I'm looking for a little help in trying to understand why I have to wash the siding of my house in the spring and the fall every year. I believe it is engineered wood siding, or fiber cement (I'm not sure the difference between the two). The siding is in great condition, and when clean, looks great on the house. However, there is a constant black dirtiness that grows on the outside, particularly heavy on the south face of the house. I found that in the back (east) face of the house, I washed the gutters which were also black and it seemed to slow down the siding getting dirty (I only washed it once this year). But the south face has no gutters, so I'm not sure where it's coming from. It is also heavier under windows, etc. I wash it with a soft brush and diluted bleach which seems to do the trick, but I'm sick of washing my 2-story house twice a year. Any ideas on what's going on or suggestions?

Post a picture of the side of the house that's giving you trouble.
No close ups needed, just stand out in the yardat a slight angle so we can see sort of a profile to check things like how much over hang there it.

A few factors that can cause this shade from over hanging trees.
Under sized gutters or gutters that were installed wrong.
No over hang or to short an over hang on the roof.
Siding needs another coat of paint, this time with a mildicide like M-1 in it.

Will do... I'll snap a couple this afternoon and post. Thanks for the help.

How wide are the setbacks between the adjacent houses? Is there a setback on the south side of the house? What's the landscaping like... large trees with a lot of overhang? From what direction are the prevailing winds?

In my area, most of the houses are on 50 - 60 foot lots, some with only 4 feet of setback. Houses are close together, ventilation isn't very good, and mold grows on the houses.

Neighboring vegetation, especially fast growing junk trees, create shade, block out the sun and seem to create an environment conducive to mold. The mold that grows on my siding on the North side is greenish, but there could be a variety that's blackish. The house to the South has even more greenish mold on the North side, perhaps caused in part by the fact that the now deceased occupants never took care of the thick bed of ivy which is now growing up the side of the house and keeping moisture from escaping.

It does sound though as if there might be some issue with the gutter, at least the one that was black. Was it recently installed? Is it sloped properly?

Okay... here is a picture of the south face.

And here is a second picture...

For one thing shrubs and trees should never be allowed to come in contact with the siding, just the shade from them and the lack of air flow will help promote mold and mildew growth.

I'm going to trim them back to try to get some more air floor through there. But can you see the amount of "grime" on the side of the house? Do you think that's because of the shrubs and trees?

There always seems to be one side of the house that just gets dirty faster.
Shade, prevaling winds, how close trees are that put out a lot of pollen, how close your house is to others can all cause an issue.
Even the sheen of the paint cause cause problums. The flatter the sheen the more dirt it's going to catch.

I agree with Joe - you need to get some airflow going there. I'm wondering if the surface is just a tad tacky. I'd take some flour and toss it against the surface and see what sticks. It just may be a slightly tacky surface. Paint sheen does make a difference too.

Agree with Joe and Mosternaz. I'm also wondering if a good power wash might help. Probably won't solve the problem, but it might take off a lot of the grime, which does look like mildew from what I can tell in the photo.

And if the tree is blocking the prevailing winds, there's little ventilation to carry away the moisture that builds up.