Estimates, Worked Examples and Checklists
Terrace waterproofing estimate: measuring the area and working out the cost
Step-by-step worked example of a terrace waterproofing estimate in India, measuring flat area, parapets, laps and wastage, with preparation, GST and payment stages.
To estimate terrace waterproofing, measure the flat area, add the parapet upturn and any tank stands, kerbs and ducts, add an allowance for laps and wastage if a sheet membrane is used, multiply by the rate for the system you have chosen, then add the lump sums for preparation, debris removal and access, and finally GST. The worked example below takes a typical 30 ft × 40 ft terrace through each step with illustrative rates, so that you can repeat it with your own measurements and the rates from your quotations.
Part of how to read a roofing or façade quotation.
Step 1: measure the flat area
Measure inside the parapets, wall to wall.
- Terrace: 30 ft × 40 ft = 1,200 sq ft
- Deduct the staircase headroom (mumty) footprint: 10 ft × 8 ft = 80 sq ft
- Deduct the water tank platform if it will be treated separately: 6 ft × 6 ft = 36 sq ft
- Flat area to treat: 1,200 − 80 − 36 = 1,084 sq ft
Keep a simple sketch with dimensions. If the terrace is irregular, split it into rectangles and add them up.
Step 2: add vertical surfaces
Waterproofing is turned up the parapet and around the mumty walls so that water cannot get behind the membrane at the corner.
| Surface | Length | Height treated | Area |
|---|---|---|---|
| Parapet inner face | 2 × (30 + 40) = 140 ft | 1 ft | 140 sq ft |
| Mumty walls (outside) | 2 × (10 + 8) = 36 ft | 1 ft | 36 sq ft |
| Tank platform sides and top | Lump measurement | 50 sq ft | |
| Total vertical and extras | 226 sq ft |
Total area to treat: 1,084 + 226 = 1,310 sq ft.
Step 3: allowance for laps and wastage
For a liquid membrane, wastage is in the material consumption, and contractors usually quote on the measured area. For a sheet membrane (APP or SBS), laps between rolls and cutting waste mean more material than the measured area; some contractors quote on the measured area and build the waste into the rate, others add a percentage. Ask which. In this example the owner chooses a PU liquid membrane, so no separate wastage is added.
Step 4: apply the rate
Illustrative rate for PU liquid membrane, primer + 2 coats + UV-resistant top coat: ₹105 per sq ft.
- 1,310 × 105 = ₹1,37,550
Step 5: add preparation and lump sums
The terrace has an old, cracked screed in one corner and some hollow areas.
| Item | Illustrative amount |
|---|---|
| Breaking out 150 sq ft of hollow screed and relaying with slope | ₹12,000 |
| Crack repair with polymer mortar, 60 running ft | ₹4,800 |
| New PVC collars at 4 rainwater outlets | ₹3,200 |
| Debris removal from the terrace to the ground | ₹4,000 |
| Total lump sums | ₹24,000 |
Step 6: add GST
- Subtotal: 1,37,550 + 24,000 = ₹1,61,550
- GST at 18% (registered contractor, supply and apply): 1,61,550 × 0.18 = ₹29,079
- Total: ₹1,90,629
Step 7: plan the payment stages
| Stage | Share (illustrative) | Amount |
|---|---|---|
| Advance for material on start | 30% | ₹57,189 |
| After preparation and primer, inspected | 30% | ₹57,189 |
| After final coat and water test | 30% | ₹57,189 |
| Retention after the first monsoon with no leak | 10% | ₹19,062 |
| Total | 100% | ₹1,90,629 |
The retention and payment calculator shows what is due at completion after the advance and stage payments.
The water test
Before any protective layer goes on, a common check is a flood or ponding test: outlets are temporarily plugged and the terrace is flooded with a shallow layer of water for the time the product's instructions allow, typically a day or more, while someone checks the ceilings below. Write the test date and result into your records. A failed test found now costs far less to fix than a leak after the monsoon.
Doing it with a different system
| System (illustrative rate) | 1,310 sq ft × rate | + lump sums | + GST 18% | Total |
|---|---|---|---|---|
| Cementitious coating (₹40) | ₹52,400 | ₹76,400 | ₹13,752 | ₹90,152 |
| Elastomeric coating (₹65) | ₹85,150 | ₹1,09,150 | ₹19,647 | ₹1,28,797 |
| PU membrane (₹105) | ₹1,37,550 | ₹1,61,550 | ₹29,079 | ₹1,90,629 |
Use the roof and façade cost estimator to run your own numbers. For how the systems differ, see terrace waterproofing cost per sq ft. Before signing, go through the contract and warranty checklist.
FAQ
Why add the parapet when the leak is in the middle of the ceiling?
Water often enters at the slab–parapet junction and travels along the slab before it shows on the ceiling. A membrane that stops at the corner leaves the weakest point untreated.
Should the mumty roof be waterproofed too?
Yes, if it is exposed. It is often forgotten and quoted later as an extra. Measure it separately.
What if the contractor's area is larger than mine?
Ask for their sketch. Differences usually come from upturn height, tank platforms or the mumty roof. Agree the final measurement method in the contract.
Is the 30/30/30/10 split a rule?
No. It is an illustrative pattern. Agree whatever stages make sense, but keep payments linked to work you can inspect.
Sources and where to check
- Bureau of Indian Standards — standards for waterproofing materials and the National Building Code
- CBIC GST portal — GST rate on works contract services
- National Consumer Helpline — if a contractor refuses to honour a written test or warranty
