Inputs
Example Data Table
| Scenario | Wall length (m) | DPC width (m) | Runs | Overlap (%) | Waste (%) | Total area (m²) | Rolls (0.30×30m) |
|---|---|---|---|---|---|---|---|
| Small room perimeter | 24 | 0.23 | 1 | 5 | 5 | 6.077 | 1 |
| Plinth wall run | 85 | 0.30 | 1 | 7 | 6 | 28.115 | 4 |
| Long boundary wall | 160 | 0.23 | 1 | 8 | 7 | 42.296 | 5 |
Example totals assume roll area = 9.0 m² per roll (0.30×30m), rounded up.
Formula Used
- Net Area (m²) = Wall Length × DPC Width × Runs
- Lap Area (m²) = Net Area × (Overlap% / 100)
- Waste Area (m²) = (Net Area + Lap Area) × (Waste% / 100)
- Total Area (m²) = Net Area + Lap Area + Waste Area
- Roll Area (m²) = Roll Width × Roll Length
- Rolls Needed = Ceiling(Total Area / Roll Area)
- Adjusted Run Length (m) ≈ Wall Length × Runs × (1 + (Overlap%+Waste%)/100)
How to Use This Calculator
- Measure the full wall run length where the membrane will be placed.
- Enter the DPC width based on your wall thickness and detailing.
- Set overlap and waste to match your site practice and complexity.
- Confirm your roll width and roll length from the product data sheet.
- Add either a roll price or an area price to estimate cost.
- Press Calculate to see results immediately below the header.
- Use the CSV/PDF buttons to share quantities with procurement.
Professional Article: Damp Proof Course Planning
1. Purpose of a damp proof course
A damp proof course (DPC) is a continuous barrier placed within masonry to interrupt rising moisture. Correct quantity planning supports consistent laps, clean junctions, and full-width coverage at plinth level and below floor buildup. Under-ordering often leads to patchwork joints, while over-ordering ties up budget and storage space.
2. Measuring wall runs correctly
Start with the total wall run length that receives the membrane, including returns and short nibs. Measure centreline for simple walls, and add extra for steps, corners, and intersections. Where multiple leaves or separate wall segments exist, treat each as a separate run and sum lengths for a single total input.
3. Selecting an effective width
DPC width should match the wall thickness and detailing. A common practice is to allow a small overhang each side to prevent capillary bridging. Where external renders, cavity trays, or insulation boards are present, confirm the required projection and any compatibility notes from the product data sheet.
4. Overlaps and site allowances
Overlaps are required at roll ends, junctions, and complex corners. This calculator uses an overlap percentage as a practical allowance. Use higher overlaps for heavily segmented walls, numerous openings, or when multiple trades may disturb the strip during installation.
5. Waste factors and handling losses
Waste covers trimming, damage, and offcuts. Tight working spaces, windy conditions, and imperfect substrates can increase losses. For controlled indoor work, 3–5% may be enough. For exposed or complex works, 7–12% is often more realistic.
6. Converting area into rolls
Membrane procurement is typically by roll. Roll coverage equals roll width multiplied by roll length. The required roll count is the total area divided by roll area, rounded up. Rounding up prevents shortfalls caused by partial rolls and allows for continuity without unplanned joints.
7. Worked example with project data
Example: wall length 85 m, DPC width 0.30 m, runs 1, overlap 7%, waste 6%, roll size 0.30×30 m. Net area is 25.500 m². With overlaps and waste, total area is about 28.115 m². Each roll covers 9.000 m², so you need 4 rolls.
8. Quality checks before installation
Verify that the substrate is level, remove mortar snots, and maintain continuity at corners and junctions. Keep the strip flat without folds, and avoid bridging across cavities. Record the calculated quantities, roll batch numbers, and installation dates to support inspection and moisture-control documentation.
FAQs
1) What DPC width should I enter?
Enter the membrane width that will fully cover the wall thickness and required projection. Many projects allow a small overhang each side to avoid capillary bridging. Confirm your detail drawings and product sheet.
2) Should I price by roll or by area?
Use price per roll when you purchase packaged rolls. Use price per m² when suppliers quote installed or supply rates per area. If both are entered, the calculator prioritizes roll pricing.
3) How do I choose overlap and waste percentages?
Overlap covers joints and junctions; waste covers cutting and damage. Simple straight walls often use 3–5% each. Complex layouts, many corners, or exposed handling may justify 7–12%.
4) Why does the roll count round up?
Membrane is supplied in whole rolls, and partial-roll availability varies. Rounding up prevents shortage and reduces the risk of extra joints or discontinuities during installation.
5) Can I use this for multiple wall segments?
Yes. Add the lengths of all segments that receive DPC and enter the total wall length. If widths differ by wall type, calculate each width separately and sum the required rolls.
6) What if my DPC is measured in linear meters?
Use the “Adjusted run length” output as a practical allowance for laps and waste. It approximates total strip length across all runs, helpful for linear-meter pricing or cutting plans.
7) Does this cover vertical DPM or cavity trays?
No. This tool focuses on horizontal DPC strips. For vertical membranes, cavity trays, or tanking, quantities depend on different areas and detailing. Use dedicated calculations for those systems.
Good detailing stops moisture and saves costly repairs later.