QUESTION -- We have a problem that I have seen on other homes as well, but I have been unable to find any information about it on the Internet or anywhere else. We have stucco on our house and two of the windows have brownish-black stains going downward from the bottom corners. This problem only started after the windows were replaced.
We replaced all the wood-framed slider windows, 11 of them, about five years ago. The new windows are triple-pane vinyl units and because they were a different design we had to trim the stucco slightly so they could be inserted all the way. The resulting gap, about three millimetres, between the edge of the stucco and the new window was caulked with silicone.
I thought that there might be moisture getting in so I pulled all the old silicone out on these two windows and re-caulked them with new silicone, about two weeks ago. I expected to find some wood rot, which I assumed was causing the stains, but found all the wood to be in perfect shape.
What causes these unsightly stains, and more importantly, how do I get rid of them?
Marv Ginter, e-mail
Answer -- Your assumption of the possibility of rotting wood behind or around windows is very possible, but this rarely causes visible stains on the stucco or siding over top. I am not surprised that the wood behind the brick mould or jamb of the newer vinyl windows is in good shape if it was well-sealed at the time of installation, but the problem may originate in a different location. I will offer some possibilities, but the answer may not be fully answerable without pulling out the windows.
The first item to address is the choice of caulking used to seal the small gap created around the newly installed windows. While silicone caulking is an excellent product for sealing around many areas in your home, particularly kitchens and bathrooms, it is not ideal for exterior use. While the silicone will adhere well to the PVC windows, it may not adhere well to the rough, porous stucco. Also, most regular silicone caulking will not resist the ultraviolet rays of the sun well and may shrink or pull away from the area where it is installed, over time. While I don't think this is the underlying cause of your problem, the remainder of your windows should be re-caulked with material designed specifically for window exteriors.
When diagnosing house problems, the first thing I always ask the homeowners is if are any changes in the home in the way of renovations or usage. In your case, changing the windows is an obvious cause of the new stains, but we must look further to determine what else was done during replacement. You stated that the stucco was cut, slightly, to accommodate the new window frames. This is the next logical place to investigate and we must look at the composition of the stucco that was cut away to see where the problem may originate.
Stucco siding has several component layers when it is installed over exterior sheathing on a home. The first component is the building paper or housewrap that is applied directly over the sheathing. This thin membrane prevents wind and air from entering the building walls and helps shed water that may get behind the various stucco layers. The next component is crimped stucco wire that is nailed directly over the building paper. The wire is crimped to provide a thin drainage plane or space behind the stucco once it cures and shrinks away from the paper behind. This space is critical to help the porous stucco dry after it gets wet from rain or snow. The next layers are a base layer of mortar or "scratch coat," followed by a coloured top coat and "splash coat," which give the stucco its textured appearance. On older homes like yours, all the components were usually installed after the windows, ensuring a good seal at the edge of the brick mould. Once you removed the windows and cut the stucco, you altered all of these layers.
My best guess for the cause of the stains is due to the stucco wire embedded in the stucco layers. This wire is normally made from galvanized steel, which has fairly good resistance to rusting, especially if it is embedded in the stucco shortly after installation. Unfortunately, when the wire is cut, the inner core may expose edges that are not sufficiently coated in the zinc used in the galvanizing process. These small areas may have begun to corrode once the stucco was cleanly cut, and when wetted by rain are bleeding through the stucco surface.
The next most common cause of the stains may have a similar origin. Most manufactured windows, particularly PVC ones, require some fasteners to secure components of the frame, nailing flange, or brick mould. If these fasteners are made from regular steel, without proper galvanizing or other corrosion-preventing coatings, they may easily rust. These are often hidden behind the window frame or brick mould, but still may be subject to periodic moisture that gets behind these areas.
No matter which of these problems is causing the unsightly stains, removal of at least one window may be required to see where the offending corrosion is occurring. Taking newly installed windows out to investigate some minor stains may be an overreaction, and attempts to clean or minimize the stains should be attempted first. If the stains are rust-related, treating them with a mild acid may help reduce the appearance. Undiluted vinegar may be painted or sprayed on the stains to reduce the appearance. If they fade considerably when this is attempted, then my suspicions should be confirmed. If the vinegar has little or no effect, then removal of the windows may be necessary to find the source of the problem.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.
