Before quoting a bathroom or kitchen renovation you need to answer two questions: which pipe material to use in each section and how many metres to really buy over what the plan measures. Getting both right avoids running short halfway through the job or buying fittings that don't match.
Which pipe material to use by section
There isn't one single "correct" material: each fits best in a given type of installation.
- PEX — the current standard for hot and cold water in new builds, especially under underfloor heating (with EVOH oxygen barrier). It isn't soldered: joints are made with mechanical fittings, which lowers the risk of leaks compared with copper.
- Multilayer (PEX-aluminium-PEX) — keeps its shape once bent by hand, so it saves fittings on exposed runs. Not suitable outdoors without UV protection.
- Copper — the traditional material for hot water and heating, soldered with a torch. It remains the reference when renovating old buildings where copper already exists.
- Pressure PVC — rigid solvent-welded pipe, usual for irrigation or swimming-pool connections. Not suitable for hot water: it deforms above 60°C.
- Waste PVC — only for gravity drainage, never for pressurised water.
- Polyethylene (PE) — the standard for buried service connections from the mains to the home; always laid below the frost line (minimum 60–80 cm deep).
What diameter each appliance needs
For waste pipes, the diameter depends on what is being drained: 32 mm for washbasin and bidet, 40 mm for shower and sink, 50 mm for bath and WC with a horizontal outlet. On main stacks and collectors 110 mm is used. Always keep a minimum fall of 2% (2 cm per metre) to avoid blockages.
What margin to add over the real measurement
The margin depends on the material and the type of installation, because each behaves differently on bends and cuts:
| Material | Concealed | Exposed | Outdoor |
|---|---|---|---|
| PEX | +12% | +8% | +10% |
| Multilayer | +10% | +6% | +10% |
| Copper | +12% | +10% | +10% |
| Pressure PVC | +10% | +8% | +6% |
| Polyethylene (PE) | +10% | +10% | +6% |
PEX carries a higher margin when concealed than multilayer because its minimum bend radius is larger: on runs with many turns more length is lost in the bends.
Worked example
For a hot and cold water distribution in PEX of 25 metres measured on the plan, concealed in conduit:
- Measured length: 25 m
- 12% margin for bends and minimum radius: 3 m
- Total to buy: 28 m
This is the same calculation the PresupuestoYa calculator applies automatically for the Plumber trade, with a line-by-line breakdown ready for the quote.
What capacity of electric water heater each home needs
Another common question in a renovation is the size of the water heater or storage cylinder. As a reference by simultaneous demand:
- 30–50 L — 1 person
- 80 L — 2–3 people
- 100–150 L — large family or home with several bathrooms
The stored water must be kept at a minimum of 60°C to prevent legionella, and always fit a safety valve on the cold water inlet of the heater.
Frequently asked questions
What water pressure is normal in a home? Between 2 and 4 bar. Above 5 bar it's advisable to fit a pressure-reducing valve to protect taps and appliances.
Can pressure PVC be used for hot water? No. It deforms above 60°C; for domestic hot water use copper, PEX or multilayer.
How much fall does a waste pipe need so it doesn't block? A minimum of 2% (2 cm of drop per metre of horizontal run).
Conclusion
Choosing the material well and calculating the real margin of each section —not just the plan measurement— is what separates a tight plumbing quote from one that falls short on site or overstates the material. Automating it avoids calculating by eye and leaves a clear breakdown for the client.
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