Exterior Mildew

Robert Jones Staff asked 2 years ago

I live in a 100 year old house with stained shake cedar shingle siding. The roof rafter tails are exposed as is the roof material on the eave overhang. (3/4″ thick beaded fir like they used to put on porch roofs.)

Five years ago I used to heat gun to totally strip the exposed rafter tails, overhanging roof eaves, and window and door trim.

I then sanded all the wood with 80 grit sandpaper on an orbital sander.

I then brushed/vacuumed all the surfaces, applied two coats of national name brand oil base primer followed by two coats of oil base gloss white paint by the same manufacturer.

One year later there was noticeable mildew growth on all roof eaves on all four sides of the house as well as on the vertical window/door trim.

In two years, the mildew under the roof eaves had turned so black it looked like the wood had been burned.

Two summers ago I treated all the surfaces with Jomax House Cleaner Mildewcide mixed with bleach according to the directions. This did not remove all the stains, so I next scrubbed these surfaces with a green kitchen pad and the Jomax solution. Some stains still remained, so I scrubbed the areas again with comet and a green kitchen pad.

In one year’s time, the mildew started returning and in two years time the eaves are almost coal black again. The house is in the city and sits at the top of a hill and gets almost constant sun.

Where did I go wrong and how am I to fix this problem? I’m not looking forward to stripping all this down to bare wood again but am also scared to ignore the problem and enclose the soffits.

The cedar shingle siding has some isolated mildew in locations, but no where near the degree as the roof eaves, rafter tails, and door and window trim.

Thank you in advance for your help.

1 Answers
crowderpainting Staff answered 5 years ago

Sounds like you did a great job with the prep. I know what you did is a lot of work. If this is mold the only thing I can think of the use of an oil base paint as the finish. Oil base paints allow more mold growth than acrylic paints, the oils act as food for mold. Also a mold inhibitor could have been added to the primer and paint. A mildewcide paint additive like Add-2 or MX-3 will help for a time.

I'm also wondering if this isn't mold you are seeing. Local auto traffic can cause a similar look from tire wear and exhaust. It will also leave stains that are hard to remove. Will this stuff mostly wash off with some soap and water?

If it is mold then yes it needs taking care of. You don't need to totally strip everything. A good washing and repainting might be all that is needed. Also a lot of work. If you decide to repaint with an acrylic paint you will need to prime so the new paint can stick to the oil base paint. Also use a mold resistant acrylic paint and add extra mildewcide for more protection.

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