When a slate roof leaks, the chimney is the first place to look. Not the slate, not the ridge, not the gutters. The chimney. A masonry chimney pushes a large, rigid obstruction straight up through the middle of a roof plane, and everything about that junction is difficult. Water has to be steered around it, metal has to be let into the brickwork, and the masonry and the roof framing do not move together.
The symptom you notice is usually inside and usually nowhere near the chimney. A damp patch on a bedroom ceiling. A dark line following a rafter. Plaster lifting off lath eight feet downslope. Water entering at a chimney runs along the top of the sheathing, down a rafter, or along a ceiling joist before it drops, so the stain marks where the water ran out of somewhere to go, not where it came in.
Understanding how the detail is supposed to be built makes it much easier to judge what you are being told about yours. The bad repairs we are asked to undo share the same few mistakes.
What a correct chimney flashing is made of
A chimney flashing is not one piece of metal. It is four or five separate parts, each doing one job, layered so water is always passed onto something below it rather than under it.
- The apron, or front flashing. A single piece across the downhill face, lapping over the slate below and turning up the brick. It takes the most direct water, because everything running down the roof above the chimney eventually arrives here.
- Step flashing. Individual L shaped pieces up each side, one per slate course, each lapped several inches over the one below. It is not one long strip. A continuous side flashing leaves water crossing a slate joint nowhere to go except behind the metal.
- The back pan, or head flashing. The piece across the uphill side, where water is driven against masonry. On a narrow chimney this can be a soldered pan. On a wide one it is not enough by itself.
- Counterflashing. A separate set of pieces let into the masonry and lapped down over the tops of the step flashing and apron, keeping water out of their upper edges.
- A cricket or saddle on the uphill side of a wide chimney, framed and metal clad to split the water and send it around both sides.
Every part overlaps the one below it in the direction of flow. That principle is the whole job. When a repair fails it is almost always because someone reversed a lap, replaced two pieces with one, or substituted sealant for an overlap.
Reglets, mortar joints, and the surface bead problem
Counterflashing has to be let into the masonry, not stuck to the face of it. The traditional method is to rake out a mortar joint about an inch deep, fold the top edge of the metal into the slot, wedge it with lead plugs, and repoint over it. The alternative is a sawn reglet, a clean kerf cut into the brick or joint, with the folded edge set into it.
What we find instead, constantly, is metal laid flat on the brick with a bead of caulk or roof cement along the top edge. Sometimes it is not even metal: a strip of self adhering membrane or aluminum tape, painted to blend in. These repairs look tidy from the ground and have a service life of a few years. Sealant shrinks, loses adhesion to masonry, and cracks with seasonal movement. Once the top edge opens, water runs down the back of the flashing and nothing below it can help.
Why older chimneys need more care
Older chimneys here were laid in soft lime based mortar, deliberately weaker than the brick so it takes up movement and can be raked and repointed without damage. Where a previous contractor filled the joints with modern portland cement, the joint becomes harder than the brick, movement is forced into the brick faces, and the faces spall. Cutting a reglet in masonry like that takes some care. This is why proper slate roof flashing repair often includes masonry work: if the brick above the flashing is spalled and porous, water passes through the chimney body and reappears below the flashing line, and new metal fixes nothing.
Crickets on wide chimneys
A chimney sitting across the slope acts as a dam. Water coming down the plane above collects against the uphill face, along with leaves, pine needles, and in winter, snow and ice. A narrow chimney with a well made soldered back pan handles that. A wide one cannot, because the water sits against the masonry instead of moving.
The fix is a cricket, sometimes called a saddle: a small framed ridge behind the chimney, shaped like a miniature gable, that divides the water and pushes it around both sides. As a general rule, a chimney more than about two feet wide across the slope should have one, and the case gets stronger on steeper roofs and long upslope runs. On older houses where a wide chimney was flashed with a flat pan and nothing else, the pattern is always the same: the area directly behind the chimney stays damp, the sheathing rots, and the back pan corrodes from standing water and debris long before the rest of the metal does.
Differential movement, and why chimney flashings wear out
A masonry chimney is founded on the ground or on a masonry base. The roof is a timber structure carried on the walls. These two things move differently, and the joint between them is never still.
- Seasonal moisture. Timber framing takes on and gives up moisture through the year and changes dimension as it does. Masonry does not.
- Thermal movement. A flue heats the masonry from inside, and sun heats the faces unevenly, so a chimney is often expanding on the south side while the north side is not.
- Settlement. Chimneys are heavy and often sit on their own footing. Slow settlement relative to the house opens the joint at the roofline over decades.
- Snow and ice loading. Load on the roof plane deflects the framing while the chimney stays put.
This is why a rigid, sealed connection between roof and chimney fails and a layered, overlapping one does not. Correct flashing does not bond the two together. It lets them move past each other while still shedding water, because the counterflashing simply hangs down over the base flashing and can slide.
That is the most useful thing to take from all of this. If somebody proposes to solve a chimney leak with sealant, tape, or a membrane wrapped around the base, they are proposing a repair that has to fail. Chimneys are the usual reason we are called for slate roof leak repair, and in most of those cases the last two or three attempted fixes were all some version of sealing the joint shut.
What we check when we get up there
- Is there counterflashing, and is it let in? Surface mounted metal with a caulk line is a finding on its own, regardless of how the rest looks.
- Is the side flashing stepped or continuous? One long strip up the side tells us the work was done by someone used to a different roof covering.
- What metal is it, and does more than one metal share water? Copper above galvanized steel corrodes the steel faster than it would age alone.
- Is there a cricket where the width calls for one? If not, we look hard at the sheathing behind the chimney for softness.
- Is the masonry sound above the flashing line? Open joints, spalled faces, a cracked crown, or a missing cap all put water into the chimney body above where flashing can help.
- What condition is the surrounding slate in? Flashing cannot be replaced without lifting slate, so the number of pieces likely to break during access shapes the scope.
Roof age runs in clusters by neighborhood. Among the brick and frame houses with slate roofs in Forest, many chimneys were reflashed once in the middle of the last century, so that metal is now at or past the end of its second cycle. The same holds on the older streets where we do slate roof work in Roanoke, where tall corbelled chimneys on steep slate roofs put a great deal of water through a small amount of metal.
Common questions
Why does my chimney only leak in heavy or wind driven rain?
Because the defect is above the normal water line. In ordinary rain the apron and step flashing handle the flow. In wind driven rain, water is pushed up the roof, against the sides of the chimney, and into open mortar joints and the top edge of surface mounted counterflashing. A leak that only shows in storms almost always means an upper edge problem, not a lower one.
Can chimney flashing be repaired without removing slate?
Counterflashing sits above the slate line and can sometimes be renewed on its own. Base flashing cannot. Apron and step flashing run under the slate, so the slate over them has to come off and go back. Anyone offering to replace the full flashing without disturbing a single slate is describing a surface repair, not a flashing replacement.
How long should a chimney flashing last?
Soldered copper, correctly let into sound masonry, will generally run for many decades and can approach the life of the slate itself. Galvanized steel is shorter lived, and its life depends heavily on how much runoff crosses it. Sealant based repairs are temporary, and they should be described that way when they are sold.
Find out what is actually happening at your chimney
A chimney leak that keeps coming back has not been diagnosed yet. We can look at the flashing sequence, the masonry above it, and the slate around it, and tell you what is failing and what the real repair involves. Chimney work and general slate roof repair usually go together on older houses. Call +15555555555 to schedule an inspection.
