Chimney flashing needs good drainage

Hey there, time traveller!
This article was published 08/02/2004 (8003 days ago), so information in it may no longer be current.

Question: Rosemary asks: The cap over our fireplace chimney was nailed in several places on top. The nails rusted and caused a leak. The roofer installed a new one exactly like the old one, (about four feet square), but the wind seems to get inside and it sounds like it is buckling. The roofer said he didn’t nail it down because the nails eventually would rust and cause another leak. He told me to lay bricks on it.

ANSWER: Whatever you do, don’t weigh the flashing down — with bricks or anything else. Besides being an eyesore, the weight might cause water to puddle, and rust the entire flashing. You need to stop the flashing from rattling by stiffening it. This will also promote watershed.

It sounds as if the normal creases (stiffening bends), which usually radiate diagonally to the four corners from the hole where the chimney penetrates the cap flashing are nonexistent. These creases also act to raise the centre of the flashing higher than the edges, allowing for proper water shed. If the creases do exist, chances of recourse against the contractor are slim. If you are forced to use the existing cap, you will find that the proper fix is exactly the opposite of what the roofer suggested.

You need to create a constant pressure on the sheet metal from below rather than from above. The rattle will disappear regardless of what side the sheet metal is braced from, but bracing it from below can be useful to improve watershed. Water sheds when the flashing is higher in the centre than at the edges, and the chance for rust is diminished.

First, carefully remove the flashing. Then install wood braces at 16-inch intervals from edge to edge at the top rim of your chimney. The four-foot-long braces should measure from zero at each end and gradually thicken to about one to two inches in the centre.

Caution: Wood braces should be no closer than one inch from the metal fireplace flu. Finally, reinstall the flashing with hot-dip galvanized nails, using the old nail holes. As an extra precaution, use 3M Gutter Seal at each nail head and all seams. The building code requires that all exposed sheet metal flashings be painted (a good rust-preventive measure).

–Associated Press