There is no single right answer, and anyone who gives you one without looking at your property is guessing. What your house actually needs depends on three things: how much shade it sits in, how close you are to the water, and which direction your worst wall faces.
I get asked this on nearly every quote, usually phrased as "how long before I need you again?" Here is how I work it out.
Why the Gulf Coast is a harder case
Mildew and algae need three things: moisture, warmth, and something organic to feed on. Our climate supplies all three for most of the year.
In a drier climate, a wall gets wet when it rains and then dries, so the growth cycle keeps getting interrupted. Here, humidity stays high enough for long enough that shaded surfaces stay damp far longer between rains. That is the difference. It is not that Houston has more dirt; it is that our conditions let living growth establish and keep going.
Which is why a house under heavy canopy in Friendswood needs attention more often than an identical house on an open lot further inland. You are not cleaning dirt off. You are removing something that is growing.
The factors that actually move the interval
Rather than a universal schedule, look at what applies to your property.
How close you are to the water
Waterfront properties in Kemah, Seabrook, Nassau Bay, and Clear Lake take constant salt-laden moisture. Salt film holds moisture against the surface and it is not doing your paint any favours sitting there. Bay-front homes generally need washing noticeably more often than inland ones.
How much tree cover you have
Friendswood is the clearest example, but older sections of League City and Nassau Bay have the same situation. Mature oaks are wonderful, and they also mean your roof and north wall rarely see direct sun. Constant shade plus constant humidity is a fast growth environment, and canopy drops organic debris continuously, which is part of what feeds the growth.
Which way your worst wall faces
Almost every house has one elevation that looks noticeably worse than the others, and in the northern hemisphere it is usually the north-facing one, because it gets the least direct sun and stays damp longest.
If your north wall is visibly green or grey again well before the rest of the house, treat that wall as being on its own schedule. There is nothing wrong with washing one elevation rather than paying for the whole house again.
How open and sunny your lot is
Newer sections in Katy, Cinco Ranch, and Fulshear often have young landscaping and little overhanging cover. These houses tend to need washing less often, but the trade-off is worse driveway staining, because full sun bakes clay and tyre marking into concrete. The house needs it less, the flatwork needs it more.
Signs worth acting on
Rather than counting months, watch for these.
- Green or black film visible from the street, not just up close
- A clear difference between elevations, one wall obviously darker than the others
- Growth on the fence at the same time as the house, which means conditions have been right for a while
- Streaking below the gutter line, which usually means the gutters need clearing too
- Slick patches on walkways after rain
The last one matters more than people expect. Algae on a shaded walkway gets genuinely slippery, and it builds up so gradually that nobody notices until someone loses their footing.
What happens if you leave it
Mildew is not purely cosmetic. It holds moisture against whatever it is growing on, and permanently damp material degrades.
On painted siding, prolonged growth works into the paint film. Past a certain point washing will not restore it and you are into repainting instead. On wood, permanently damp timber is what rots. On a roof, algae can progress to moss, which lifts shingle edges and lets water underneath.
The economics are lopsided in the obvious direction. Washing is maintenance; repainting and reroofing are capital jobs. Staying ahead of it is one of the cheaper things you can do to a house.
The honest answer
Have someone look at it. I would rather tell you that you can wait another season than book work you do not need, and I would rather set an interval based on your actual property than repeat a number I read somewhere.
I quote for free across League City, Katy, and the surrounding Greater Houston area. Call or text (404) 324-9177 and I will come take a look.
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