Composite decking has become the dominant choice for new deck construction in the Atlanta metro over the past decade. Trex, TimberTech, AZEK, and similar brands promise decades of low-maintenance outdoor living without the painting, staining, and sealing that wood requires. That promise is largely accurate — but "low maintenance" still means regular cleaning, and cleaning composite decking requires a different approach than wood. Getting it wrong can permanently damage the cap layer that makes these products perform as advertised.
This guide covers the specifics of cleaning and maintaining the three major product lines most common in Atlanta-area homes, explains why composite decks still get mold and staining despite manufacturer claims, and provides a practical annual maintenance schedule for the Georgia climate.
Understanding How Composite Decking Is Built
Most modern composite decking is a "capped" product — a wood-plastic composite (WPC) core surrounded by a polymer cap layer. The cap is what provides the product's primary protection: it resists moisture absorption, UV fading, and surface staining far better than the composite core would on its own. Understanding the cap is critical because the cap is also what's most vulnerable to improper cleaning technique.
Trex
Trex is the most widely recognized composite brand and is found throughout the Atlanta metro. Trex's current product lines (Transcend, Select, Enhance) all use a mineral-based polymer cap over a recycled wood-plastic core. Transcend is the premium line with deeper embossed wood grain texture; Enhance is the entry level with lighter texture. The deep texture on Transcend, while beautiful, creates the narrow grooves where mold and debris most commonly accumulate. Trex's warranty covers fading and staining, but the warranty is voided by pressure washing above 3,100 PSI — a threshold easy to exceed with standard contractor equipment.
TimberTech
TimberTech (now owned by the same parent company as AZEK) offers both wood-plastic composite and full PVC products. The TimberTech AZEK line is 100% cellular PVC — no wood fiber at all — which gives it superior moisture resistance and makes it genuinely more mold-resistant than WPC products. TimberTech PRO and Edge lines are capped composite. The PVC products are more forgiving of cleaning chemistry than wood-fiber-containing products, but still require soft wash technique to protect the surface texture.
AZEK
AZEK is the premium PVC decking brand and is common on higher-end construction in Atlanta's northern suburbs — Alpharetta, Milton, Johns Creek, and Roswell. AZEK boards contain no wood fiber, which means there's no organic nutrient source for mold growth within the board itself. However, the surface texture still traps organic debris from leaves, pollen, and the environment, and mold grows on that deposited organic material. AZEK's Harvest collection has the deepest texture profile in the line and requires the most attention to groove cleaning. AZEK warranty coverage is among the most comprehensive in the industry, but it specifies cleaning with low pressure and approved cleaners only.
Why Composite Decks Still Get Mold
The marketing for most composite decking products implies that mold is not a significant concern. The reality, particularly in Atlanta's humid climate, is different. Here's what's actually happening:
The Groove Trap Effect
The embossed wood grain texture on most composite products creates a surface with small ridges and valleys. These valleys are typically 1–3 mm deep and 2–4 mm wide. In Atlanta's climate, these grooves become accumulation points for:
- Pollen (which packs tightly into grooves and acts as a nutrient source)
- Leaf debris and tannin-rich decomposing organic matter
- Dirt and soil tracked from the yard
- Algae spores, which colonize any surface with adequate moisture and nutrients
The groove surfaces stay wetter longer than the exposed board surface because they're sheltered from air circulation and direct sun. Even on a PVC product that contains no wood fiber, this groove environment — organic debris plus retained moisture plus shade — is sufficient for black mold to establish and grow visibly. This is the mold you see as dark spotting or streaking across composite deck surfaces, most visibly after a wet winter or spring in Atlanta.
End Grain Moisture Entry on WPC Products
On capped composite products (Trex, TimberTech PRO, etc.), the cap covers the four long faces of the board but the cut ends are exposed composite core. These cut ends, which occur at every board end in a deck installation, are the primary moisture entry points into the core. Water absorbed at end grain can swell the core over time and, in extreme cases, cause the boards to cup or bow. End caps (plastic end pieces that cover the cut ends) are the solution, and many homeowners skip them. Keeping end grain sealed with end caps significantly reduces moisture entry and the resulting mold that grows from inside the core outward.
What Damages Composite Decking During Cleaning
High Pressure Washing
This is the most common source of cleaning damage on composite decks. The polymer cap layer on WPC products has a texture that is pressure-sensitive. A standard pressure washer at 2,500+ PSI held at close range (under 12 inches) can erode the cap texture, leaving a lighter-colored, fuzzy surface that permanently changes the appearance of the board. This damage is visible and is not covered under warranty because it results from improper maintenance.
The mechanism: high-pressure water impacts the cap surface and physically removes or deforms the polymer texture. The result is not uniform — you see streaks or bands across the deck where the wand was held too close or moved too slowly, alternating with darker bands where the pressure was more distant or the movement faster. This "wand stripe" pattern on composite decks is one of the clearest indicators of improper pressure washing and cannot be corrected without replacing the affected boards.
Incompatible Cleaning Chemicals
Composite deck warranties specify approved cleaner types. Strong alkaline degreasers (pH above 10) can attack the cap layer chemistry on some WPC products, particularly older formulations. Solvent-based cleaners are universally contraindicated — they attack the polymer cap directly. Undiluted bleach at high concentration on prolonged contact can cause color change on some products.
Approved cleaners for most composite products are pH-neutral to mildly alkaline deck cleaners, or dilute sodium hypochlorite solutions (1–2%) with appropriate dwell time and thorough rinsing. Always check the specific manufacturer's cleaning recommendations for your product line before applying any chemistry.
The Correct Cleaning Approach by Product Type
For Trex Transcend, Select, and Enhance
Trex recommends low-pressure washing (maximum 3,100 PSI, fan tip, held at distance) or soft washing. For mold and mildew in grooves, their approved approach is: apply a composite deck cleaner solution, scrub with a soft-bristle brush (not wire brush, which scratches), allow appropriate dwell time, then rinse thoroughly. The brush step is important for getting chemistry into the grooves — spray application alone often doesn't penetrate the groove depth effectively enough to kill mold at the base.
For TimberTech and AZEK
TimberTech and AZEK (now one company) approve soft washing and low-pressure cleaning. For PVC products (AZEK), dilute sodium hypochlorite solutions work well and don't have the same oxidation sensitivity as wood-fiber-containing products. For WPC products, the same groove-scrubbing approach as Trex applies. AZEK's care instructions specifically caution against any abrasive scrubbing tools and recommend soft-bristle brushes only.
Our Professional Approach
Our composite deck cleaning service uses a soft wash system delivering chemical solution at under 100 PSI, followed by hand scrubbing of heavily mold-affected groove areas with soft-bristle brushes, followed by a thorough low-pressure rinse. This approach kills mold in the grooves at the root rather than just removing the visible portion, and it stays well within manufacturer pressure limits to protect the cap layer. We identify end grain exposure issues and can install end caps as part of the service where boards are missing them.
Cleaning Specific Stain Types on Composite Decking
Tannin Staining from Wet Leaves
Oak and sweetgum leaves are the primary culprits in Atlanta. Wet leaves sitting in deck grooves leach tannin, leaving dark brown staining. On capped composite, tannin stains the groove rather than the board surface (the cap resists staining better than the groove walls). Fresh tannin staining responds well to deck cleaner chemistry. Tannin that has been present for multiple seasons may require oxalic acid-based deck brightener chemistry and patience — it doesn't always come fully clean in one treatment.
Grease and Cooking Stains
Around grill areas, composite decks develop cooking grease staining that bonds strongly to the cap texture. Degrease with a pH-neutral to mildly alkaline degreaser, allow full dwell time, scrub, and rinse. Multiple treatment cycles may be needed for built-up grease. Preventing the buildup with a deck mat under the grill is the most practical long-term solution.
Rust Staining
Rust stains on composite decks come from metal furniture or planters without drainage holes sitting on the deck surface. Oxalic acid-based cleaners treat rust staining on composite effectively, but the result depends on how long the stain has been present. Fresh rust staining (less than 6 months) responds very well. Staining from years of the same planter sitting in the same spot may not fully resolve.
Annual Maintenance Schedule for Atlanta Composite Decks
The maintenance cycle that works for Atlanta's climate:
- Spring (late April–May): Full cleaning — remove all furniture and planters, soft wash or low-pressure clean the entire deck surface including grooves, scrub mold-affected areas, rinse thoroughly. This is the primary annual cleaning event.
- Summer: Promptly remove leaf and organic debris after storms. Don't allow wet debris to sit in grooves for extended periods. This preventive step dramatically reduces mold accumulation.
- Fall (October): A secondary cleaning after leaf fall to remove tannin-depositing leaf debris before wet winter. This is particularly important for decks under oak trees.
- Winter: Clear snow if it occurs (rare in Atlanta, but ice storms do happen). Don't use rock salt on composite — it can cause surface damage and the runoff can affect landscaping. Sand or kitty litter for traction if needed.
We provide composite deck cleaning throughout Atlanta, Marietta, Roswell, Alpharetta, Decatur, and the full metro area. Call (678) 748-3578 for a free estimate specific to your deck brand and condition.