Capping Slabs on Disused Flues: Concrete, Slate and Vented

Three common cap types

A flue that is finished with needs its top closed so that rain, birds and debris stay out. The simplest answer is a flat slab laid over the opening, and the usual materials are precast concrete and slate. A third family, vented caps, are purpose-made fittings that close the top to weather while leaving a gap or louvre for air to move through.

Concrete slabs are heavy, cheap to cast and usually grey or buff. Slate slabs are thinner, darker and neater on a period stack, and they are more at home on brick heads that were originally topped with slate or stone. Vented caps come in clay, metal and moulded concrete patterns, some with a raised hood and a gap beneath, others with louvres built into the sides.

Which of these suits a given stack depends on the width of the head, the number of flues beneath, and whether the pot is staying or going. Whether to cap at all is dealt with in Capping a Flue You No Longer Use, and Why Sealing It Solid Causes Damp, so this guide sticks to the slab itself.

Why a flat slab alone is not enough

The temptation with a disused flue is to lay a slab straight across the pot or the brickwork and bed it down solid. It looks tidy and it keeps the weather out. It also shuts the flue at the top, and where the fireplace has been closed at the bottom, the whole flue is now sealed at both ends.

A sealed flue does not stay dry. Brick takes up moisture from the roof line, soot residue holds damp, and with no air movement the moisture lingers and works into the chimney breast. Many of the damp patches that show up on bedroom breasts in newer-estate houses in Runcorn trace back to exactly this arrangement. The stain on the breast is the visible end of a problem that starts at the cap.

The principle for any cap is therefore the same: shed water from above, allow air to move through. A slab can do that if it is raised on small spacers or supports to leave a gap beneath, and if the flue is also given some way of drawing air from below, such as an air brick or a grille in the breast.

Ventilated caps and how the air gets in

A purpose-made vented cap does the slab’s job and the ventilation job in one fitting. The top is solid enough to shed rain, and the sides or the underside have openings sized to let a steady trickle of air through. Some stand over a pot; others sit directly on the stack head after the pot has been removed.

The choice between a raised slab and a proper vented cap is partly one of appearance and partly of exposure. On a tall, windy stack, a low-profile vented cap with sheltered openings tends to work better than a slab on stilts. On a short, sheltered estate stack, either can do the job if the fit is good.

Whatever the type, the ventilation only works if the bottom of the flue is also able to breathe. That is a matter for the room below: a small grille or air brick in the chimney breast, fitted by whoever opens up the old fireplace opening. We work on the outside of the stack, so the top is our part of that arrangement; the bottom needs agreeing separately.

Fitting a slab or cap so it stays put

A slab should be bedded on the top course of brickwork or on the flaunching, using mortar pads or spacers at the corners rather than a continuous bed across the whole opening. It needs to overhang the brickwork enough to throw water clear, and it needs to be level enough that water runs off rather than pooling. If the head is too uneven, the flaunching is re-formed first so that the slab has something true to sit on.

The size matters more than people expect. A slab that only just covers the opening leaves little margin for movement, and a slab that is too small can slip or crack at a corner. For multi-flue heads, one large slab is often replaced by a pot-by-pot arrangement, since a wide slab spanning several openings is more likely to crack in the middle.

Fitting is done from a scaffold platform at head height. Concrete caps are heavy, and slate is easily chipped, so both need handling with two hands and a proper working area. A ladder alone is not a safe place to do it.

Cracks, staining and when to replace

A cracked slab usually tells a straightforward story. A single crack across the middle often means the slab was bedded rigid over a gap and flexed under load or frost. A crack at the corner suggests a slab that was too small or sat on a damaged brick. A network of fine cracks across a concrete slab is more likely to be an ageing surface than a structural issue, but it does let water in.

Dark staining beneath the overhang, or a green crust on the top surface, is a sign that water is sitting rather than draining. That can come from a slab that has settled out of level. White deposits on the brickwork below suggest water is passing through the stack and drawing salts with it.

A cracked slab is generally replaced rather than patched, since repairs in thin mortar do not last on a surface that moves. If the stack itself is in poor condition, the better question may be whether to rebuild or remove it, which Repoint, Rebuild or Remove? How the Decision Gets Made covers. Our guidance for the surrounding area is on the chimney repairs in Runcorn page.