Slow Damage Caused by a Missing Chimney Cap (Part I) – Risks and Solutions
Two weeks ago, here on our blog and website, we took a look at a an issue at the side termination of a low-slope / flat roof that led to serious masonry damage to a wall below the roof. Ostensibly, without really knowing much about it, it looks like this is simply a brick masonry problem.
If you really trace through and look more closely though you’ll find out this is actually not a problem that originates with the brick, it comes from the chimney above the roof. Chimneys might not be a roofer’s problem, in most cases, but for flat roofing contractors like ourselves, here with historic buildings in Washington DC, the portion of the chimney above the roof is normally an aspect of the building that we will look after or repair, in many cases, particularly where the chimney interfaces with the roof membrane.
The next picture of the wall shows that same brick facade but directly, head on, at a direct view. If you look, you can see efflorescence, a form of deterioration of masonry, caused by excessive hydration. The efflorescence in this case expands to a large area of the wall, but farther below will show additional pictures with a greater wider angle view of the wall which show that this large area of efflorescence is actually limited to the area below and beside the base of the chimney.
The next picture below shows that wider view, as you step back and get farther away from the building, you can see the majority of the brickwork does not have this damage. It’s isolated to the area around the chimney. For years, this problem has been ongoing without remediation or mitigation. For an important and valuable historic building like this, they should have had their roof simply checked by a professional roofing contractor like Dupont Roofing, just to make sure elements like this were not slowly leaking.
As water slowly entered between the top of the chimney and the adjacent roof membrane, it saturated the brick, dissolved the materials around the aggregate and binder within the mortar, and then leech those materials onto the side of the building as the water escaped and dissipated through evaporation.
Here in these wider view angle pictures you can see that the area of damage is in fact limited to the area around the base of the chimney. Although it’s still a large area and definitely a problem, it’s at least good that the entirety of the facade has not been damaged in the same way. This example shows how water disperses and permeates through masonry as the water slowly (over years) absorbs into the brickwork from the chimney.
The next picture below shows a similar rooftop chimney being restored in a similar way.
During the time of the picture, the chimney top has been wrapped to prevent water entry while the chimney cap has been removed for repointing. Repointing involves fixing the damaged and deteriorated mortar, an important step before reattaching the roof membrane, in this case to the base of the chimney. At the base of the chimney, the roof membrane transitions to counter flashing which is, in most cases, set into the side of the brickwork. That counter flashing spans, normally from a reglet or raggle, over the base of the chimney onto the upturn in the roof membrane where the roof meets the side of the chimney.
Here the chimney cap is removed during restoration. This particular chimney stack is in the middle of repointing process and here the mortar has been applied but not yet cleaned and excessive mortar is shown around each of the bricks.
The next picture below shows a different chimney with a concrete cap. Although this concrete cap will keep the majority of water out of the chimney, we prefer a chimney cap that extends farther beyond the sides of the opening into the chimney to prevent a greater degree of wind driven rain.
People often don’t realize that above a rooftop, there are several elements other than just the roof itself. The roof itself covers the majority of the overall area, in most cases, but these separate elements like chimney stacks, plumbing vent stacks, HVAC piping / vents, electrical cabling penetrations, even antennas, and other accessory details can be points of significant leakage if not properly maintained. A full service roofing contractor should at least be able to look at these details and rule out or spot obvious problems that could lead to leakage. These problems are very common. In fact the majority of leaks happen at the interface between the roof and accessory elements such as chimneys and HVAC equipment.
The next picture below shows a chimney cap with an extending flange at the perimeter edge of the cap to protect the opening more from the majority of typical rainfall.
We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com.
