Q.: I've been told that your issue of last October had an article on waterproofing, but I don't know if it had anything to do with bricks. I am having difficulty with water seeping through my chimney. I would appreciate any information you could send me about coatings that would stop moisture seepage through bricks.
A.: We have little information on brick masonry since we concentrate on cast-in-place concrete and associated subjects. In April 1981 we published a series of three articles on the general subject of waterproofing concrete which you might find informative:
- Waterproofing: Who Needs It? pages 305, 307
- Dampproofing: Why, Where and How? pages 309, 311
- Waterproofing: Establishing a Barrier, pages 313, 315, 317-319
- Waterproofing Checklist, page 321
However, waterproofing a brick chimney may not be a simple matter of choosing and applying the right coating. Although untreated bricks do absorb water and although water does move through untreated bricks, it is probably much more common for seepage to come through cracks between the mortar and the bricks. If this is what is happening in your chimney, perhaps the problem can be remedied by tuckpointing. Another common cause of water problems in chimneys is cracking of the chimney cap or a separation between the chimney cap and the flue. Rainwater penetrating a cracked cap may later appear on the chimney surface at a considerably lower level. The same tradesman who does tuckpointing repairs should be able to repair the chimney cap.
If it turns out that you do need to treat the whole chimney surface you might want to use a clear, breathable coating. Or you might choose a water repellent such as a silicone or a stearate. These will reduce permeability of the brick to liquid water, yet allow water vapor to escape. They will not seal cracks between brick and mortar.