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E1 – Chimney stacks

November 19, 2010

Chimney stacks are a reminder that prior to the advent of central heating solid fuel heating was the norm and the chimney was a vital element of the home.

However, because chimneys are perched above the roof line, they are particular exposed to the weather. Moreover, being difficult to access and rarely used, they seldom receive adequate attention.

Common chimney stack defects include: cracked cement mortar at the base of the pots (flaunchings), badly weathered mortar pointing and defective flashings.

Chimney stacks located on the side wall of the back addition are very problamatic with regards dampness.

Cracked flaunchings around pots

The Flaunchings (mortar support at base of pots)

The large mass of mortar around the base of the clay chimney pots is important because it anchores the pots to the stack and throws rain clear of the flue.

Due to access problems it is often left for many decades without ever having been properly inspected. Cracks in the flaunchings will lead to penetrating dampness.

Badly eroded cement mortar pointing

Mortar pointing

Pointing is the name given to the sacrificial mortar that fills the first 20mm of the vertical and horizontal gap around the bricks.

The pointing will eventually erode and fall off requiring repointing. If the mortar pointing is not replaced, the mortar joints keeping the bricks apart will deteriorate and eventually the bricks will start to become loose as illustrated in the photo opposite.

By far the best material to use for repointing is lime mortar, which will allow the bricks to dry out. The lime mortar will tend to suck moisture out of the brick and in turn, the air will suck moisture out of the lime mortar. Avoid cement based mortars as the hard impervious compound will entrap moisture thus exposing the damp bricks to frost damage in winter.

Felt underlay was positioned under the lead soaker

Chimney flashings

Chimney flashings are the weatherproof joint sealing the gap where the chimney passes through the roofing material.

Flashings are usually made from lead but zinc, copper, cement mortar and tiles are also used. However, I like to see a well formed lead flashings which should last a good 70-80 years.

Internal dampness around chimneys stacks is almost always a flashing problem. In the photo opposite, my insistance that the flashing detail was defective and allowing rain into the bedroom below almost got me chucked off the scaffolding by the roofer. I was even willing to pay him to replace the flashing! However, he sworn blind that I’d be wasting my money.

However, when I got a proper roofing contractor in, it quickly became apparent that the felt under the concrete tiles had been dressed under the lead flashing behind the chimney. This meant that any moisture getting through the tiles would run down the felt and fall onto the ceiling below.

Chimney stacks is Element E1 on the RICS Homebuyer Report

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