Masonry chimney flues are designed to vent the front opening of the fireplace. The larger the size of the fireplace, the larger the area is that is requires for the flue to be. The standard chimney sizes are measured from the outside distance of the flue liner. Some liners are square and some are rectangular. The most common sizes of chimney liners are 13×13 (very common), 8×18, 13×18 (vents larger fireplace openings), 13×8 (very small fireplaces and furnaces) and 8×8 (furnace flues.)
Flue tiles are typically stacked on top of each other and mortared together with high temperature mortar. The tiles are terra cotta and heat fired to produce great insulating materials for masonry chimneys. The biggest problem with terra cotta flue tiles are that they are very brittle and can crack if they experience drastic temperature changes. If a terra cotta chimney has a chimney fire, the flue gas quickly increases in temperature. This quick temperature change typically causes damage to the chimney liner. Damaged chimney liners can sometimes be difficult to spot because cracks can look hairline in nature. Nearly all homeowners insurance companies will pay for chimney liner replacement if the damage was a result of a chimney fire. Good documentation and understanding of thermo dynamics of a chimney fire is very important.