General Rules of Thumb for Plumbing in Construction

​When it comes to sizing and installing domestic water plumbing, there are some general rules of thumb to follow. Here are a few guidelines:

1. Supply Line Sizing: For the main supply line, it's usually recommended to use a 3/4-inch pipe to ensure adequate water pressure. However, if the house is large or has many fixtures, a 1-inch pipe may be necessary.

2. Branch Line Sizing: Branch lines, which feed individual fixtures, typically use 1/2-inch pipes for most residential applications. However, certain fixtures like showers or appliances may require larger pipe sizes based on their flow rate requirements.

3. Fixture Flow Rates: Determine the flow rate requirement for each fixture in your home (e.g., faucets, toilets, showers, etc.). This information is usually available from the manufacturer's specifications. Ensure that the pipe size can deliver the required flow rate without a significant drop in pressure.

4. Pressure Considerations: The minimum recommended pressure for domestic water supply is typically around 30 psi (pounds per square inch), while the maximum should not exceed 80 psi. Install a pressure regulator if the pressure exceeds this range to prevent damage to plumbing fixtures.

5. Drainage Sizing: For drainage pipes, the size is generally determined by the fixture's outlet size. For example, most residential sink drains use a 1 1/4-inch or 1 1/2-inch pipe, while toilets use a 3-inch pipe. Ensure proper slope of drain pipes to facilitate efficient drainage.

6. Venting: Vent pipes are essential to prevent negative pressure in the drain system and ensure proper drainage. The size of vent pipes depends on the number of fixtures being vented and their distance from the main vent stack. Typically, 1 1/2 or 2 inches is used for most residential applications branch venting and connected to a 3 inch (min. size to prevent frost closure in cold climates) vent pipe through roof.

7. Material Considerations: The two most common materials for water supply pipes are copper and PEX (cross-linked polyethylene). Copper is durable and has excellent corrosion resistance but can be more expensive. PEX is more cost-effective, versatile, and easier to install. Choose the material based on your budget and preferences.

8. Code Compliance: Make sure to follow local building codes and regulations when sizing and installing domestic water plumbing. The requirements may vary depending on the jurisdiction, so consult with local authorities or a licensed plumber to ensure compliance.

Remember that these guidelines are general and may vary based on specific circumstances and codes in your area. It's always best to consult with a professional plumber or a licensed contractor for a thorough assessment and to ensure the proper sizing and installation of your domestic water plumbing.