In this post, I’m going to walk through common blunders that lead to leaks, clogs, foul odors, structural headaches, and safety hazards, then show you how a professional approaches each situation. Whether you’re a homeowner trying to make smart decisions, a DIYer learning the ropes, or a new apprentice building good habits, this is your field guide to doing it right.
The Joist You Don’t Get to Cut
Let’s start with a classic: someone needs to set a toilet, the drain location clashes with a floor joist, and out comes the saw. Three‑quarters of a 2×12 gets carved out like a canoe—and the “fix” is to cover it with a flange and hope nobody notices. That’s not ingenuity; that’s a structural failure waiting to happen.
Why it’s bad:
Joists carry live loads (people, furniture, appliances) and dead loads (the structure itself). Over‑notching or drilling the wrong part of a joist weakens the entire span. It can lead to bouncy floors, cracked tile, squeaks, and in severe cases, structural compromise. No responsible trade wants to be the one who caused that.
What a pro does instead:
- Coordinate early. Before rough‑in, confirm the toilet centerline relative to joists. If there’s a conflict, talk to the framer or carpenter before you cut anything.
- Use an offset closet flange when needed. These exist for a reason and can move the waste line just enough to clear a joist without gymnastics.
- Box out or header off the joist—but only the framing crew or a qualified pro should do it. This distributes load around the opening correctly.
- Honor bore/notch rules. Every jurisdiction has limitations for where and how much you can cut. When in doubt, stop and call the framer or an engineer.
If you’re a plumber, don’t hack someone else’s work. If you’re a builder, don’t drop a joist right under a water closet rough and expect the plumber to “figure it out.” Communicate and plan—everybody wins.
“Wrong 90s” and Other Fitting Sins
Another crowd‑favorite mistake is using the wrong elbow in the wrong place. I see tight, short‑turn 90s jammed into drainage lines as if water and waste bend like wire. They don’t.
Short‑turn vs. long‑turn:
A short‑turn 90 creates turbulence and invites clogs on drainage lines. Long‑turn 90s (or two 45s) give waste a smooth path, preserving flow and reducing stoppages. Use short‑turn where it belongs—on venting or certain above‑floor transitions if allowed—not on your main waste path.
Side notes on fittings that cause trouble:
- Sanitary tees on their backs for horizontal drainage: nope. Use a wye and 45 (or combo) to keep flow moving.
- Shoulder‑loaded, over‑glued joints: a giant purple ring isn’t a badge of honor. It’s often a sign the fitting wasn’t deburred or seated right.
- “Lettered” drain assemblages where fittings look like a puzzle toy: if it takes alphabet stickers to remember how it goes back together, it’s probably over‑complicated and likely to trap solids.
Keep your drainage geometry graceful. Waste should “want” to go where you’re sending it.
Slope: Back‑Grading Turns Sinks Into Swamps
I also run into kitchen drains sloped the wrong way—down, then up, then down again. That “up” is a belly that holds water, grease, and food particles. It stinks. It clogs. It drives people crazy.
Rule of thumb:
Provide consistent, positive slope on horizontal drainage runs—commonly 1/4″ per foot for smaller lines. Maintain that slope through changes of direction. If you accidentally create a belly, cut it out and reset the run. There’s no spray, chemical, or wishful thinking that will fix a back‑grade.
Quick checklist for kitchen sink drains:
- Uniform slope from trap arm to the stack or branch.
- Long‑turn fittings for direction changes.
- Cleanout access where required so future maintenance is sane.
- Support the piping—no “hanging spaghetti” that sags into bellies over time.
Traps, “S‑Traps,” and the Vent That’s Missing
Nothing ruins a good drain faster than bad venting. I see trap configurations that look like the letter “S,” “X,” or a pretzel—anything except what’s needed: a proper P‑trap with a vented trap arm.
Why traps exist:
A trap holds a water seal that blocks sewer gas. If you pull a siphon through that trap because there’s no vent, it loses its seal and you get gurgling, odors, and slow drainage.
Avoid these offenders:
- S‑traps: They’re notorious for self‑siphoning because the outlet drops straight down with no vent to break negative pressure.
- Double traps or X‑traps: Two traps in series or oddly mirrored traps slow flow and double the risk of losing the seal.
- No vent at all: The line burps and bubbles, yanking the water seal out of the trap.
The fix:
Run a proper vent that ties in at the right height and distance for your trap arm. If a traditional roof vent isn’t practical, some jurisdictions allow an Air Admittance Valve (AAV) if it’s installed correctly, accessible, and permitted by local code. AAVs aren’t magic; they’re a tool. Use them where allowed—and only as designed.
“Purple Primer Everywhere” Isn’t Proof of Quality
I love primers and solvent welding done right. I hate when “purple everywhere” is treated like proof the job is correct.
Solvent welding best practices:
- Dry fit first, then mark orientation so your fitting lines up during the actual glue‑up.
- Deburr and clean the pipe—rough edges catch waste and weaken joints.
- Primer, then cement, both evenly, then push and twist a quarter‑turn to fully seat.
- Hold it for several seconds so the joint doesn’t push itself apart.
- Wipe excess—inside “shoulders” create snags that catch solids; outside drips aren’t a trophy.
If it looks like a kindergarten art project, it was probably rushed.
Push‑to‑Connect and Shark‑Style Fittings: Use Wisely
Push‑to‑connect fittings are brilliant for service work, emergency restorations, and temporary setups. I carry them. I use them. But I don’t bury them in walls or ceilings unless local code expressly allows it and I still have a way to access them later. Accessibility is the key word.
Golden rules:
- Deburr the pipe, mark the insertion depth, and push to the line.
- Support the line—wiggle is the enemy of seals.
- Prefer permanent methods (press, crimp, expansion, solder) for concealed spaces and long‑term reliability.
- Transition fittings matter. Copper‑to‑PEX transition? Use the proper adapter and a controlled connection method, not a collection of “good enough” pieces.
If you’re seeing drips from a push‑fit, the problem is often prep, support, or a damaged pipe end. Fix the cause, not just the symptom.
PEX vs. “Looks Like PEX”
Another trap for the uninitiated: flexible hose that looks like PEX but isn’t. Real PEX tubing is marked with its type (A, B, or C), size, pressure rating, temperature rating, and certifications. If your “PEX” is soft, rubbery, and missing markings, you’re not working with PEX—you’re gambling.
How to verify:
- Look for the markings—they should be continuous along the tubing.
- Know your system method: PEX‑A expansion, PEX‑B crimp/clamp, PEX‑C methods—use fittings designed for that system.
- Use rated tubing and fittings from reputable manufacturers.
- Follow support spacing requirements; PEX needs proper supports to prevent sagging, which creates bellies and noise.
If a supplier hands you “red and blue” hose that feels like a garden hose, walk away.
RVs, Mobile Homes, and Specialty Systems
A lot of plumbers decline RV and manufactured housing work for a reason: the materials and layouts are different, and parts availability can be a minefield. Flexible hoses, proprietary fittings, tight cavities—what works in a site‑built home may not fit here.
If you work on these systems:
- Verify the OEM specifications for tubing and fittings.
- Stock the right adapters—don’t force a home plumbing solution where it doesn’t belong.
- Educate owners on maintenance and winterization.
If you’re an RV owner, don’t assume “regular house parts” are drop‑in replacements.
Water Heater Piping and Electrical Panels: Keep Them Apart
One of the ugliest sights is messy water piping draped over or behind an electrical service panel. Water and electricity aren’t friends, and leaks don’t announce themselves before they happen.
What good practice looks like:
- Respect clearances around electrical equipment. Keep water lines routed away from panels and provide clean, serviceable paths.
- Label hot and cold and verify heat source connections are correct. Cross‑connecting hot and cold can cause temperature swings, long wait times for hot water, or worse.
- Install shutoffs, unions, and expansion control where required so maintenance doesn’t become demolition.
A $900,000 home with bargain‑basement mechanicals is still a bargain‑basement job. Price doesn’t equal quality—craftsmanship does.
Don’t Send Waste to the Gutter
I’ve seen fixtures tied directly into storm drains, downspouts, or yard drains. It’s a neat idea only if you want sewer odors on your sidewalk and a direct pathway for pests. Sanitary and storm are separate systems for a reason.
Better solutions:
- Run the fixture to the sanitary system with proper trap, vent, and slope.
- If you need a condensate or similar drain outside, terminate it as permitted—often with visible discharge and air gap—never into a storm line that conveys roof runoff.
- Check local rules on what can and can’t connect where. Cross‑connections can lead to citations and costly corrections.
Fewer Fittings, Fewer Problems
One homeowner counted five fittings before a single tee on a hot line. That’s five opportunities for leaks, pressure drops, and noise. Sometimes those extra fittings are band‑aids over planning mistakes.
Plan the run:
- Minimize direction changes and avoid “zigzag” routes that create pressure loss.
- Use sweeping turns instead of tight elbows where allowed.
- Consider manifolds or home‑run PEX layouts for balanced distribution.
- Strap and support everything. Unstrapped lines move, rub, and eventually wear through or loosen joints.
Every fitting should have a reason to exist. “That’s where the pipe ended up” isn’t one.
“Just Because It Works” Doesn’t Mean It’s Right
I hear this all the time: “It works, so it must be fine.” Function today doesn’t guarantee safety or longevity. Codes aren’t busywork; they’re the accumulated lessons of what has failed in the past.
What you’re paying a pro for:
- Compliance: Installed to code so you pass inspections and your insurance stays happy.
- Safety: No gas leaks, no scald risks, no electrical hazards from water intrusion.
- Performance: Adequate flow, good drainage, and fixtures that behave.
- Accountability: Licensed, insured professionals stand behind their work.
The cheapest bid often costs the most once you pay someone else to rip it out and do it over. I tell customers up front: if you hire the lowest number without checking credentials, you may pay double later—first for the “deal,” then for the repair.
A Pro’s On‑Site Mindset
Whether I’m called for a specific repair or a broader project, I’m scanning the system. If I see a crimp that’s barely on the barb, a valve installed backward, or a strapped‑like‑a-noodle water line, I’ll recommend fixing it while I’m there. That conversation matters.
How I handle it:
- Explain the issue in plain language, show the risk, and offer options.
- Get approval for additional work if it’s beyond the original scope.
- Document what I did and why.
- Leave it better than I found it.
Professionals don’t just replace parts; they fix systems.
Homeowner’s Quick‑Start Checklist
If you’re hiring, here’s a streamlined checklist to protect your investment:
- Verify license and insurance. Ask for proof.
- Ask about scope and code compliance. “What permits are needed?”
- Discuss materials. What type of pipe/fittings? Why those?
- Clarify venting and slope for drains. Listen for specifics.
- Look for serviceability. Are there cleanouts, unions, shutoffs?
- Check workmanship on visible piping: neat, supported, labeled.
- Get a clear warranty in writing.
The right pro won’t dodge these questions.
Apprentice / DIY Field Notes
If you’re learning or tackling small projects at home, build these habits early:
- Measure twice, drill once—especially in structural members.
- Keep the slope consistent and check with a level as you go.
- Use the correct fittings for drainage vs. venting.
- Vent properly so traps keep their seals.
- Deburr and clean every cut.
- Support your lines to prevent sagging and noise.
- Know your limits. Gas lines, major structural changes, and complex repipes are not “learn as you go” projects.
A little humility now saves a lot of money later.
Pulling It All Together
Bad plumbing wears a lot of disguises—hacked joists, wrong 90s, back‑graded drains, S‑traps, unvented sinks, “mystery PEX,” push‑fits in buried spots, and water lines strung over electrical gear. Each mistake is fixable, but the best solution is avoiding them in the first place with careful planning, the right materials, and proper coordination between trades.
Do it once. Do it right. Respect structure. Preserve slope. Vent properly. Choose fittings that help flow, not fight it. Keep water away from electricity. Use push‑to‑connect as the tool it is—smart and accessible—not as a universal shortcut. And if you’re hiring out the work, choose skill and accountability over the lowest number on a page.
That’s how you turn #Badplumbing into good plumbing—and a house that stays dry, safe, and comfortable for the long haul.
