"Understanding Your Home" by Building Inspector Mark Visser
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Preventing "Black Edge"
It is recommended to remove the baseboards and seal the space between the plywood sub-floor and drywall to stop any air movement between rooms.
The soot-like black discoloration is almost impossible to remove with common carpet cleaners and even professional carpet cleaning companies have a hard time removing the stains.


Dark Stains on Hardwood Floors
Dark Stains on Carpet Along Baseboards
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Hardwood floor stains. Refinshing hardwood floors. Carpet stains. Carpet cleaning.

Dark stains on hardwood floors
are often caused by wet boots, overflowing planters, leaking radiator valves or pet urine. The dark stains can be removed as follows:
A concentrated solution of oxalic acid crystals will bleach out the water stains very quickly. Cut a piece of absorbent cloth large enough to cover the stain. Dissolve 15 grams of the crystals in 120 ml. of water and soak the cloth in this solution, then lay it on the stain. When the stain is gone (usually in 1/2 hour) remove the cloth and sponge the area with a damp cloth, to which a few drops of household ammonia have been added, to neutralize the acid. Allow the spot to dry and rub with #000 steel wool to smooth out any raised wood grains, then apply a little paste wax and buff.
Dark carpet stains along baseboards. 'Draft markings' or 'black edge' is a common problem with light coloured carpets. Exterior walls are not affected. The stains are caused by dirty air blowing from room to room underneath interior walls. The carpet catches the dirt particles which, over time, form a dark area along the edge of the carpet in front of the baseboards.

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