Posted on 2026-02-08 20:39:18

Homeowners call me when the ceiling stains, the attic smells like a locker room, or the new high‑efficiency furnace short cycles in January. Nine times out of ten, the root cause traces back to venting choices and how those vents pass through the roof. Heating contractors and roofing contractors are both specialists, but venting often lives in the gap between trades. Myths fill that gap, and those myths cost comfort, fuel efficiency, and sometimes the roof itself.

This is a plain‑spoken tour through the most common misunderstandings around furnace, boiler, and water heater venting, focusing on what happens at and above the roof deck. I will reference the real tradeoffs I’ve seen over the years, where mechanical code meets shingles, and where a seemingly small decision during a roof replacement echoes for decades.

Why venting myths stick around

Venting lives behind drywall and under shingles, so it rarely gets a homeowner’s attention until something goes wrong. Codes evolve every few years, manufacturers update instructions frequently, and equipment now ranges from legacy 80 percent furnaces to sealed‑combustion boilers with plastic venting. Mix in local weather, snow loads, and wind exposure, and you have a web of variables that resist simple rules of thumb.

Roofers and heating contractors approach the task from different angles. Roofers think in terms of penetrations, waterproofing, and attic airflow. Mechanical contractors think in terms of combustion air, flue temperatures, pressure, and condensate. When they coordinate early, the system tends to perform well and the roof stays tight. When they do not, you get wobbly caps, condensate dripping onto rafters, or melted shingles beside a metal B‑vent that runs too hot for the clearance provided.

Myth 1: “A pipe through the roof is a pipe through the roof.”

I still hear this on jobs where the homeowner assumes one flashing detail fits every penetration. Not true. The right flashing, termination height, and clearance depend on the pipe’s function and temperature. A plumbing vent stack, a furnace exhaust, and a dryer termination behave differently in heat, moisture, and flow rate. Treat them the same and you risk leaks or unsafe operation.

For example, a Category I gas furnace or atmospheric water heater that uses metal B‑vent has strict clearance to combustibles, often 1 inch all around the vent, which must be maintained across the roof plane. That clearance affects the size of the roof opening, the type of storm collar, and how insulation is kept back in the attic. By contrast, a Category IV condensing furnace that vents with PVC or polypropylene runs at a much lower flue gas temperature but produces acidic condensate. The roof penetration must protect against UV on the pipe, anchor securely against wind uplift, and allow the condensate to drain back to the appliance or out a proper trap, not into the attic.

Even termination details vary: a power‑vented exhaust may need to terminate a set distance from operable windows and away from soffit vents to prevent re‑entrainment, while a plumbing stack is sized to relieve sewer gases and equalize pressure, not to eject combustion byproducts.

Myth 2: “High‑efficiency equipment can vent anywhere.”

Sealed combustion gives a lot of flexibility, but not carte blanche. Manufacturers publish specific routing rules, maximum equivalent lengths, elbow counts, termination heights, and separation between intake and exhaust. Ignore those and the unit can trip on pressure switches, ice up at the termination, or corrode prematurely.

I walked a roof last winter where a pair of PVC pipes for a condensing boiler poked out near a dormer because the installer wanted the shortest run. The exhaust plume blasted the dormer wall with warm, moist air. In subfreezing temps, frost built up, then thawed, then stained the siding and soaked the sheathing. The fix was not exotic: reroute the exhaust above the roof surface to a location with clear dispersal, use a concentric kit rated for rooftop terminations, and maintain the manufacturer’s minimum distances from architectural features. The roof penetration itself needed a UV‑resistant boot and positive slope back to the appliance to keep condensate from pooling.

On coastal houses, wind direction changes matter, too. A vent that meets code on paper can still drive exhaust into the intake under certain crosswinds if the spacing and orientation are poor. Small adjustments at the roof line, like rotating a snorkel cap or increasing separation by a foot or two, can make the difference between nuisance lockouts and smooth operation.

Myth 3: “B‑vent is obsolete now that everyone installs condensing furnaces.”

Plenty of homes still run Category I appliances, either by design or because a full retrofit wasn’t in the cards. B‑vent, a double‑wall metal chimney designed for natural draft gas appliances, remains common and perfectly acceptable when installed to code and manufacturer instructions.

What trips people up are clearances and support. I find B‑vent shoved tight against framing or spray foam, or exiting the roof without the proper flashing and storm collar, which lets water chase down the flue and show up as “mysterious” attic stains months later. B‑vent needs to be plumb, supported at prescribed intervals, and kept off combustibles. Penetration kits exist that simplify the roof crossing and maintain the annular clearance through the sheathing. The termination must extend to a specific height above the roof surface based on roof pitch, often with a listed cap to improve draft and block wind‑driven rain. None of this is hard, but it must be deliberate. Roofers who plan skylights and vents like a chessboard sometimes let the heating flue fall wherever the mechanical sub cut the hole. It pays to coordinate.

Myth 4: “One hole is better than two: combine intake and exhaust.”

The appeal is obvious. Fewer holes mean less time flashing, fewer potential leaks, and a cleaner look. But combined terminations are only safe and effective if they are designed as such. That typically means a listed concentric vent kit where intake air is pulled around a central exhaust, with certified spacing between the flows.

I see makeshift “two pipes in one boot” penetrations without the internal separation necessary to prevent exhaust from rolling into the intake. This can set up a recirculation loop, reduce available oxygen, and foul the heat exchanger. In shoulder seasons, the appliance may light but run poorly, soot up, and trip safety limits later. If you want a single roof penetration for a two‑pipe system, use the manufacturer’s concentric kit, size the roof opening to the outer diameter, and flash it correctly. If you cannot source the kit or the run is too long for concentric venting, make two penetrations and separate them by the required distance, usually measured edge to edge at the terminations, not center to center.

Myth 5: “Any roofer can flash a vent if the boot fits.”

Many roofers do excellent work with penetrations, but I have also repaired more than a handful of “fits, so it must be fine” jobs. The right boot is not only about diameter. Temperature rating, UV resistance, and compatibility with the pipe material matter. A standard EPDM plumbing boot on a hot metal flue can cook to a brittle ring within a year or two. Conversely, a high‑temperature metal jack with soldered flashing might be overkill on a PVC pipe but the rubber collar can still degrade if it is not rated for sun and ozone.

Shingle layout matters as well. On steep slopes, I prefer step‑flashing on the upslope sides of metal jacks and careful counterflashing to prevent water from driving under the shingles. The storm collar must be snugged down and sealed with a high‑temp compatible sealant, not general purpose caulk that peels by the second season. On standing seam metal roofs, clamp‑on curbs or purpose‑built round penetrations with flexible base flashings are cleaner than butyl mastic alone. Details vary, but the principle is constant: match the flashing to the pipe and the roof system, and install it as a system rather than an isolated part.

Where venting meets roof replacement

A roof replacement is the perfect time to correct venting sins. Unfortunately, it is also a time when speed and staging drive decisions. Tear‑off crews want a clear deck. Mechanical contractors may not be on site. If you are the homeowner, ask for a quick coordination meeting. If you are the general contractor, put specific notes in the scope.

On older houses in my region, I often see abandoned metal chimneys that once served an atmospheric furnace or water heater, now unnecessary after upgrades. Pulling that chimney during roof replacement reduces penetrations, eliminates a thermal bypass in winter, and frees up attic space for insulation. But only remove it after your heating contractor confirms nothing else uses the flue and that Roof replacement any orphaned appliance will be properly vented.

If you are transitioning equipment, like going from an 80 percent furnace to a 95 percent, plan where the new intake and exhaust will penetrate. For a steep slope or heavy‑snow climate, rooftop terminations need enough height to avoid burial and enough rigidity to resist drifting snow and ice. Some roofers will build a small cricket upslope of a rigid vent to shed drifts. Others will recommend sidewall termination to keep the roof clean. Sidewall can be excellent, but do not choose it if it discharges onto a deck, walks people past a plume, or puts the intake near soffits that pull attic air. A short discussion between roofers, roofing contractors who manage the crew, and the heating pro avoids those pitfalls.

Condensate, corrosion, and the hidden path of water

High‑efficiency appliances condense water out of flue gases. That water is acidic, typically with a pH in the 3 to 5 range before neutralization. If the exhaust pipe slopes incorrectly toward the roof termination, condensate can pool at the cap, run cold in winter, and form icicles that stress the boot. If it runs the other way, toward the appliance, it must drain to a proper condensate trap, not out of a loose joint into your attic insulation. I find stains around PVC roof penetrations that are not roof leaks at all, just condensate that backtracked through a marginal joint. The fix involves correct pitch, clean gluing of joints, and in some cases, a drain tee near the appliance.

Metal venting has its own water stories. Natural draft appliances can create condensation if the chimney is oversize or too cold. Lining a masonry chimney with an appropriately sized metal liner and insulating it reduces the risk. On a roof replacement, it is tempting to abandon and cap chimneys that look unused. Verify, then cap with a proper metal cap that sheds water and blocks animal entry without restricting draft on any remaining connected appliance. The cap height and design matter, especially in windy sites.

The attic tells on you

If you want to know whether venting and penetrations are healthy, spend fifteen minutes in the attic with a flashlight. I look for dark rings around vent pipes, crusty white or green residue on metal, insulation matted down from past wetting, and fungal streaks on the underside of the roof deck near terminations. A frosty attic in January points to air leakage or vent misrouting. A sour smell near a PVC exhaust can indicate recirculation problems or a cracked joint leaking fumes. If I see nails rusted to a uniform orange and sheathing nails dripping in cold weather, there is an attic ventilation problem that may be compounding any venting issue.

This is where coordination with the roofing crew matters again. Balanced attic ventilation helps dry any minor intrusions that do happen and keeps roof deck temperatures in check, but powered attic fans can actually pull combustion byproducts back down through an inadequately sealed penetration under negative pressure. Balance is the word: soffit intake, ridge exhaust, and air sealing at all penetrations.

Sidewall versus roof termination

Homeowners often ask whether sidewall venting is simpler and less risky. It can be. Sidewall exhaust for condensing appliances avoids a roof hole, simplifies service access, and may keep the plume away from roof snow. But sidewall venting brings its own concerns: required distances from doors and windows, potential staining on siding, frost at the termination that can block the outlet, and noise or visible vapor where people live and walk.

Roof terminations, when done right, disperse exhaust above the living area and shield the route from accidental damage. They may require taller snorkel caps in deep snow country and stiffer support to resist wind. If your property has tight lot lines and many windows, the roof is often the cleaner solution. If your roof is tile or slate and penetrations are expensive and complex, sidewall may be preferable. Work with both your heating contractor and roofers to decide, not just one or the other.

Plastic pipe myths and realities

PVC, CPVC, and polypropylene vent systems are not interchangeable. Many furnace and boiler manufacturers list PVC and CPVC in their instructions, but operating temperatures and local code may narrow your choices. CPVC and polypropylene typically handle higher flue temperatures than PVC and are often required on condensing boilers during warm‑up or at high fire. Some jurisdictions no longer accept cellular core PVC for venting at all. Read the manual for the appliance on your wall, not the one you remember from five years ago.

UV matters. PVC that sits above a roof without protection will chalk and embrittle over time. Use a UV‑rated boot, paint the exposed pipe with a compatible latex paint as allowed by the manufacturer, or select a listed termination kit that shields the pipe. Solvent welding requires clean, square cuts, proper primer, and full‑depth glue coverage. Rushed joints are the silent cause of exhaust leaks and intake infiltration, especially where the pipe passes hot‑cold cycles at the roof line.

The roofer’s perspective, and how to use it

Good roofers think in layers: deck, underlayment, flashing, shingle. They also think in time. Every nail hole is a potential future leak if placed poorly. When I work with a roofing contractor I trust, I ask them to lay out penetrations on paper before a tear‑off and to tell me where they want to see the vent come through so water shedding is natural. They tell me how much clearance they need to work shingles around a jack, and I tell them the minimum offset I need from valleys and hips to keep elbows gentle.

Many roofing contractors now include a line item for “mechanical coordination of penetrations.” If yours doesn’t, ask for it. The extra hour of labor to align a flue on the upper third of a shingle course, install an ice and water shield apron under the flashing, and stage a dedicated boot for a high‑temp vent is cheap insurance. If the roofers discover rotten sheathing around a penetration during tear‑off, stop, photograph, and fix it with solid blocking so the boot doesn’t rock.

What inspectors really flag

Inspectors do not nitpick cosmetic details. They look for safety and performance. I often see tags or correction notices for these issues:

Termination too close to a window, door, soffit, or property line, in violation of clearance tables in the appliance manual or mechanical code. Intake and exhaust terminations too close to each other for the appliance and site conditions, leading to potential recirculation. B‑vent clearances to combustibles violated, or vent not supported and not plumb through the roof, creating draft instability. Improper roof flashing or storm collar on a metal flue, allowing water entry along the pipe. Plastic vent materials not in accordance with the manufacturer’s listing, or joints assembled without primer where required.

If you are preparing for a roof replacement or equipment swap, ask your contractor to point to the exact manual pages for termination clearances and permitted vent materials. The good ones already have those pages bookmarked.

Snow, wind, and life at the ridge

In climates with heavy snow, I plan vent terminations at least 12 inches above the expected maximum snow load on the roof surface, with restraint that resists a snow slide. Short snorkels that sit kisses above the shingles disappear in a storm and leave the appliance starving for air. Wind also creates surprising effects near the ridge. Negative pressure zones downwind of a ridge can tug on flue gases, while positive pressure on the windward side can pressurize an intake. I have relocated terminations just two feet downslope to calm those forces.

Ice dams deserve a mention. Heat leaking around a warm metal flue can melt snow and carve channels that refreeze at the eaves. Proper insulation clearance to B‑vent isn’t optional, but you can still air seal the ceiling around the framed chase and install heat‑resistant insulation baffles to limit heat bleed into the attic while maintaining the vent’s clearance.

Fire safety is part of venting

Penetrations through fire‑rated assemblies like garage ceilings and multi‑family party walls require listed firestop components and correct sealing. I have seen nice‑looking roof penetrations paired with sloppy firestopping at the attic floor. That gap is more than an energy leak, it is a fire and smoke pathway. Use fire‑rated collars or sleeves where required, and seal with approved fire caulk, not foam that melts away at the first hint of heat. Inspectors are unforgiving here for good reason.

Working with the right team

A successful project starts with a contractor willing to explain. Ask your heating contractor to sketch the vent path and mark the termination point on an elevation or roof plan. Ask your roofer to show you the specific boot or jack they plan to use, with the brand and temperature rating. A ten‑minute conversation may reveal that the roofer expects to install a standard plumbing boot, while the mechanical contractor expects a metal high‑temp jack. Better to catch that mismatch on paper than on a windy day during tear‑off.

Homeowners often choose roofers based on shingle brand and warranty length. Add one more criterion: experience coordinating mechanical penetrations. The best roofing contractors speak the language of venting clearances and do not treat penetrations as an afterthought. During a roof replacement, they protect existing vents from damage, stage new boots, and call the heating contractor when surprises arise.

A short homeowner checklist before the ladder goes up

Confirm which appliances vent through the roof today and whether any will change with the project. Ask for the manufacturer’s venting instructions for each appliance, and verify termination locations against windows, doors, and soffits. Decide sidewall versus roof termination with both the heating contractor and roofer at the table, accounting for snow, wind, and aesthetics. Match the flashing or boot to the vent type and temperature. Do not let a generic plumbing boot land on a hot flue. Inspect the attic around existing penetrations for staining or rot and budget for repairs and air sealing.

Edge cases and judgment calls

Every house has quirks. On low‑slope roofs with membranes, penetrations demand another level of care. I coordinate with the membrane installer to use welded pipe boots or pre‑made curbs, then add a mechanical support below the roof line so the vent does not wiggle and fatigue the membrane. On tile roofs, I prefer factory‑made adjustable flashings or custom lead jacks formed to the tile profile, never a stack of mastics hoping to keep up with expansion and contraction.

Historic districts sometimes restrict visible rooftop elements. You may need to route vent terminations behind a parapet or to a rear slope. That can work, but keep the equivalent length within the appliance’s limits and maintain slopes for condensate. On multifamily buildings, clustering penetrations through a single curb can look tidy and improve waterproofing, but you must preserve separation between exhausts and intakes and label the penetrations for service.

Final thoughts from the ridge

Venting is a quiet system. When it is right, no one notices. When it is wrong, you chase odd smells, small leaks, intermittent lockouts, and early equipment failures. Myths persist because simple rules are attractive, but venting choices live in details: pipe temperature, clearance, slope, distance to openings, wind, snow, and the hands that flash the hole.

Bring your heating contractor and roofers into the same conversation before you cut. Use the manufacturer’s instructions as the backbone, and adapt details to your climate and roof system. During a roof replacement, take the opportunity to remove abandoned penetrations, repair rotten decking around active ones, and air seal the attic floor. Afterward, give the attic a look each season. Small problems announce themselves there first.

The payoff is quieter equipment, cleaner ceilings, fewer callbacks, and a roof that ages at the slow, predictable pace you paid for. That is what good coordination buys: not just a dry house, but a safe and efficient one.

The Roofing Store LLC (Plainfield, CT)

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Landmarks Near Plainfield, CT

Moosup Valley State Park Trail (Sterling/Plainfield) — Take a walk nearby, then call a local contractor if your exterior needs attention: GEO/LANDMARK
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Heating Contractor Myths: Venting and Roof Penetrations Explained