Can a Chimney Cause a Roof Leak?

Why the chimney is the weak spot in a roof

A roof is built to shed water in one direction: downhill, shingle over shingle. A chimney is a large block of masonry sitting in the middle of that flow. Water running down the roof hits the uphill side of the chimney and has to be steered around it, and every joint between the masonry and the roofing is a place for it to slip underneath.

The chimney also catches rain on all four faces above the roof. Brick and mortar absorb water, and in wind-driven rain, which nor'easters deliver for hours at a time, a lot of it. Some of that water moves down through the masonry until it reaches the roof line and the framing inside, where no amount of good roofing can stop it.

Where the water gets in, from most common to least

The usual culprit is flashing. Step flashing is woven into the shingles along the sides of the chimney, and counter-flashing is set into the mortar joints of the brick and folded down over it. When the counter-flashing pulls out of its joint, when it was only caulked to the face of the brick, or when the metal rusts or cracks, water runs straight behind it.

Less often, but still common, the water comes from above: a cracked crown or mortar wash, open mortar joints on the weather side, or porous, spalled brick that soaks up rain like a sponge. A missing cap lets rain fall down the flue itself, but that water usually shows up in the fireplace rather than on a ceiling.

• Counter-flashing pulled out of the mortar joint or held only by caulk
• Rusted or cracked step flashing along the sides
• No cricket on the uphill side of a wide chimney, so water and debris pond there
• Cracked crown or mortar wash letting water into the brick
• Open joints and spalled brick on the weather-facing side

Crickets on wide chimneys

A cricket, sometimes called a saddle, is a small peaked roof built on the uphill side of a chimney. It splits the water coming down the roof and sends it around both sides instead of letting it pile up against the brick. Without one, water, leaves and in winter a dam of snow and ice sit against the back of the chimney, and the flashing there is under water every storm.

Wider chimneys, common on older colonials and homes with two or three flues side by side, need a cricket far more than a narrow one does. If the back of your chimney has no cricket and the stain is on the uphill side, that is a strong lead.

A cricket is framed and sheathed like a small roof, then covered and flashed so it ties into both the main roofing and the chimney. It is usually added when the roof is replaced, since it is built under the roofing, which is one more reason to look at the chimney before a reroof rather than after.

Flat-roof rowhouses work differently

On brownstones and Philadelphia rowhouses with flat or low-slope roofs, the chimney usually rises from the party wall or the parapet, and the roofing membrane is turned up against the brick and covered with metal or a membrane flashing. Water does not run past the chimney so much as sit around it after every storm.

Leaks there tend to come from the joint where the membrane meets the brick, from soaked brick above the flashing, or from the half of a shared chimney that belongs to the neighbour. Because the roof deck is nearly flat, water can travel a long way along it before dripping, and the ceiling stain often appears in the middle of a top-floor room, nowhere near the chimney breast.

Reading the signs inside

Where and when the water appears is the most useful information you can give whoever comes out. A stain several feet from the chimney with damaged roofing above it is likely a roof problem. A stain hugging the chimney, or one that only shows when rain blows from a certain direction, points to the chimney or its flashing.

What you can check from inside

If you have an attic, shine a flashlight along the wood around the chimney during or right after rain. If the wettest wood touches the brick, the water is most likely coming from the chimney or the joint around it. Look for dark streaks on the rafters running down from the chimney, damp insulation against the brick, and white mineral deposits on the masonry itself.

Water does not always drip straight down. It can run along a rafter or the top of a ceiling joist and show up a few feet away. Note whether the leak appears in every rain or only in certain storms, and whether it shows in the fireplace at the same time. Keep to the attic and the rooms below; the roof itself is a job for someone with the right equipment.

What happens if it is left alone

A slow leak beside a chimney soaks the roof deck and framing every time it rains. Wood that stays wet softens and rots, insulation flattens and stops working, and plaster ceilings stain, sag and crack. In the masonry, trapped water freezes on cold nights and loosens the brick, the mortar and the crown, which lets in even more water the next season.

Replacing the roof does not stop it if the chimney is the source. New roofing goes on, the old counter-flashing and the porous brick stay, and the stain comes back within a few storms.

Who should fix it, and in what order

When the leak is at the chimney, it helps to have someone who works on both the masonry and the flashing, because the two are tied together: counter-flashing is cut into the mortar joints, and a crown or brick repair above it changes how water reaches it. If a roof replacement is already planned, schedule the chimney work first or alongside it, so new flashing goes onto sound masonry.

We check the crown, the cap, the brick and the flashing together, then set out each repair, priced in writing at no charge. That way you are not paying a roofer to patch around a chimney that is still letting water in from the top.