Most rigid chimney liner sections are made from 304 or 316 stainless steel alloys. The 316 is more resistant to corrosion and runs a little higher in price. Rigid pipe comes in sections ranging from 6 inch to 4 foot sections and is usually round in shape. The most common wall thickness is 24-gauge or 22-gauge stainless steel.The seam running down the length of each liner section has factory smooth laser welded seams. Individual liner sections are joined together with the male end facing down. Stainless steel pop rivets and / or stainless steel screws secure the joints. Pop rivets are recommended since screws may work themselves loose from the expansion and contraction of the liner. A storm collar over the support clamp at the top plate, above the chimney, prevents moisture from entering the chimney along the outside of the liner.A smooth wall rigid liner offers the most efficient venting due to decreased turbulence, and is simple to clean. You can shape it to take full advantage of every cubic inch and offer maximum draft. It can be shaped into rectangular, square or oval. The rigid liner can become heavy when trying to install, and while attaching additional sections. Rigid liner can only be used when there are no bends or offsets in the system.Flexible liners, unlike the rigid pipe, are one-piece liners. Flexible liners should be used if your chimney is less than perfectly straight with jogs or offsets, and are flexible enough to form to the shape of the flue. The flexible 316Ti is a stainless steel alloy with a small amount of titanium added. Its make-up allows it to resist acids and the stresses of hot and cold cycles. It can be used with solid fuels (wood, coal and pellet), gas and oil. It can withstand temperatures up to 2100 degrees. Flexible liner can also be shaped into rectangular, square or oval cross section.Flexible liners are available in three thicknesses ranging from .005 for better flexibility, .006 (the long time standard) and the new .013 Double Wall Smooth Wall liner. The smooth wall interior cuts down on creosote buildup which can cause devastating chimney fires and untold loss. It also increases chimney draft by 20% and makes cleaning a breeze.Even though flexible liners have a corrugated surface, they tend to collect less creosote when used to vent wood fireplaces and stoves, because they flex as they expand and contract with temperature fluctuations, causing buildups to loosen and fall away. The ease of installing flexible liner far outweighs any difference between the rigid and flexible liners.