafters sistered and ties added
- Rot in rafter tails, top plates or sill plates
- Shingle layers to tear off and decking to replace
- Whether an engineer's design is needed for a failed beam or moved walls
Photos from inside looking up at the framing tell us the most. A tape measure across the tops of the side walls at each end and at the middle shows how far they have spread.
How long it takes
Most one- and two-car detached garages: two to four days for shoring, sistering, ties and the reroof. Longer when rot repair or an engineer's detail is involved.
How your project works
- 1
Look and decide
From your photos, we say whether the roof can be straightened or should be rebuilt, and whether an engineer is needed first.
- 2
Shore and jack
Temporary posts and a beam under the ridge and rafters, then the roof is raised a little at a time and the walls pulled back toward plumb.
- 3
Sister and tie
New rafters fastened alongside the old ones, collar ties or ceiling joists across, and ties at the walls so the roof holds its shape.
- 4
Reroof
Old shingles off, bad decking replaced, membrane at the eaves, underlayment, drip edge and new shingles.
- 5
Inspect and photos
Permit inspection when one was pulled, shoring out, and photos of the framing and the finished roof in your job folder.
Sagging Garage Roof Fix — Sister Rafters, Add Ties, Reroof: questions, answered
Why is my detached garage roof sagging in the middle?
Usually because the rafters are small for the span and there are too few ceiling joists or collar ties holding the walls together. Snow load pushes the rafters down and the tops of the walls out, and the ridge drops in the middle. Extra layers of shingles and rot at the rafter tails make it worse.
How do you fix a sagging garage roof by sistering rafters?
The roof is shored with temporary posts, jacked back up slowly, and new rafters are fastened full length alongside each old one. Collar ties or ceiling joists go in so the walls cannot spread again. Sistering without the ties treats the symptom; the ties are what stop the sag from coming back.
Can a sagging garage roof be jacked back up?
Often, but slowly. Old framing that has sat in one shape for years is raised a little at a time so nothing cracks, and it may not come all the way back. The goal is a straight, supported roof, not a perfect ridge line. When a beam has broken or walls have moved far, an engineer designs the repair first.
Do I need a permit to fix a sagging garage roof?
Structural roof repair like sistering rafters and adding ties usually needs a building permit, and many cities want one with any reroof. JMB pulls the permit when the job needs one and schedules the inspection. Rules vary by city, so we check yours when we quote.
Is it worth fixing an old garage roof or should I tear it down?
Fix it when the walls, sill plates and slab are sound and the problem is undersized or untied rafters. Rebuild the roof, or the garage, when the rafters are rotted through, the sills are gone, or the slab has heaved badly. We tell you from the photos which one you are looking at before you spend anything.
Should I reroof the garage at the same time as the rafter repair?
Yes. The shingles have to come off to straighten the roof plane anyway, and the decking over a sagged roof is often cracked or delaminated. Doing both in one job means the new shingles go on a flat, solid deck and nobody pays for the tear-off twice.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners and for the builders and dealers we sub for. Counts come straight from our job log; no customer names.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
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Read morePage reviewed September 26, 2026 · JMB Builders LLC, MN license BC809005 · All roofing pages
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