Guest guest Posted October 20, 2006 Report Share Posted October 20, 2006 Steve: You ask a fair and reasonable question, and I agree with you that good flashing is typically adequate to mitigate water seepage between the vertical chimney and the sloped roof system. Most builders will do just that, i.e., flashing. It is a walk-away system, and for the most part it works; at least in the beginning. This said, I have no rule of thumb for you. From my perspective, 6 to 8 linear feet, or more, of roof system generates quite a velocity of flow in a moderate to heavy rainfall on slopes greater than 4-in-1. The higher the velocity the greater the potential for water to blow past (up-slope) good flashing. I believe this velocity element is more critical than freeze-thaw conditions, but I don’t have sufficient experience in cold climates to comment on that issue. I see the cricket mitigating the velocity of flow very well. Moreover, the transition between the roof membrane and the vertical chimney takes some significant expansion-contraction forces due to temperature changes and dynamic loads due to wind. This transition needs to be flexible, but water tight. Which brings me to my biggest and most compelling factor why a cricket is almost always needed/beneficial....this transition needs routine maintenance. Lets face it, most folks just don’t service or maintain their roof system on a regular basis; be it gutter cleaning, mastic on vent pipes, or flashing inspections. A cricket will help move the flow of water away from a chimney, even if not maintained, better than flashing could (when equally not maintained). I have even installed sheet-metal crickets on round (stove-pipe) metal chimney pipes in heavy snow-load areas, solely to slice the snow pack into two pieces so it does not push the pipe off the roof. Works very well. Do I have a cricket on my home...No. My roof is 1-in-1 slope and built to withstand a snow pack of 3-ft; which almost never happens, though it did in 98 in a truly bizarre circumstance. When I build my 2nd story with my 5-in-1 sloped roof, I will definitely install a cricket because my chimney is near an exterior wall and 12-ft from the ridge. I would loose the stove pipe to the snow pack if I didn’t. Given the benefits of a cricket, it is surprising how few are installed at the time of construction, and it is because the damn things are expensive to construct; especially when tile roof systems are installed. I would guess that whenever water leaks around a chimney, it is almost always on the up-slope side. Crickets should mitigate most of this. Anyone care to share a story when the water seepage originated from a side-slope, or down-slope side of a chimney? -- Geyer, PE, CIH, CSP President KERNTEC Industries, Inc. Bakersfield, California www.kerntecindustries.com And, yes, there will be those that are suffering from the lack of a chimney cricket. Don Fugler CMHC Policy and Research Is there a rule of thumb for when a chimney cricket should be used? Based upon roof slope or width of chimney maybe? In my area, I have only seen a couple in hundreds of inspections. As long as water isn't being held by the upslope chimney wall exacerbating freeze-thaw damage, good flashing is typically adequate. Steve Temes (wishing upon a star) Quote Link to comment Share on other sites More sharing options...
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