Plumbing Basics Every Home Renovator Should Know

Plumbing is often overlooked — yet when it fails, the problems can be huge. As one famous homeowner-builder noted, “Walking on the moon may be important… but plumbing is more important.” If you’re doing a renovation: adding a bathroom, finishing a basement, or repiping your home, this blog helps you understand the system, what you can fix yourself, what you should watch out for, and how to avoid big mistakes.

In this article you’ll learn: how water-supply lines work, how drain-vent systems work, the key fittings, how to rough-in plumbing under a concrete slab, and how to bring new venting through a roof. Let’s dive in.

1. Water Supply Basics

PEX vs Copper

When it comes to water supply lines in modern homes, PEX (cross-linked polyethylene) has become very popular.Why? It’s flexible, easy to cut, easy to install, and the connections are very strong. If installed correctly, many manufacturers claim PEX connections are even stronger than soldered copper. Key rules:

• Cut the PEX cleanly — some say twist slightly as you cut so you don’t “collapse” the pipe.

• Use quality tools — don’t rely on cheap crimpers with pinch rings which have higher failure rates.

• Use copper ring crimp fittings (even for PEX) for a proven reliable connection.

• If you’re converting from existing copper pipe, there are transition fittings (PEX on one side, copper on the other).

• Ensure supply lines are placed at least 1¼ inch from the front of studs behind drywall. This prevents screws or nails from penetrating them when drywall is attached.

A Few Additional Supply-Side Essentials

• When using a PEX to copper transition fitting, you might sweat weld the copper side and then run PEX afterwards.

• Some systems use a push-on connector (e.g., “SharkBite”) for copper to PEX transitions; these often come with a long guarantee.

• When installing supply lines and drywall later, always install a steel protection plate over the pipe when it is close to the drywall edge for added nail protection.

2. Drain, Waste & Vent (DWV) Basics

The Drain-Vent System

While water supply moves “clean” water to fixtures, the DWV system moves waste away from fixtures and also provides an air path (vent) so drains work properly.From a high level:

• Waste flows through drain pipes.

• Vent pipes allow air into the system to prevent vacuums which slow flow or lead to trap siphoning.

• Drain pipes must have proper slope to keep solids and water moving together.

• Vent pipes must rise higher than fixture outlets to avoid flooding vent lines.

Important Fittings

• Tee (T): A fitting where a branch joins a main line. It has a slope built in when used for drains — orientation matters.

• Wye (Y): A branch angle fitting (usually 45°) that’s smoother for waste flow than a 90°.

• Long-sweep 90°: Better than a standard 90°, especially in horizontal drain runs where you want less resistance. Some codes now require this.

• Bushings: Fittings that allow change of pipe size (e.g., from 3″ to 1½″). Useful in transitions or tight spaces.

• Trap adapter: Used under sink drains. For systems under concrete slab, a glued trap adapter is required when there’s no access.

• Flexible couplings (rubber-lined): Allow transition between pipe types (ABS, PVC, copper) or repair situations under slab where space is tight.

Materials & Solvent Weld

Many drain systems in modern homes use ABS or PVC. When using solvent weld:

• Use purple primer to clean and soften surfaces.

• Use heavy-duty cement that melts the material surfaces so they fuse.

• Always dry-fit first, mark alignment, then glue.

• Hold for 15-20 seconds to allow set.

Venting & Wet-Vents

• A wet vent is a drain pipe that also acts as a vent (allows air). If the run is short (e.g., under 3 feet from stack) you may not need a separate vent.

• Under concrete, toilets usually drain into a 3″ pipe, then sink/shower may branch off as 2″ with a proper vent path.

• The vent must always rise higher than the highest fixture branch. If you run vent horizontally or below fixture level, air flow may reverse, causing poor drainage or trap siphoning.

3. Rough-In Plumbing Under a Concrete Slab

When you’re building a bathroom where the slab will be poured after plumbing is laid out, you follow a specific process.

Layout & String Lines

• Before digging, run string lines to mark walls, offsets, and references.

• Mark where the outside of walls will be, offsets for fixtures, etc.

• These lines help direct where drains will come up, and where they’ll sit relative to walls.

Dig & Grade Trenches

• After layout, dig trenches for the main drain line and branches.

• Ensure consistent base, proper depth, and correct slope (approx ¼″ per foot).

• Mark specific distances from string lines for fixture centers — e.g., toilet center 39″ from a reference string.

Install the Cleanout (Test Tee)

• Install a test clean-out tee at the main drain (before concrete).

• Cap temporarily with a test plug including a valve stem for pressure testing later.

• This allows you to pressurize or fill the system and validate no leaks after backfill but before the slab.

Select Proper Pipe Type

• Use solid Schedule 40 PVC under the slab (not cellular core unless your code allows).

• Schedule 40 solid rated for pressure (~260 psi) and more rigid.

• Avoid cheap cell-core under slab.

Cutting & Measurements

• Use a miter saw with a standard wood blade for fast, clean cuts.

• Deburr inside and outside edges after cutting.

• Mark pipe and fitting alignment for glued assemblies.

• Example: If the space between fittings is 9⅝″ plus insertion depths on each end (1½″ both sides) → the actual cut length = 12⅝″.

Installing Drain Branches

• For sinks: use 1½″ pipe with appropriate combo-wyes or long-sweep elbows.

• For toilet: branch off 3″ main with combo-wye and long-sweep elbow.

• Use a torpedo level to maintain slope (¼″ per foot) while dry-fitting.

• Dry-fit all components before gluing.

Elevating Through Walls or Blocks

• When drains need to go through block walls: mark top of pipe height using level.

• Use rotary hammer to open cavities.

• Install long-sweep 90s and glue through the wall.

• Keep vent pipe penetrations at proper heights (18–20″ above slab for a typical vent stub-up).

Backfill & Prepare for Slab

• Once plumbing passes inspection (e.g., holding water/pressure) remove test plug, install final cleanout cap.

• Backfill trenches with gravel around pipes – avoid damaging pipes or fittings.

• Grade surface flat and prepare for slab pour.

Fixture Rough-Ins

• Toilet flange center typically 12″ from wall (standard).

• Keep fixture branch tee within 3′ of main stack if you want to avoid additional venting.

• Shower/trap must have a glued trap adapter if no access will be available after slab.

4. Fittings and Connections – A Closer Look

Choosing the Right Fitting

• Read the label: Interior dimensions matter more than outside diameter.

• For example: A fitting may say 1½″ → this refers to interior pipe size.

• Some adapters allow connecting copper to ABS or PVC; this is helpful during renovation of older homes.

• Flexible couplings help transition or repair in tight slab conditions.

Support & Protection

• Use metal strapping or pipe hangers around pipe in floor joist cavities – don’t rely on solvent joints alone for support.

• Supply lines near drywall: install nail protection plates 1¼″ from stud front.

• Install access panels if using push-lock connectors (e.g., SharkBite) behind finished surfaces.

Miscellaneous Tools & Materials

• Pipe cutter for supply lines.

• Plumber’s putty for sink drains and vanities.

• Teflon tape on threaded connections to prevent sewer gas entry.

• Temporary end caps (often orange) for open drains before slab pour.

• Always confirm local building code requirements (especially if you’re DIY) and obtain inspections.

5. Common Mistakes & How to Avoid Them

Mistakes in Slope

• Horizontal drain runs must have proper slope – too little = solids settle, too much = water outruns solids.

• Rule of thumb: ~1″ drop for every 4′ of run.

• Ensure slope with a level or laser, not eyeballing.

Using the Wrong Elbows

• Avoid short-radius 90° elbows on horizontal drains – many codes now require long-sweep 90° or two 45s plus pipe.

• This reduces resistance, improves flow, decreases clog risk.

Access Mistakes

• Never bury a trap adapter or push-on fitting behind drywall without access.

• If you use push-on fittings or transition fittings behind finished walls, install a hidden access door.

Venting Errors

• Running a vent pipe too low or too far before turning vertical may lead to air pressure problems, slow drainage, or trap loss.

• For drains that travel more than 3′ horizontally from main stack, add a dedicated vent.

Using Inappropriate Pipe Under Slab

• Do not use cell-core PVC or other unauthorized materials under a concrete slab unless code allows.

6. Adding New Venting Through the Roof

When adding a bathroom or modifying the DWV system, you often need to bring a new vent pipe through the roof. Key steps:

1. Determine pipe size – often 1½″ or 3″ depending on duties.

2. Use a proper roof flashing designed for the pipe diameter (usually 3″ flashing even if pipe is 1½″).

3. Penetrate roof under a shingle course near the ridge or near water deflection line (just above highest shingle row).

4. Ensure pipe rises vertically above highest fixture level before any turn.

5. Apply roofing sealant around flashing nails or pipe penetration.

7. When to Call a Professional vs DIY

You can DIY a lot: adding a bathroom, repiping with PEX, rerouting drains. But know your limits:

• Hire a licensed plumber if structural changes are needed (joist drilling, slab cutting).

• Always pull permits and get inspections – they protect resale value and ensure compliance.

• If your system is complicated or you’re unsure about local code, get professional help.

Conclusion

Plumbing may not be glamorous, but it is absolutely essential. Whether you’re finishing a basement, adding a bathroom, or just replacing supply lines – get it done right. Understand the flow of water in and out, the importance of slope and venting, the right materials and fittings, and when to call in a pro.

If you follow these steps: proper layout, correct materials, accurate slope, thorough preparation, and effective backfill – your plumbing will be reliable and meet inspection. And if you’re doing it yourself, you’ll save a lot of money while doing quality work.