Chimney repairs should be undertaken only by competent and confident people, using the correct access equipment. Depending on the scale of the repairs, it may be necessary to haul up mortar, bricks and even new chimney pots, and a variety of bricklaying tools may be needed. The job will be a comparatively lengthy one, including putting up and taking down the access equipment. Stout rubber-soled shoes, thick trousers and gloves are recommended.
Chimney pots are secured on top of their stacks with a bed of mortar called flaunching which is shaped round the base of the pot and sloped away to the edges of the stack. If this has cracked or crumbled away, it must be hacked off and replaced with fresh mortar. If the pot is cracked or broken, it should be replaced at the same time or removed if the flue is no longer in use.
With the old flaunching removed, the pot should be lifted off — it may be very heavy — and either secured to the ladder or lowered to the ground by rope if no longer needed. Then a bed of new mortar, 1 part cement to
3 parts sharp sand, should be laid on the top of the stack, and the pot replaced. If the stack brickwork and the pot are wetted first, the adhesion of the mortar will be improved. Then the mortar can be built up around the pot to a thickness there of about 75mm (3in), and trowelled to a smooth slope towards the edges so that water will run off. Ideally, when dry it should be given a coat of silicone water-repellent to stop water penetrating it in future.
If the stack is of brick or stone, you may find that the pointing is crumbling. Rake the old mortar out to a depth of at least 18mm (3/4in) and damp the brickwork before re-pointing with a mixture of 1 part cement to
4 parts soft sand. Press the mortar firmly into the joints, and finish off neatly with your trowel.
Where the stack has been finished off with rendering or pebbledash, you may find that this has cracked or fallen off altogether. In this case it should be hacked off completely, and the stack re-rendered.
Sealing the flue
If a flue is no longer used, the pot should be removed and the flue capped with a piece of slate over the flue opening, or with a small ventilating cowl. If slate is used, it should be set in mortar and covered over with sloped flaunching; to maintain some ventilation in the flue, an airbrick should be let into the side of the flue in the roof.
The junction between the chimney stack and the roof slope is waterproofed with an apron flashing, usually of lead or zinc. These are mortared into the brickwork of the stack, are tucked under the tiles above the stack, and laid over them below it. Repoint if necessary where the flashings meet the stack, and press the flashing down over the tiles if it has been lifted by the wind. Then brush bitumen emulsion over the surface of the flashing to seal any pinholes and make the apron completely waterproof. If the sheet has deteriorated badly, you will have to remove it completely and use it as a pattern to cut a new apron from sheets of lead, zinc, aluminium alloy or semi-rigid bitumen-based felt.
Making new flashings
Flashings are formed either from sheet material — lead, zinc, bituminised felt — or from mortar. The former may be torn or porous, while the latter simply cracks and breaks away. It is comparatively straightforward to repair or replace flashings, as long as they can be reached safely.
If a sheet flashing is letting in water, there are three possible reasons. The upper edge of the flashing, which should be let into a mortar course in the wall, may no longer be secure because the pointing is defective. In this case the old pointing should be raked out, the flashing wedged securely in place and the pointing replaced, using a mortar of 1 part cement to 4 parts soft sand.
The lower edge of the flashing should either be pressed down firmly over the surface below, or should be tucked under the upper layer of a multi-layer felted roof. In the first case wind pressure may have lifted the flashing, which should be gently pressed back into place. Bitumen mastic can be used to bed the flashing in place and prevent it lifting in future. In the second case, an imperfect seal may be allowing water to penetrate, and the join should be sealed with one or two applications of bitumen emulsion.
If the flashing is too badly damaged for this sort of repair to be effective, you will have to remove the entire flashing and use it as a pattern for a new flashing. While you can use traditional metal sheet materials for this, you will find it easier to use a proprietary flashing strip which can be cut to length and shape with old scissors and is bedded into a coat of bituminous emulsion. Provided adhesion to the wall above the flashing is good, there is no need to turn the top edge into the mortar course. The strip can be formed around corners and over minor obstructions that may be present.
Mortar flashings that have cracked and crumbled should be hacked away completely and replaced. Use a mortar of 1 part cement to 4 parts soft sand, mixed with a PVA additive to improve adhesion, and apply it in two layers to lessen the tendency for it to crack as it dries out. Cross-hatch the surface of the first layer, and when it has dried apply the second layer, trowelling the surface smooth. Try to avoid feathering out the edge of the mortar too thinly; it will only crack all the more easily. To prevent (or at least discourage) water penetration, brush a couple of coats of silicone water-repellent over the new flashing when it has dried.