The Three Water Defenses on a Flat-Roof Rowhome Chimney
On a Northeast Philadelphia rowhome with a flat or low-slope roof, three parts keep water out of the chimney: the crown, the cap, and the flashing. Here is what each does and why a flat roof makes them matter more.
Why a flat roof changes the water problem
Most chimney advice assumes a pitched roof, where water hits the chimney and runs off quickly. But a great many Northeast Philadelphia rowhomes have flat or low-slope roofs, and that changes the water problem in an important way. On a flat roof, water does not race off the way it does on a steep one. It sits, it pools in low spots, and it works at any seam, joint, or gap for as long as it stands there. The chimney passing through that flat roof is surrounded by standing or slow-draining water far more often than a chimney on a pitched roof ever is, which puts more pressure on every part that is supposed to keep water out.
That is why, on a flat-roof rowhome, the chimney's water defenses are not a detail to take for granted. Three parts do the work of keeping water out of the chimney and out of the house, the crown at the very top, the cap over the flue, and the flashing where the stack passes through the roof. On a pitched roof a weakness in one of these might be forgiven by how fast the water runs off. On a flat roof, where the water lingers, a weakness in any of the three gets found and exploited. Understanding what each one does is the first step to keeping a flat-roof chimney dry.
The crown: the chimney's roof
The crown is the masonry or concrete cap that covers the very top of the chimney, sloping slightly so that rain landing on the top of the stack runs off rather than soaking straight down into the masonry. Think of it as the chimney's own little roof. When the crown is sound, it sheds water off the top of the stack and protects everything below. When it cracks, and on the older chimneys out here crowns crack readily from years of freeze and thaw, water goes straight down into the body of the chimney, washing out mortar joints and saturating the brick from the top down. A cracked crown is one of the most common and most damaging water faults we find, precisely because it lets water in at the highest point and lets gravity carry it through the whole stack.
The good news is that a crown problem caught early is one of the more affordable fixes. A crown with hairline cracks can often be sealed, and one that has deteriorated further can be rebuilt. Either way, restoring the crown stops water at the top of the chimney, which is the most efficient place to stop it, since water kept out of the top never gets the chance to do damage lower down. On a flat-roof rowhome, where the chimney already faces more standing water than most, a sound crown is the first and arguably most important of the three defenses, because it handles the water that comes straight down from above.
The cap and the flashing: the other two lines
The cap sits over the flue opening itself, and while its better-known job is keeping out animals and embers, it is also a water defense, keeping rain from going straight down the open flue and into the smoke shelf and the masonry from the inside. A flue with no cap takes water directly down its throat every time it rains, and on a flat-roof home where that rain may be driving or pooling, an uncapped flue collects a remarkable amount of water. A properly fitted stainless cap closes that path while still letting the flue breathe, which is why we recommend one on any flue that lacks it, especially on a flat roof where the weather has more chances to get in.
The flashing is the third defense, and on a flat roof it is the one under the most pressure. Flashing is the metalwork and sealing where the chimney passes through the roof surface, and its job is to make that junction watertight. On a flat or low-slope roof, where water stands and drains slowly, the flashing around the chimney base has to hold back not just running water but pooling water, and any gap or failure there lets water straight into the roof and the home around the chimney. Failed flashing is one of the most common sources of the interior stains homeowners blame on the chimney, and on a flat-roof rowhome it deserves particular attention. When all three defenses, the crown, the cap, and the flashing, are sound, the chimney stays dry. When any one fails, the standing water on a flat roof will find it.
- Crown: sheds rain off the top of the stack
- Cap: keeps water from going down the open flue
- Flashing: seals where the chimney meets the roof
- A flat roof lets water pool and linger against all three
- A weakness in any one is what standing water finds
Keeping a flat-roof chimney dry for the long run
Because a flat-roof chimney faces more standing water than a pitched one, keeping all three defenses sound is the whole game, and it is far cheaper than the masonry repairs that water intrusion eventually forces. An annual inspection that checks the crown, the cap, and the flashing together is the simplest way to stay ahead of it, since these parts tend to fail gradually and a yearly look catches the first crack or the first lifted flashing while it is still a small, sealable fault. On a flat-roof rowhome especially, where you cannot easily see the top of your own chimney from the ground, that yearly look is the only realistic way to know how the defenses are holding up.
When we inspect a flat-roof chimney, we read all three defenses as a set, because they work together and water that gets past one often reveals weakness in another. We tell you plainly which are sound and which need attention, with photographs from the roof so you can see the condition of parts you would otherwise never lay eyes on. The goal is to keep water out at the top and at the roofline, before it ever reaches the masonry and the living space, which on a flat-roof Northeast Philadelphia rowhome is the most cost-effective chimney care there is.
On a flat-roof rowhome, the crown, the cap, and the flashing are the three things standing between your chimney and the standing water above it. We will inspect all three from the roof, show you the photos, and tell you honestly what is sound and what needs attention. Call 215-602-7623.
When it is time, reach us at 215-602-7623 and a real person will pick up.