Whenever it’s raining and someone sees evidence of water leaking, the first thing they think of is a roof leak — and they're probably right. However, it can also be a masonry leak. If the leak shows up next to a brick chimney or at an opening in a masonry wall and the flashings above the area are installed properly and in good condition, the masonry is probably the problem.
But first, check for any opening in a wall above the leak; windows especially can be the source of a lot of problems. Is the sill angled properly and is the seal around the window in good shape and tight? Is the storm window in good shape and tight? Storm windows are good if used properly, but make sure the glass is in the top and bottom with the top glass lapped over the bottom glass. A lot of people leave the storm windows open in the summer and this is just asking for trouble. The rain driving in is trapped by the storm window frame and the water will work its way down through any crack or joint in the wood that is not tight. Weep holes in the frame allow the water to dribble out while the rain is pouring in.
If the leak is at a chimney and the flashings are in good condition, the first thing to ask is if they have changed to gas as their fuel source. If they have switched to gas, was a stainless steel flue liner installed? If no liner was installed, they will need to get one. Gas furnaces are so efficient today that the gas fumes are not hot enough to escape the chimney and will start condensing on the masonry. This condensation can be enough to saturate the brick and eat away at the mortar.
If the flashing is tight and gas fumes are not a problem, it is probably the masonry absorbing water. If an addition has been added onto a house, a flashing leak will show up on the addition side of the wall. If the masonry is a problem, the leak will show up at the opening in the wall.
To keep water from entering the masonry, we have had very good success with siloxane based Chimney Saver water repellant. ChimneySaver does not trap water the way water proofing products do. It still allows the masonry to breathe naturally so if any moisture does get into the masonry it can vent out. We have done entire church walls and have had no problems with them for several years now. The product comes in a solvent and water based form and carries a 10 year warranty. We have had better success with the solvent based product, but with either one, follow the instructions carefully.