Roof Sagging Repair Calculator

Diagnose the cause of a sagging roof, determine the correct repair method from sistering to reframing, and estimate structural repair costs by sag severity and state

Describe the sag location, depth, and roof characteristics to determine the likely cause

Quick presets

ft

Count

27 pieces

24" spacing • 15.7 ft length

PRO

Professional Calculator

Line-by-line cost breakdown with regional pricing

sq ft

Estimated Project Cost

$12,643 – $17,526

Cost per sq ft

$8.42

Roof Area

1,792 sq ft

Cost Breakdown

Materials: $10,303 (68%)
Labor: $1,344 (9%)
Tear-Off: $2,688 (18%)
Disposal: $450 (3%)
Permits & Other: $300 (2%)

Detailed Breakdown

Materials$8,063 – $12,543
Unit cost$4.50 – $7.00/sq ft
Labor$1,344
Rate$75/sq × 1x complexity × 1x stories
Tear-Off$2,688
Layers1 layer(s)
Disposal/Dumpster$450
Permits$300
TOTAL$12,643 – $17,526
How to Use This Calculator
The Roof Sagging Repair Calculator helps homeowners and contractors diagnose the cause of a sagging roof, determine the correct repair method, and estimate the cost of structural repairs. Roof sag is a structural issue that ranges from cosmetic concerns to urgent safety hazards, and the correct repair depends entirely on accurate diagnosis.

Diagnosis tab: Start by identifying where the sag is located — mid-span sag (center of rafters) has different causes and repairs than ridge sag (ridge board failure) or localized sag (water damage). Enter the sag depth to classify severity: minor sag under 1 inch is often stable and cosmetic, moderate sag of 1-3 inches indicates active structural deflection, and severe sag over 3 inches may be at risk of failure. The rafter span and home age help narrow the cause. Homes over 50 years old with long rafter spans are most likely sagging from undersized rafters that were acceptable under older building codes but insufficient by modern standards.

Repair Method tab: Based on the diagnosis, select the repair approach. Sistering is the most common and least disruptive method for rafters that are intact but undersized. Post-and-beam adds a new support point for long spans. Jack-and-sister first corrects the sag position before reinforcing. Partial reframing is needed when the existing members are damaged beyond reinforcement. The calculator adjusts cost based on repair length, jacking requirements, and whether a structural engineer is needed for design and permits.

Cost tab: Material costs depend on whether you use standard dimensional lumber, engineered LVL beams, or steel. Hardware costs cover structural fasteners — through-bolts, structural screws, or Simpson connectors. Attic access difficulty is a major labor multiplier because tight crawl spaces with blown insulation dramatically slow the work. The state selector adjusts structural labor rates, which are typically $50-$100/hour — higher than general roofing labor because structural framing requires specialized skills.

The Formula
The roof sagging repair calculator uses these formulas:

Sistering Cost Lumber cost = repair length x lumber price per LF Dimensional: 2x8 = $1.50/LF, 2x10 = $2.00/LF, 2x12 = $2.75/LF Hardware cost = (repair length / 2) x fastener cost (bolt = $4, screw = $6, plate = $12) Labor hours = repair length x 0.5-1.0 hr/LF (varies by access difficulty) Access multiplier: Easy = 1.0, Moderate = 1.5, Difficult = 2.5 Total sistering cost = Lumber + Hardware + (Labor hours x structural labor rate x state multiplier)

Post and Beam Cost Beam material: LVL = beam length x $6-$12/LF, Steel = beam length x $10-$25/LF Posts: 2-4 posts at $50-$150 each (dimensional or steel) Hardware: post bases, beam saddles, connectors = $100-$300 Labor: 8-16 hours for beam installation Total post-beam cost = Beam + Posts + Hardware + Labor

Jacking Cost Minor (< 1"): $500-$1,000 (1-2 days jack rental + labor) Major (1-3"): $1,000-$3,000 (1-2 weeks gradual jacking + monitoring)

Engineering Cost Consultation: $300-$500 Stamped drawings: $500-$800 Site visits during repair: $150-$300 per visit

Total Repair Cost Total = Repair method cost + Jacking cost (if needed) + Engineering cost (if needed) State multiplier applied to all labor components
Example Calculation
Example: Moderate Ridge Sag Repair — Sister 6 Rafters + Post and Beam in Ohio

Tom notices a visible sag along the ridge line of his 1960s ranch home. A structural engineer inspects and finds the ridge board has dropped 2 inches because the original 2x6 rafters spanning 18 feet are undersized by modern code and the ridge board has no vertical support post.

Step 1: Diagnosis
• Location: Ridge line sag
• Depth: 2 inches (moderate)
• Span: 18 feet (2x6 is undersized for this span)
• Age: 60+ years
• Cause: Undersized rafters + unsupported ridge

Step 2: Engineer Recommendation
• Structural engineer fee: $500 (stamped drawings for permit)
• Repair plan: Sister the 6 most affected rafters with 2x10 lumber AND install a ridge support beam with a post to a bearing wall below

Step 3: Sistering 6 Rafters
• 6 rafters x 18 ft = 108 LF of 2x10 sistering lumber
• Lumber: 108 LF x $2.00/LF = $216
• Through-bolts: 54 bolts (every 2 ft) x $4 = $216
• Labor: 108 LF x 0.75 hr/LF (moderate access) x 1.5 = 121.5 hours
• Labor cost: 121.5 hours x $60/hr = $7,290

Step 4: Ridge Support Post and Beam
• LVL beam: 12 ft x $10/LF = $120
• Steel post with base plate: $150
• Hardware (saddle, base, fasteners): $120
• Labor: 12 hours x $60/hr = $720
• Subtotal: $1,110

Step 5: Jacking
• Minor jacking to lift ridge 1.5 inches gradually over 1 week
• Jack rental and monitoring labor: $800

Step 6: Total Cost
• Engineer: $500
• Sistering: $7,722
• Post and beam: $1,110
• Jacking: $800
• Permit: $150
• Total: $10,282

The repair eliminates the sag cause (undersized rafters and unsupported ridge) and restores the roof structure to meet modern code. Without the post-and-beam addition, sistering alone would slow but not stop future deflection because the fundamental problem of an unsupported 18-foot span would remain.

Frequently Asked Questions

What causes a roof to sag?
Roof sagging has five primary causes. The most common is undersized rafters — especially in homes built before modern building codes required engineered span tables. Rafters that are too small for their span will gradually deflect under the weight of roofing materials, snow, and their own dead load. Water damage from chronic leaks rots rafter wood and reduces its load-bearing capacity, causing localized sag. Overloading occurs when heavy materials like multiple shingle layers, solar panels, or HVAC equipment exceed the design capacity. Removal of a load-bearing wall during interior remodeling eliminates a support point, causing rafters to sag from the sudden increase in unsupported span. Finally, age-related lumber fatigue causes older homes to develop cumulative deflection over decades.
How much does it cost to fix a sagging roof?
Roof sag repair costs range from $1,000 to $15,000+ depending on severity and method. Minor sag repair by sistering a few rafters with dimensional lumber costs $1,000-$3,000 including materials, labor, and hardware. Moderate sag requiring jacking and sistering or adding a post-and-beam support runs $2,000-$5,000. Severe sag needing partial reframing of a section costs $5,000-$15,000. A full rafter replacement for widespread failure can reach $15,000-$40,000. Add $300-$800 for a structural engineer if required. These costs are for the structural repair only and do not include re-roofing the exterior, which may be needed if the repair requires removing shingles to access the framing.
Do I need a structural engineer for a sagging roof?
A structural engineer is recommended for any sag deeper than 1 inch and is required by most building departments for sag over 3 inches or any repair that modifies the structural framing system. The engineer determines the cause through inspection, calculates the required repair members using load tables and engineering analysis, and produces stamped drawings that serve as the repair plan and satisfy permit requirements. The fee of $300-$800 is a small investment compared to the cost of an incorrect repair that fails or a repair that does not address the actual cause. For minor cosmetic sag with an obvious cause like a single undersized rafter, an experienced framing contractor can typically handle the repair without engineering involvement.
Can a sagging roof be jacked back into position?
Yes, hydraulic or screw jacks can lift a sagging roof back toward level, but the process must be done gradually and with caution. Minor jacking of less than 1 inch can often be done in a single day by placing jacks under the sagging rafters and gently raising them. Moderate jacking of 1-3 inches should be done incrementally — typically 1/4 inch per day over 1-2 weeks — to avoid cracking interior plaster, separating drywall joints, or damaging the roof surface. Once the desired position is reached, the correction is locked in by sistering new rafters or installing permanent posts. Jacking more than 3 inches is rarely attempted because the existing shingles, flashing, and interior finishes cannot accommodate that much structural movement without damage.
What is rafter sistering and how does it fix a sagging roof?
Rafter sistering involves bolting a new rafter alongside an existing one to increase the combined cross-section and load capacity. The new rafter (called the "sister") is typically the same depth or one size larger than the existing member. It is placed tight against the existing rafter and connected with through-bolts or structural screws at 16-24 inch spacing. Sistering effectively doubles the load capacity of the repair area. For example, sistering a 2x8 with another 2x8 gives the combined section nearly the same capacity as a single 2x12. The sister must extend at least 3 feet past the damaged area on each end and bear on the same support points as the original rafter. Sistering costs $20-$40 per linear foot installed including lumber, hardware, and labor.

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