Philadelphia Row House Chimneys: Common Problems

How these chimneys are built

Most row houses and twins have one or two chimneys, often toward the back or along a party wall shared with the neighbor. They rise a short way above a flat or low-slope roof covered with modified bitumen, rubber or a coated membrane. Many no longer serve a fireplace and vent only the boiler, furnace or water heater in the basement.

The brick is often old and soft, laid in lime-rich mortar, and the flues may be unlined, lined with clay tile, or relined at some point with metal. Because the stacks are short and the roofs are flat, the chimney sits in the full force of wind-driven rain and snow that drifts against it, with nothing above it to shelter the top.

The usual problems, most common first

These come up again and again on row house roofs, and an older chimney often has several at once.

• Soft, washed-out mortar and loose brick near the top of the stack
• Leaks at the base where roof coating was used instead of proper flashing
• Cracked or missing crowns and no cap on the flue
• Unlined flues eaten by boiler and water heater exhaust
• A water heater left alone in a big flue after a furnace was replaced
• A stack now below a taller neighbor's new addition

Why flat roofs are hard on chimneys

On a pitched roof, water runs past the chimney. On a flat roof, it can sit against the base after every storm, and snow drifts pile against the windward face and melt slowly. Each roof replacement adds another layer of coating at the base, often smeared up the brick a few inches. That coating cracks as the chimney and roof move at different rates, and water runs behind it into the wall and down the flue.

The proper detail is a base flashing turned up the brick and tied into the roof membrane, with a counter-flashing let into a mortar joint and lapped over it. Where the roofer and the chimney contractor coordinate, the joint stays dry for many years. Where they do not, the leak tends to come back with every hard rain.

Flues that serve the basement

A row house chimney that vents a gas boiler or water heater works hard and stays wet on the inside. Gas exhaust is mostly water vapor; in a cold, oversized masonry flue it condenses and soaks the mortar. Over years the inner brick crumbles and drops debris at the base, and white stains and damp patches show up on the chimney wall in the basement or upstairs.

The fix is usually a stainless liner sized to the appliance, a cap, and a cleared base. When the furnace has been replaced with one that vents out the wall, the water heater may have been left alone in the old flue, which is an orphaned water heater and needs its own sized liner or a different water heater.

When the house next door builds up

A new floor or addition next door can leave your chimney too short, which hurts draft and can push exhaust back into your house. L&I regulations on protecting adjoining property cover this. When a new or modified building rises above your roofline and your chimney or vent is within 10 feet of it, the person doing the construction must take reasonable steps to have your chimney or vent extended or offset to meet the code's clearance and termination requirements.

The process runs on written notice. The builder must deliver an initial notice to the owner's address on file with the Office of Property Assessment at least 60 days before work starts, describing the work and offering to secure permits and fund or perform the chimney work. If you reject it or do not respond, the regulation says you may be held responsible for your building's compliance. A separate mechanical permit, authorized by you, is needed for work on your chimney.

One more point from the same regulation: correcting deficiencies that existed on your chimney before the extension is your responsibility, not the builder's. That is a good reason to have the chimney inspected when the notice arrives.

Repair or take it down?

A chimney still venting a boiler or water heater needs a sound, correctly sized flue, which often means a liner, plus a crown and cap and proper flashing. A stack that serves nothing can be repaired and capped, or taken down below the roof and the roof closed over, after confirming no appliance or neighbor's flue uses it.

Party wall chimneys need extra care, since flues from both houses may share the stack and its weight may help hold the wall. Taking down half of a shared stack, or capping a flue the neighbor still uses, causes problems that are expensive to undo.

• Stack still venting a boiler or water heater: repair and reline, with the liner sized to the appliance
• Unused stack with sound brick: repoint, rebuild the crown and cap it, and watch the flat-roof joint
• Unused stack that is badly deteriorated: remove it below the roof, after checking for the neighbor's flues
• Neighbor added a floor nearby: extend or offset, following the L&I notice and mechanical permit process

What you can check from inside and from the street

From the street or a rear window, look at the top of the stack for missing bricks, a leaning top course, or a flue with no cap. Inside, look for water stains on the ceiling beside the chimney after rain, peeling paint or white powder on the chimney wall, and debris or water at the cleanout in the basement. Flat roofs are easier to walk than pitched ones, but they are still roofs: leave the close inspection to someone with fall protection.

When you call, have a few facts ready: which appliances run on gas or oil and where their vent pipes enter the wall, whether the roof was redone recently and by whom, and whether the neighbors share the stack. A technician on a row house job checks the flue from the basement connection to the top, the crown, the cap and the flashing joint, and notes which flues belong to which house. Those answers decide whether the job is a repair, a reline or a removal.

Permits and the city

L&I's building permit page says that if a property is not on the Philadelphia Historic Registry, a permit often is not needed for repairs related to regular maintenance or for pointing masonry. Bigger work, like rebuilding or removing a chimney, can be a different matter, and historic properties need Historical Commission approval first. Confirm with L&I for your specific job before work starts.