This stainless steel Collar plate seals the top of masonry chimneys, supports the liner and attaches with a band connectionThe collar plate base measures 13" x 13", has a hole for an 8" pipe and is 4" tallNeed a larger base plate?We can also supply you with a collar plate that has a base dimension of 13" x 18" part# 34282 or 18" x 18" part# 34294 for a slightly higher price. If you do not find these parts currently listed on our site you can order this one and put instructions in the comments box during checkout to change it with the part number and size of your selection with authorization to charge you no more than $7 more. Ceramic wool is usually used for a seal below the collar plate to keep heat or air from escaping around the liner pipe. You can find this product in bulk by clicking here.