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What is the Purpose of a Cement Chimney Crown/Cap?

#1 When a chimney is without a cement crown or mortar wash/splay on top.

The rain hits the top of the bricks and soaks into the bricks causing wet walls inside and causing the mortar to loosen and pop out from between the bricks. The latter is caused by constant freezing and thawing (contraction & expansion ), which can sometimes be repaired by tuckpointing, but usually will need to be rebuilt. Also with no crown, water can seep between the flue tiles and bricks, will run down inside the chimney and deteriorate the firebox which results in costly repairs to the damper, firebricks, ceilings, etc.

#2 Cement crown will have an expansion joint.

The reason for an expansion joint is to prevent the damage caused by constant freezing and thawing.

#3 The mortar wash/splay prevents water from seeping in through the top of the chimney (for a while).

…but the water still runs down the sides of the bricks, makes them wet and eventually the faces of the bricks pop off which can only be repaired by rebuilding the chimney. Without an expansion joint around the tile the thermal shock (heat) usually cracks the mortar wash/splay during the first season of use.

#4 Finish off the top of your chimney properly and give your chimney many extra years of life.

We also suggest that a rain cover with screening be installed over the flue tile to prevent rain from pouring directly into the firebox and to keep animals and birds out.

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Plainfield, IL

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