How to Winterize Your Pool in Wisconsin

Wisconsin winters are hard on swimming pools. Temperatures routinely drop below zero, snow accumulates for months, and freeze/thaw cycles can damage improperly winterized equipment. But here's the good news: fiberglass pools are engineered to handle Wisconsin's climate better than any other pool type. This guide walks you through the complete winterization process—when to close, how to do it right, and why fiberglass pools make winter maintenance easier than concrete or vinyl pools.

When to Close Your Pool in Wisconsin

Ideal Timing: Mid-October

Most Wisconsin pool owners close their pools in mid-October. This timing balances two factors: you want to enjoy warm fall days, but you need to close before the first hard freeze (when nighttime temps drop below 25°F for more than a few hours). A hard freeze can damage plumbing lines if water is left in the system.

In southern Wisconsin (Madison, Janesville, Beloit), the first hard freeze typically occurs in late October or early November. In northern Wisconsin (Wausau, Rhinelander), it comes 2-3 weeks earlier. Check your local 10-day forecast in early October and schedule closing when temperatures are still above freezing but trending downward.

Don't Wait for Freezing Temps

Here's a common mistake: waiting until it's already freezing to winterize. Closing a pool properly requires running the pump, adding chemicals, and working with hoses and valves—all of which become miserable (and sometimes impossible) when it's 30°F outside. Close your pool when daytime temps are still in the 50s-60s. Your future self will thank you.

Can You Close Too Early?

Yes. If you close in early September when water temps are still 70°F+, algae can bloom under the cover during warm fall days. The ideal water temperature for closing is 55-65°F—cool enough to slow algae growth, but warm enough that your winterizing chemicals disperse properly.

Step-by-Step Winterizing Process

Step 1: Balance Water Chemistry (7-10 Days Before Closing)

A week before closing, test and balance your water. You want:

• pH: 7.4-7.6
• Total Alkalinity: 80-120 ppm
• Calcium Hardness: 180-220 ppm (slightly lower than summer levels to prevent scaling)

Balanced water is critical—it prevents corrosion, scaling, and staining during the 6-month winter dormancy. If your pH is off by 0.5 or more, adjust it now and retest in 24 hours.

Step 2: Shock the Pool

Add a heavy dose of chlorine shock (2-3 times the normal amount) to kill any bacteria or algae. Run the pump for 24 hours to circulate the shock throughout the system. This step sanitizes the water and gives your winterizing chemicals a clean slate to work with.

For a 25,000-gallon pool, use 3-5 pounds of calcium hypochlorite shock. Wait until chlorine levels drop below 5 ppm before proceeding to the next step—you don't want to mix high chlorine with algaecide.

Step 3: Add Winterizing Chemicals

Once chlorine levels drop, add your winterizing kit:

• Winter algaecide: Prevents algae growth under the cover (use 2x the normal dose)
• Pool antifreeze: NOT the same as automotive antifreeze—pool antifreeze is non-toxic propylene glycol
• Enzyme treatment (optional): Breaks down oils and organic matter, reduces spring cleanup

Run the pump for 2-4 hours to distribute chemicals. Fiberglass pools have an advantage here—the smooth surface doesn't absorb chemicals like porous concrete, so you need less algaecide.

Step 4: Lower the Water Level

Lower the water level to 4-6 inches below the skimmer opening. This prevents the skimmer from freezing and cracking during winter. Use a submersible pump or the pool's drain setting to remove water.

Do NOT drain the pool below mid-skimmer level. Fiberglass pools rely on water weight to counteract groundwater pressure. If you drain too much, the shell can pop out of the ground—especially in areas with high water tables (common in parts of Wisconsin). Leave enough water to anchor the shell.

Step 5: Blow Out the Plumbing Lines (Critical Step)

This is the most important step in Wisconsin winterization. Any water left in plumbing lines will freeze, expand, and crack pipes or fittings. Here's how to blow out lines properly:

1. Turn off the pump and filter system completely
2. Open all drain plugs on the pump, filter, heater, and chlorinator to release trapped water
3. Connect an air compressor to the skimmer or dedicated blowout port (if your pool has one)
4. Blow compressed air through each line individually—main drain, skimmer, return jets, cleaner line, water features
5. Watch for bubbles in the pool—when you see continuous bubbles from a return jet, that line is clear
6. Plug the lines immediately after blowing using rubber expansion plugs or threaded winter plugs

Use a 1-2 HP air compressor set to 10-15 PSI. Higher pressure can damage fittings. Blow each line for 1-2 minutes until water stops draining. If you don't have a compressor, you can use a shop vac on blow mode—it's less effective but better than nothing.

Step 6: Add Pool Antifreeze to Lines

Even after blowing out lines, there's usually a small amount of water trapped in low spots. Pour 1-2 quarts of pool antifreeze into each line through the skimmer or return jets. This ensures any residual water won't freeze. Pool antifreeze is pink and non-toxic—it's safe for fiberglass and won't damage seals or gaskets.

Step 7: Remove and Store Equipment

Remove the following items for indoor storage:

• Pump lid and drain plugs: Store in the pump basket so you don't lose them
• Filter pressure gauge: Freezing can damage the internals
• Heater drain plugs: Critical—heater heat exchangers are expensive to replace
• Skimmer baskets and weir doors: Prevents ice damage
• Ladder, handrails, diving board: Prevents warping and UV damage
• Automatic cleaner: Store indoors to extend its life

Leave the filter and pump outdoors (they're built for it), but drain all water and remove drain plugs. Cover the pump motor with a weatherproof cover or tarp to prevent snow and ice buildup.

Step 8: Install the Winter Cover

Your last step is installing the winter cover. In Wisconsin, you have two options:

Mesh Safety Cover (Recommended for Wisconsin)

Mesh covers are our top recommendation for Wisconsin pools. Here's why:

• Handles heavy snow load: Mesh covers support 400+ lbs per 5x5 foot section—no problem for Wisconsin snow
• No water accumulation: Rain and snowmelt drain through the mesh into the pool, so you don't need a cover pump
• Safety: Mesh covers are anchored with springs and meet ASTM safety standards—a child or pet can't fall through
• Durability: Quality mesh covers last 10-15 years in Wisconsin conditions

The downside: mesh allows some fine debris and sunlight through, so you may see some algae growth by spring. The winterizing chemicals prevent heavy blooms, and spring cleanup is minimal.

Cost: $800-$1,500 for a quality mesh cover sized for a typical 40' x 16' pool.

Solid Safety Cover

Solid covers block all sunlight and debris, resulting in cleaner water in spring. But they require a cover pump to remove rain and snowmelt—and Wisconsin gets a lot of both. If you go with a solid cover, budget for a quality automatic cover pump ($150-$300) and check it weekly during warm spells to prevent water accumulation.

Solid covers are heavier and can sag under snow load if water accumulates. In heavy snow years, you may need to shovel snow off the cover to prevent damage.

Cost: $900-$1,800 for a solid safety cover.

Tarp Covers (Not Recommended)

Old-style tarp covers held down with water bags are a bad choice for Wisconsin. They sag, collect water, freeze into ice dams, and tear under snow load. They're also unsafe—they won't support weight if someone walks on them. Spend the money on a proper safety cover.

Why Fiberglass Pools Handle Wisconsin Winters Better

Flexibility Under Freeze/Thaw Pressure

Concrete pools are rigid. When the ground freezes and expands around the pool shell, concrete has no give—it cracks. We've seen concrete pools in Wisconsin develop structural cracks after just 5-7 winters. Repairs are expensive (often $5,000-$15,000) and never as strong as the original shell.

Fiberglass pools flex. The one-piece shell can handle seasonal ground movement without cracking. Thursday Pools fiberglass shells are engineered with composite reinforcement that distributes stress evenly across the structure. After 20 Wisconsin winters, a properly installed fiberglass pool looks and performs like new.

Smooth Surface = Less Chemical Damage

Winterizing chemicals are harsh—high-dose shock and algaecide sit in your pool for 6 months. Concrete and plaster are porous, so these chemicals slowly degrade the surface, accelerating the need for replastering. Fiberglass is non-porous and chemically inert. Winter chemicals don't harm the gel-coat at all.

No Risk of Liner Shrinkage

Vinyl liner pools have a unique winter problem: liner shrinkage. When water levels drop and temps freeze, vinyl liners can shrink and pull away from the walls. If the liner dries out, it becomes brittle and cracks. You have to maintain precise water levels all winter to prevent damage.

Fiberglass pools don't have this issue. The shell is the surface—there's no liner to shrink or crack. Lower the water, close the pool, and forget it until spring.

Common Winterizing Mistakes in Wisconsin

Mistake 1: Closing Too Early

Closing in early September when water temps are still 70°F+ invites algae blooms. Even with winterizing chemicals, warm water under a cover creates a greenhouse effect. Wait until water temps drop below 65°F.

Mistake 2: Not Blowing Out Lines

This is the #1 cause of expensive winter damage. A cracked pipe or freeze-damaged pump costs $500-$2,000 to fix. Spend the 30 minutes to blow out lines properly, or hire a professional. It's the best $150 you'll spend all year.

Mistake 3: Wrong Chemical Balance

If you close with unbalanced water—especially low pH or high calcium hardness—you'll open to stained walls, scaling, or corrosion damage. Test your water a week before closing and correct any imbalances. Bring a sample to a pool store if you're not confident in your testing.

Mistake 4: Draining the Pool Too Much

Never drain a fiberglass pool below mid-skimmer. In areas with high groundwater (common in Wisconsin due to proximity to lakes and springs), an empty pool can pop out of the ground like a boat. The hydrostatic pressure from groundwater underneath the shell is stronger than the weight of the empty pool. Keep 80-90% of the water in the pool all winter.

Mistake 5: Not Checking the Cover Mid-Winter

If you have a solid cover, check it after every major snowfall or thaw. Water accumulation can overload the cover and damage the anchors. Pump off standing water, and gently brush off heavy snow. Mesh covers are more forgiving—just check that the anchors are secure.

Spring Opening Checklist

When to Open: Late April to Early May

In southern Wisconsin, most pools open in late April when nighttime temps consistently stay above 40°F. Opening too early means you're heating cold water for weeks—expensive. Opening too late means you miss Memorial Day weekend. Mid-to-late April is the sweet spot.

Opening Process

1. Remove the cover and clean it: Hose off debris, let it dry completely, fold it, and store indoors
2. Refill the pool to normal level: 6-12 inches below the coping
3. Reinstall equipment: Pump lid, drain plugs, pressure gauge, skimmer baskets, ladder
4. Remove line plugs and reconnect plumbing: Make sure unions are hand-tight
5. Prime and start the pump: Check for leaks and proper flow
6. Test and balance water chemistry: pH, alkalinity, hardness, chlorine
7. Shock the pool: 2-3 lbs of shock to kill any algae or bacteria that developed over winter
8. Brush and vacuum: Remove any debris or algae that settled on the bottom
9. Run the filter 24 hours: Clear out any cloudiness
10. Retest water and adjust: Fine-tune chemistry and you're ready to swim

Opening takes 2-4 hours if you're experienced. First-time pool owners may want to hire a professional for the first spring opening to learn the process ($200-$350 for professional opening service).

Professional vs DIY Winterizing

DIY Winterizing

If you're mechanically inclined and have an air compressor, closing your own pool is absolutely doable. You'll save $300-$500, and the process gets easier each year. Budget 3-4 hours your first time, 2 hours once you're experienced.

DIY Costs:

• Winterizing chemical kit: $50-$100
• Pool antifreeze (2 gallons): $20-$30
• Line plugs and skimmer cover plate: $30-$50
• Air compressor rental (if needed): $40-$60/day
• Total: $140-$240

Professional Winterizing

Professional closing is worth it if you don't have time, don't own a compressor, or want the peace of mind that it's done correctly. We include a warranty on our winterization service—if a line freezes due to improper blowout, we cover the repair.

Professional Service Includes:

• Water chemistry testing and balancing
• Adding winterizing chemicals
• Lowering water level
• Blowing out all plumbing lines with professional-grade compressor
• Adding antifreeze to lines
• Removing and storing equipment
• Installing winter cover (if provided)

Cost: $300-$500 depending on pool size and features

Final Thoughts: Winter is Easier with Fiberglass

Wisconsin winters are tough, but they don't have to be hard on your pool. Fiberglass pools—especially Thursday Pools with their reinforced composite shells—are engineered to handle freeze/thaw cycles year after year without structural damage. Follow this guide, use quality winterizing chemicals, and blow out your lines properly, and your pool will come back to life in spring looking as good as the day you closed it.

If you're considering a new pool and worried about winter maintenance, fiberglass is the lowest-maintenance option available. No liner replacements, no replastering, no freeze cracks. Just close it in October, cover it, and forget it until April. That's the Pooltopia promise.