A sagging ridge or roofline usually means rafters are spreading because ceiling joists or ties are missing or cut, framing is undersized for the span or load, trusses have been altered or damaged, or the roof has been overloaded by snow or extra layers of roofing. Heavy snow on a roof that is visibly sagging is urgent. Truss uplift, a winter ceiling gap, is not a structural failure.
- Rafters push outward at the walls; ceiling joists or rafter ties hold them together. Cut the ties and the ridge drops.
- Never cut, notch or remove truss webs or chords; repairs need the truss designer's or an engineer's detail.
- Truss uplift opens winter gaps at interior walls and closes in summer; it is not sag.
- Design snow loads vary by location; the NBC climatic tables list values for over 600 Canadian locations.
- Heavy snow plus new sag, cracking sounds or sticking doors upstairs means clear the rooms below and call for help.
01 /How does a roof carry its load?
Houses in Canada are roofed in one of two ways. In a stick-framed roof, pairs of rafters lean against a ridge board, and the ceiling joists or rafter ties across the bottom tie the rafter feet together so they cannot push the walls apart. In a truss roof, factory-built triangular trusses combine rafters, ceiling joists and web members into one engineered unit, usually spanning from exterior wall to exterior wall.
The triangle is what makes both work. Snow and roofing push the rafters down; because they are sloped, they also push outward at the top of the walls, a force called thrust. A continuous tie across the bottom of the triangle resists that thrust. Remove or cut the tie, for a vaulted ceiling, an attic conversion or a new duct, and the rafter feet slide out, the walls lean, and the ridge drops. A roof with a structural ridge beam carried on posts is the exception: there the ridge carries the load itself and rafter ties are not needed in the same way.
02 /Why does a roofline sag?
A sagging roofline usually has one of these causes, and the shape of the sag often tells you which.
| What you see | Likely cause |
|---|---|
| Ridge dips in the middle, walls lean out at the top | Rafter spread: ties missing, cut, too few or poorly connected |
| Wavy roof plane between rafters | Undersized or overspanned rafters, thin or delaminated sheathing |
| Local dip over one area | Broken or rotted rafter, cut truss member, leak damage |
| Sag after a renovation | Ceiling joists removed for a vault, a bearing wall removed, trusses altered |
| Sag that appeared after a heavy winter | Snow overload, drifting, or rain on snow |
| Gradual sag on an old roof with several shingle layers | Added dead load plus decades of creep in undersized framing |
Measure a ridge sag by sighting along it from the ground at each end, or in the attic by pulling a string from gable to gable under the ridge. Measure rafter sag the same way from the ridge to the wall plate. The Floor Sag Calculator works for a rafter or ceiling joist span too, but roof members have different design criteria, so treat the result as a flag.
03 /What goes wrong with roof trusses?
Trusses are engineered as a system. Every web and chord is sized for its force, and the metal connector plates (gusset plates) at each joint are designed for the loads at that joint. That efficiency is why altering them is dangerous.
- Cut or removed webs. Webs cut to make attic storage, run ducts or fit a furnace or a skylight. A single missing web can overload the rest.
- Damaged or loose gusset plates. Plates pulled out by rot, leak damage, fire, rough handling during construction, or long-term movement.
- Notched chords. Top or bottom chords notched for wiring or plumbing.
- Overloading the bottom chord. Heavy attic storage, equipment or a floor built on trusses not designed for it.
- Missing bracing. Permanent lateral bracing specified by the truss designer, omitted or removed, can let slender webs buckle.
- Girder truss or bearing problems. A girder truss carrying other trusses, or a truss bearing on an interior wall that has been altered.
Truss repairs are not improvised. The usual route is a repair detail from the truss manufacturer's designer or a structural engineer: plywood or lumber gussets glued and nailed in a specified pattern, replacement webs, or added members. Photograph the truss stamp or label if there is one; it helps trace the design.
04 /Is it truss uplift or real sag?
Truss uplift is a seasonal, moisture-driven movement, not a failure. In winter, the top chords of trusses sit in the cold, moist attic air and gain moisture and length, while the bottom chords are buried in warm, dry insulation and shrink slightly. The truss arches upward, lifting the ceiling off interior partition walls and opening a crack or gap at the wall-ceiling joint. In summer the gap closes.
Uplift is common in cold climates. It is managed by attaching ceiling drywall near partitions with clips that let the ceiling float, and fastening crown mouldings to the ceiling only, so the gap hides behind the trim. Do not screw trusses down to partitions to stop it; the force can pull the partition up or crack the ceiling.
05 /How much snow can a roof hold?
Roofs in Canada are designed for the snow load at their location. The National Building Code's climatic data give design snow load values for hundreds of locations, and the NRC's Design Value Explorer presents climatic design values, including design snow loads, for over 600 locations in the same format as the code's climatic table. The designer then adjusts for roof slope, shape, exposure and drifting. The load that matters is weight, not depth: dense, wet or wind-packed snow and ice weigh far more than the same depth of fresh powder, and rain falling on a snowpack adds weight quickly.
- Drifts form against higher walls, dormers, parapets and on the downwind side of the ridge, often much deeper than the snow elsewhere.
- Valleys and lower roofs collect sliding snow from above.
- Flat and low-slope roofs hold snow longer than steep ones.
- Older, altered or damaged roofs may have less capacity than they were built with.
Snow removal is dangerous. Use a roof rake from the ground if you can reach safely, leave a thin layer to protect the shingles, keep clear of falling snow and power lines, and hire a crew with fall protection for anything else. Ice dams and attic moisture are covered in depth by our sister site CalgaryAtticRain.ca.
06 /How are sagging roofs and damaged rafters repaired?
- Assess from the attic
An engineer or experienced framer checks rafter and truss condition, ties, bearing, moisture, rot and alterations, and measures the sag.
- Shore if needed
Temporary posts to the ceiling joists or a strongback under the ridge carry load while repairs are made, especially in winter.
- Restore the ties
Install rafter ties or ceiling joists at the specified spacing and connections; sometimes walls are pulled back in gradually with cables before the ties are fixed.
- Strengthen members
Sister cracked or undersized rafters, add purlins and struts to bearing walls, or install a ridge beam on posts where the design calls for it.
- Repair trusses to a detail
Gussets, replacement webs or added members exactly as the truss designer or engineer specifies.
- Fix the cause
Leaks, attic moisture, removed walls or excess roofing layers; strip old layers at the next reroof.
| Repair | Typical planning range |
|---|---|
| Engineer's attic assessment and letter | $500–$1,500 |
| Sister a few rafters or add rafter ties | $1,500–$6,000 |
| Truss repair to an engineered detail | $1,000–$5,000 |
| New ridge beam with posts and footings | $8,000–$25,000 |
| Professional roof snow removal | $300–$1,200 per visit |
See the structural repair cost guide for more ranges, and send a request with photos from the attic hatch and of the roofline from the street.
FAQQuestions people ask
Is a sagging roof dangerous?
It can be. An old, stable sag may be a long-term repair item, but new sag, sag that is growing, or any sag under a heavy snow load should be assessed urgently, with people kept out of the rooms below.
Can I cut a truss web to make attic storage?
No. Truss members are engineered as a system and cutting any of them can overload the rest. Alterations need a design from the truss manufacturer's designer or an engineer.
Why does a crack open between my ceiling and wall every winter?
Most often truss uplift: the trusses arch slightly in winter as their top chords take on moisture in the cold attic. It closes in summer and is not a structural failure.
How do I know my roof's snow load capacity?
The design snow load for your location comes from the building code's climatic data, adjusted for roof shape and exposure. An engineer can check whether your roof, as built and as altered, meets it.
Should I shovel snow off my roof?
Only if you can do it safely from the ground with a roof rake. Climbing on a snowy roof is dangerous; hire a crew with fall protection. Remove snow evenly and leave a thin layer.
What does it cost to fix a sagging roof ridge?
Adding rafter ties or sistering rafters often falls in the $1,500–$6,000 planning range for 2026; a new ridge beam with posts and footings can run $8,000–$25,000, plus engineering.