New Chimney Crown

[12handyman]

Hello to everyone. Saw an old thread that is now closed on installing a new crown and have a couple of questions that I can't seem to find an answer for so hoping someone can shed some light on this subject. Have a single 8x8 flue in a 16x16 brick chimney that vents a gas hot water heater and gas furnace and will be pre-casting it in my basement. I built a form in which I will be using bag fiber impregnated cement mix with rebar placed in it for overkill with a 3/8" gap between the back of the crown and the flue pipe then coat it with a water repellent after it cures and all the other stuff--- back to front pitch--- overhang--- drip groove. 1) Going to put backer rod in that gap before caulking and need to know if regular closed cell rod would work. Not sure how hot the top part of the flue gets but the rod I was looking at is rated at 180*. 2) I know I will need a bond breaker between the brick and the crown and have seen roof felt and sheet metal used. When it said sheet metal not sure if it meant aluminum or galvanized or doesn't matter. I have a roll of aluminum coil stock available and wanted to know if the alum would ok to use or how about using self adhesive window and door rubber flashing and stick it to the bottom of the crown before placing it in place? Will see what responses I get before asking anymore questions. Thanks for any help. LARRY

 

[bholler]

regular backer rod will be fine and really no need for a bond break in this case i usually just set precast top rings with silicone which stays flexible enough that it is not an issue. If you where pouring it in place then yes you need that bond break

 

[12handyman]

thanks for the reply. do you put a bead of silicone around the whole bottom or say just the corners?