The Content Of The Article:

A chimney strain that is no longer sealed can cause serious disturbances in the frame. Its repair is within the reach of any handyman, provided you work methodically and safely.

Repair a chimney stump

Practical advice

• Masonry cements are only suitable for repairing flashing. To renovate the crown, prepare mortar by mixing 1 part cement, 2 parts sand and 3 parts chippings.
• Whatever the height of the roof you are working on, watch out for safety! Do not neglect equipment and materials: a well-secured ladder on the ground, a roof ladder and a safety harness if necessary.

The chimney stump

Visible external part of the chimney, the strain (or exit of roof) is protected from bad weather by a hat (or mitron) which also favors the draw. In our example, it is made of blocks of 10 cm thick on a corrugated sheet roof that houses a small technical room.

Analyze and understand before intervening

The junction between the duct and the roof (called flashing) was made with mortar, as is generally the case on tiled roofs. But on a sheet, zinc or slate roofing, the flashing must consist of a metal flap that seals and flushes the rainwater. Here, the absence of this connection resulted in infiltration due to cracks in the mortar.

Restore the waterproofing with the roof

The work therefore consists in laying a sealing flap, zinc sheet generally available in the shops. Material rather malleable, zinc can slip under the cover, be modeled according to the shape of the junction with the roof, and go up along the stump.

The sealing mortar must be flexible enough to withstand temperature changes. It is composed of sand and masonry cement ("Baticem" from Ciments Calcia, "Multibat" from Lafarge...).

Repair the crowning of the stump

On this stump, the hat also suffered from bad weather: the water was infiltrated by a crack and the gel completely broke part of the belly. To repair it, remove all the damaged part and rebuild it with reinforced concrete. The reinforcement, here a concrete iron, can also be made using long screws pegged in the support.

Remove the cracked part

Remove the cracked part

Draw the cracked part with a chisel and a mallet. Attach a plank to the stump with large clamps to retain the rubble. For your safety, use a harness.

The sealing lip

The sealing lip

Make the sealing lip: cut a sheet of zinc (15 cm wide), pinch it between a cleat and a metal tube and print a twisting motion to form a rounded edge.

Coat with mortar

Coat with mortar

Wet and coat with mortar (2 half-vol of fine sand for 1 volume of masonry cement). Let dry 2 days. Apply a bead of special roof sealant (Rubson).

Fix the zinc sheet

Fix the zinc sheet

Drill pilot holes (Ø 4mm) in the upper part of the bib. Drill and peg the lower body of the stump. Screw the zinc sheet. Its curved edge must flatten the sealant seal.

Cover the bib

Cover the bib

Finish the flashing by trowelling (from the bottom up) a sealing mortar (see photo composition 3) that covers and connects the bib to the masonry.

Repair the coronation

Repair the coronation

To repair the cap, drill each side of the corner and insert an angled concrete wrench (Ø 6mm). Apply with a brush a mixture half water / resin ("Sikalatex").

Pour concrete

Pour concrete

Clamp a formwork using large clamps: two planks and a cleat nailed square (oiled beforehand). Pour the concrete and vibrate it by tapping the hammer on the walls.

Touching up

Touching up

Remove the form after two days of drying. If the concrete is well vibrated, the surfaces require little touch-up: sand them with the corundum paper (grain 40) to attenuate the connections.


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