Estimates, Worked Examples and Checklists
Metal roof replacement estimate: sloping area, sheets, structure and GST worked out
Worked example of replacing a sloping sheet roof in India, from plan area to slope area, ridges, gutters, old-roof removal, a new steel frame option, GST and EMI.
To estimate a metal roof replacement, convert the plan area (what the roof covers on the ground, including overhangs) into the sloping area using the roof pitch, multiply by the per-sq-ft rate for the sheets and fixing, add the running-foot items (ridges, flashings, gutters), the removal of the old roof, and, if the frame is not sound, a new steel structure, then add GST. In the illustrative example below the same roof costs about ₹1.7 lakh if the frame can be reused and about ₹3 lakh if a new frame is needed, which is why an inspection of the structure comes before any comparison of sheets.
Part of how to read a roofing or façade quotation.
The roof in this example
A single-storey rear extension with a gable (two-sided) sheet roof:
- Building footprint: 20 ft wide × 30 ft long
- Overhang: 1.5 ft on each side at the eaves and 1 ft at each gable end
- Pitch: the ridge is 5 ft higher than the eaves over a half-width of 10 ft
- Existing roof: old corrugated sheets on a steel frame, some purlins rusted
Step 1: plan area including overhangs
- Width with overhangs: 20 + 1.5 + 1.5 = 23 ft
- Length with overhangs: 30 + 1 + 1 = 32 ft
- Plan area: 23 × 32 = 736 sq ft
Step 2: convert to sloping area
For a rise of 5 ft over a run of 10 ft, the sloping length of each side is √(10² + 5²) = √125 ≈ 11.18 ft, so the slope factor is 11.18 ÷ 10 = 1.118.
- Sloping area: 736 × 1.118 ≈ 823 sq ft
Flatter roofs have a smaller factor (close to 1), steeper roofs a larger one. Quotations should state whether the area is plan area or sloping area. A rate on plan area and a rate on sloping area are not comparable.
Step 3: sheets and fixing
Illustrative rate for colour-coated steel sheets with fasteners, supplied and fixed: ₹120 per sq ft of sloping area.
- 823 × 120 = ₹98,760
Ask the supplier to confirm thickness (and whether it is TCT or BMT), coating, profile and BIS marking. See metal roofing sheet cost per sq ft.
Step 4: running-foot items and removal
| Item | Quantity | Illustrative rate | Amount |
|---|---|---|---|
| Ridge cap | 32 ft | ₹250 per ft | ₹8,000 |
| Gutters with downpipes, both eaves | 64 ft | ₹300 per ft | ₹19,200 |
| Removal and disposal of old sheets | Lump sum | ₹10,000 | |
| Total | ₹37,200 |
If the old sheets are asbestos cement, removal needs more care: they should not be broken or cut dry, and disposal should follow your local body's instructions. That can add cost.
Step 5: Option A, reuse the frame
The fabricator replaces three rusted purlins (included in a small repair lump sum of ₹6,000) and reuses the rest.
| Amount | |
|---|---|
| Sheets and fixing | ₹98,760 |
| Running-foot items and removal | ₹37,200 |
| Purlin repair | ₹6,000 |
| Subtotal | ₹1,41,960 |
| GST 18% | ₹25,553 |
| Total | ₹1,67,513 |
Step 6: Option B, new steel structure
If the frame is badly corroded or undersized for the wind exposure, a new structure is needed. Illustrative rate for a new steel frame (trusses, purlins, bracing, primer and paint): ₹140 per sq ft of sloping area.
| Amount | |
|---|---|
| Sheets and fixing | ₹98,760 |
| Running-foot items and removal | ₹37,200 |
| New steel structure: 823 × 140 | ₹1,15,220 |
| Subtotal | ₹2,51,180 |
| GST 18% | ₹45,212 |
| Total | ₹2,96,392 |
The new structure is more than a third of the total. That is why the inspection of the frame, ideally with a structural engineer for anything larger than a small shed, decides the budget.
Step 7: if you finance it
Suppose you pay ₹46,392 from savings and borrow ₹2,50,000 for Option B at an illustrative 11% a year over 2 years. The EMI is about ₹11,652, total repayment about ₹2,79,650, so interest is about ₹29,650 plus any processing fee. Try your own figures in the repair loan EMI calculator.
Adding insulation
If the room below will be used daily, an underlay or insulation, or an insulated sandwich panel instead of single sheets, reduces heat and rain noise. In this example, at an illustrative ₹80 per sq ft more for a sandwich panel, the extra is 823 × 80 = ₹65,840 before GST. Whether it is worth it depends on how the room is used.
Before you sign
- Confirm permission is not needed, or obtain it; see building permission for roof and façade alterations
- Agree the area basis (sloping area) and the measurement at the end
- Specify sheet thickness, coating, profile and BIS marking
- Ask for wind-uplift details in cyclone-prone areas: purlin spacing, fastener spacing, anchors
- Use the contract and warranty checklist
FAQ
How many sheets will I need?
It depends on the sheet's effective cover width and the sloping length, plus end laps. The supplier works this out; you only need to check the area and the length of the sheets.
Can I keep the old sheets on and overlay new ones?
Overlaying is sometimes done on sound frames, but it adds load and can trap moisture. Ask the fabricator and, for older frames, an engineer.
How do I reduce leaks at screws?
Correct fastener spacing, sealing washers not over-tightened, fixing at the crest of the profile where the system calls for it, and long single-length sheets to reduce end laps.
Should I consider a repair instead?
If only a few sheets are damaged and the frame is sound, a repair can be enough. See repair or replace a roof.
Sources and where to check
- Bureau of Indian Standards — IS 277 and IS 14246 and licence status
- Ministry of Steel — quality control orders for steel products
- CBIC GST portal — GST rate on works contracts
- Reserve Bank of India — Key Facts Statement rules for retail loans
