Drips, Overhangs and Stops on a Rendered Stack Head

Why the shape of the head matters

Rain landing on a stack does two things. Some of it soaks in, and the rest runs down the face. On an unprotected flat face, that running water reaches the roof junction in a continuous sheet, carrying dirt with it and finding every crack on the way. A good head interrupts that path. It carries the water out beyond the face and lets it drip clear, so that only the rain that falls directly on the face has to be dealt with by the render.

A bare brick stack often does this naturally, because older chimneys were built with corbelled courses stepping out near the top. Rendered stacks, particularly the plain, quickly built kind, often have no such feature at all. The render is simply taken straight up to the pot and finished flush with the face, and the entire exposed top of the chimney has to cope with whatever falls on it. The flat top edge of the render takes in water readily, and from there it tracks down into the body of the stack and into the joints behind.

On the exposed, wind-blown ground that our chimney repairs in Deeside page describes, that matters even more. Rain is driven across the face, not just down it, and a head that does not throw water clear takes the brunt of it.

Oversails, drips and bells

An oversail is a projection at or near the top of the stack, where the face steps outwards before the flaunching and pot. The projection is usually a few courses of brick set slightly proud, or a rendered band that does the same job. Taller projections throw water further, but they also need to be well bonded, because they are the first part of the stack to be hit by wind. Whatever its form, its purpose is to shelter the face beneath and to carry water over the edge.

A drip is a small groove or a sharp lower edge that stops water creeping back under a projection along the underside. Without one, water follows the underside of the overhang by surface tension, runs back to the face and defeats the purpose of the projection. A formed drip, in the render or in a separate lead or slate component, forces the water to detach and fall.

A bell, sometimes called a bellcast, is the sloping flare at the base of a rendered stack, which finishes the render above the lead flashing so that water is thrown out onto the lead. It is usually formed with a slight outward slope and a clean, hard edge, so that the drip point is sharp and the water cannot cling to it. It stops the render from being buried over the top edge of the lead, which is a common cause of leaks. We cover the lead side of that junction in Lead Flashing or a Cement Fillet? How to Tell Which You Have.

Stops, beads and where they go wrong

A stop is the neat finished edge of a render coat: a bead or profile fixed to the wall before the render goes on, which gives a straight line to work to and protects the edge from knocks. At the head and at the corners, stops give a clean line, and at the base they help to terminate the render above the roof covering.

The trouble is that many beads are galvanised or plain steel, and in a wet, exposed position they corrode. As the metal rusts it expands, which pushes the render away from the brick and creates a line of cracking that follows the bead. A rusty streak below the head, or a perfectly straight crack running horizontally, can mean that this is happening.

If you see those signs, the remedy is to cut the affected render back, remove the corroded metal and replace it with a stainless or plastic profile, or to form the edge without a bead at all. Patching over rust without removing it only delays the problem. The same corrosion problem also appears in the render lower down the stack, and if it is found, the replacement is specified in the quotation.

What happens when the details are missing

The pattern is predictable. With no overhang, the head of the stack stays wet for longer than it should. The render around the pot cracks first, then starts to lift. Water finds its way behind and the top courses of brick begin to decay. Dark streaks run down below the head, dirt collects along the edge of every crack, and the lower face stays damp long after the weather has cleared.

The fix is often easier than people expect. A new head can be formed with a small projecting band, a properly shaped flaunching to throw water away from the pot, and a drip at the lower edge. A bell can be formed above the lead. It is work on the outside of the chimney, and it is something we do as part of rendering repairs and head rebuilds. The shape is worked out on site from what is already there, because every stack is a little different.

Before agreeing to any work, check that the quotation says how the water will be shed at the top and at the bottom, not just that “the render will be repaired”. Our guide to Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need? covers the other things that sit on the head, and any work on the flue itself is for a HETAS or Gas Safe registered installer or a sweep. Please do not climb to inspect the head; the ground and a good zoom lens will do.