Exposure at the Top of the Valley: Draper Caps

Wind uplift is the design load

Sitting up against the Traverse Mountains at the closed end of the valley, these homes take sustained wind that houses lower down do not. The load on a chimney cap is mostly uplift: air moving fast over a horizontal lid generates lift in the same way it does over a wing. A cap held down with two sheet-metal screws into thin material will eventually let go, and when it does it usually goes at night in a storm and nobody notices until water appears somewhere inside. Fastening into sound material, in the right number of places, with stainless hardware that will not corrode away, is the difference between a cap that lasts and one that is a consumable.

Sun is the quiet destroyer

At this elevation the ultraviolet load is high and the daily temperature swing is large. That combination is hard on every flexible material up there. The sealant at the storm collar, which is the band that seals where the flue pipe passes through the chase top, becomes hard and brittle and then cracks. From that moment water runs straight down the outside of the pipe into the chase. It is a five-minute maintenance item and one of the most common sources of a mystery leak we get called to. We check it every time we are on a roof.

What a chase top is doing wrong

Factory chase covers are commonly flat galvanized steel. Flat means water sits, sitting water means rust, and rust means holes. The correct specification is stainless with a slope or a cross-break so water runs off, an overhang past the chase sides, and a drip edge so runoff falls clear rather than tracking back under and down the stucco. If you have dark streaking down the chimney below the top, that drip edge is what is missing.

Dampers, and the ones that are not there

Worth knowing which system you have. Factory-built fireplaces do not have a throat damper in the masonry sense; the flue is sealed by design and there is nothing for you to open or close. Masonry fireplaces do, and on an older Draper home that plate is often warped enough to leak permanently. If a wood fireplace has been converted to a vented gas log set, the damper will have been fixed permanently open with a clamp, which is a safety requirement and not a fault, but it does mean the chimney is open to the sky year round. A top-sealing damper is the way to close that path back up without interfering with the appliance.