Chimney head
The top section of a chimney above the roof, including the cover slab, drip edge and flue outlet, and the part most exposed to weather.
The chimney head is the top section of a chimney: the part that rises above the roof, carries the flue outlet, and takes the full force of rain, snow, wind and frost. In Slovak practice it is called komínová hlava. It starts where the chimney flashing seals the junction with the roof, so it is the part of the chimney that sits in open weather.
What does the chimney head consist of?
The masonry stack carries the flue walls up through the roof. A cover slab or cap closes the top of the stack around the flue openings. A drip edge throws water clear of the brickwork below. The flue outlet, sometimes fitted with a terminal, releases the gases from the flue.
| Part | Job | Typical problem |
|---|---|---|
| Stack top | Carries the flue walls above the roof | Spalled bricks, eroded mortar joints |
| Cover slab or cap | Closes the top of the stack around the flue | Cracks that let rain into the stack |
| Drip edge | Sends water off the face of the stack | Missing or blocked, so water runs down the brick |
| Flue outlet and terminal | Releases gases, may carry a rain cap or cowl | Corrosion, blockage, or a terminal unsuited to the appliance |
How do the cover slab and its drip protect the stack?
The cover slab is a flat or gently sloped top, usually in concrete, stone or metal, that covers the stack except for the flue openings. Its slope moves rainwater outward, away from the flue and the brick. The drip is the lower edge of the slab, which should project past the face of the stack so that water falls clear of the wall. Without that projection, water runs down the brick face and soaks the top courses, and that is where spalling usually starts.
The cap should not be locked rigidly to the flue. The flue heats and cools far more than the surrounding masonry, so the joint between them has to be able to move. A cap bedded hard against the flue tends to crack at that junction over time.
What do rain caps and cowls change about draught?
A rain cap is a hood over the flue outlet that keeps rain, snow and debris out of the liner, but it must leave the outlet open enough for the gases to leave. A cowl is a terminal that uses the wind to help outflow or to limit the downdraught that wind can push down the flue. Its effect depends on wind direction and speed, so a cowl is a tool for a specific site, not a guaranteed fix.
Spark arrestors, called lapač iskier in Slovak, are fine mesh screens on solid-fuel chimneys that stop embers leaving the flue. Any terminal should follow the appliance manufacturer's instructions.
| Terminal | Main purpose | Points to check |
|---|---|---|
| Plain cap | Keeps rain out of the stack | No moving parts, but no wind assistance |
| Rain cap (hood) | Stops rain, snow and debris entering the flue | Outlet must stay open, check that it has not shifted |
| Static cowl | Uses wind pressure to help outflow | Performance depends on site and wind direction |
| Rotating cowl | Turns to face the wind and limit downdraught | Bearings can seize, so it needs periodic checks |
| Spark arrestor | Stops embers leaving a solid-fuel chimney | Reduces free area, must remain cleanable |
How does the height of the head affect draught?
Draught depends on the height of the hot gas column and on the temperature difference between the flue and the outside air. The head adds to that height, but its position relative to the roof ridge matters as much as its total length. A head that sits low beside the ridge is exposed to turbulence as wind passes over the roof. That turbulence can create a zone of high pressure at the outlet, which pushes gases back down the flue. This is downdraught, and it often shows up as smoke spilling into the room in windy weather.
Nearby obstacles cause the same problem. A tall tree, a neighbouring house or a hillside can deflect wind onto the outlet, even when the chimney itself looks well placed.
| Situation | Likely effect | Typical response |
|---|---|---|
| Head clear of the ridge on open ground | Good draught, wind helps outflow | Standard cap or rain cap |
| Head low beside the ridge | Turbulence and downdraught in windy weather | Raise the stack or fit a cowl |
| Tall obstacle close by | Wind pressure zone at the outlet | Raise the head above the obstacle or fit a cowl |
| Flue too short for the appliance | Weak draught, condensation, smoke spillage | Review the flue design with an appliance specialist |
Why do brick chimney heads fail first?
The top of a brick stack has the worst exposure in the building. Water soaks into the brick and the mortar joints, and when that water freezes it expands. Repeated freeze and thaw cycles break the outer face of the bricks and wear out the joints, which is why spalling, loose bricks and open joints tend to appear first at the head. Frost and weather damage are the most common reasons for repair at the chimney head.
Earlier repairs often make things worse. Hard cement mortar and cement render are stiffer and less permeable than the soft lime mortar used in older brickwork, so the brick face takes the strain instead of the joint, and moisture gets trapped inside the stack. Frost resistance of masonry materials is covered under freeze-thaw resistance.
How is a damaged chimney head repaired?
Repair starts with diagnosis: the specialist checks which part has failed and whether water has reached the flue lining. Loose or spalled bricks are replaced with units matched in size and strength, and the surrounding joints are repointed. On older brickwork the repointing should use a lime-based mortar softer than the brick, so that moisture can leave through the joints rather than through the face of the brick. Cracked caps are replaced, and the drip is restored so that water clears the stack again. A cosmetic skim of cement over a failing head only traps water. If water has reached the flue, the chimney liner needs its own assessment.
How does the chimney sweep reach the head?
The kominár has to clean the full length of the flue, which means access from both ends. Sweeping normally starts at the appliance end, through a cleaning door or access point at the base. The top is reached through the outlet, using rods that pass through the terminal. If the cap or cowl is permanently bedded or welded in place, the sweep cannot pass through and the top of the flue is left uncleaned. The head should be detailed so that the terminal can be lifted, unscrewed or removed without damage.
Inspection at the top also needs safe roof access. A fixed ladder or roof hatch is safer than improvised climbing, and it makes the head easier to check after storms.
Frequently asked questions
- What is the difference between a chimney head and a chimney cap?
- The chimney head is the whole top section of the chimney above the roof, while the cap is one part of it: the cover slab that closes the top of the stack around the flue. The head also includes the outlet and any terminal fitted to it. When discussing repairs it helps to say which part is meant, because a cracked cap and spalled brick at the top need different work.
- Does a rain cap reduce the draught of a chimney?
- A rain cap keeps rain, snow and debris out of the flue, and a correctly sized one has little effect on draught. Problems arise when the cap is undersized, damaged or shifted so that it restricts the outlet. Any terminal should match what the appliance manufacturer specifies.
- Why do chimney heads crack in winter?
- Water enters pores and cracks in the brick, cover slab and mortar during wet weather, then freezes and expands when temperatures fall. Each freeze and thaw cycle widens the damage a little more. Cracks in the cap are often the first sign, because they let water reach the masonry below.
- Can a cowl be fitted to an existing chimney?
- In many cases a cowl can be fitted to an existing outlet, but the right choice depends on the flue size, the appliance and the site. A cowl that suits one roof may perform poorly on another, so have the flue checked before buying. Confirm the fitting with the appliance supplier and a chimney specialist.
- How often should a chimney head be checked?
- The head should be looked at during every sweep or inspection visit, and after severe storms or heavy snow. Brick and mortar damage builds up slowly, so regular checks catch loose bricks and open joints early. Anything that looks loose or cracked should be assessed before the heating season starts.
- Who should inspect and repair the chimney head?
- A chimney specialist, the kominár in Slovak practice, should check the head as part of the flue inspection. Brickwork repairs are best done by a mason who works with lime mortar, and cap or flashing work may need a roofer. If water has reached the flue lining, the specialist should assess the lining as well.