Seasonal ADU Care for San Jose: Year-Round Homeowner's Guide

Seasonal ADU Care for San Jose: Year-Round Homeowner’s Guide

Wood-frame ADUs in the South Bay expand and contract roughly a quarter inch across a typical year. That movement is not a flaw; it is what wood does when six months of drought heat give way to six months of rain. The caulk joints and flashing details that ignore that movement are the ones that show up as water intrusion calls in January. This guide walks through what a San Jose ADU owner actually needs to do, season by season, to keep the structure tight, the systems running, and the warranty intact. For ADU Warning Signs: A San Jose Homeowner’s Reference Guide, see our related resource.

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Quick Answer

Seasonal ADU care in San Jose comes down to four tasks: clean and check gutters before the rains arrive in late October, service mini-split filters and condensate lines before the summer heat, inspect exterior caulk and flashing twice a year as the structure expands and contracts, and document everything to protect your warranty. For details on ADU Permits, Codes & Inspections in CA: What You Need to Know, see our code compliance guide. Most maintenance takes under two hours per season and costs less than $100 in materials. The expensive problems come from skipping the small checks.

Table of Contents

• Why San Jose’s Climate Punishes ADUs Differently
• Fall Prep: The Six-Week Window Before the Rains
• Winter: Water Intrusion and Ventilation Checks
• Spring: Foundation, Drainage, and Post-Rain Assessment
• Summer: Mini-Split Care and Dry-Season Tasks
• Which Maintenance Tasks Protect Your Warranty (and Which Void It)
• Inspecting Your ADU Foundation: Cosmetic Crack vs. Settlement
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions
• The Bottom Line

Why San Jose’s Climate Punishes ADUs Differently

San Jose gets about 15 inches of rain a year, nearly all of it between November and April. That means a detached ADU in Willow Glen or Evergreen sits dry for six straight months, then takes repeated soakings for the next six. The soil swells. The framing moves. The exterior caulk that looked tight in September can open a hairline gap by January. National maintenance advice written for Seattle or Minneapolis does not line up with this pattern. San Jose’s cycle is drought, then deluge, then drought again. Every gap a contractor left behind gets found during that first wet winter.

We build and service ADUs across San Jose, and the pattern holds in every neighborhood from Almaden Valley to Berryessa. The calls we get in January are almost never about structural failure. They are about water finding a quarter-inch gap that opened in December. The fix is usually simple if caught early. The damage is expensive if ignored until the next rain.

Start by understanding the three things San Jose’s climate does to an ADU. First, the long dry season bakes exterior sealant brittle. Second, the sudden soil saturation after the first big storm moves the ground under your foundation slab. Third, the mild temperatures mean owners do not notice small leaks because there is no freezing to force the water out. A slow leak behind siding can run for months before you see the stain on drywall. By then, the framing is already compromised.

This guide is calibrated to the actual weather pattern of Santa Clara County, not a generic national template. If you follow the seasonal schedule below, you will catch most problems before they cost real money. For homeowners considering a new build, ADU Builders in San Jose who understand this climate will detail the flashing and drainage before you sign anything.

Fall Prep: The Six-Week Window Before the Rains

Late September through mid-November is the critical period in San Jose. The first real storm usually arrives in late October or early November. Whatever you have not done by then will stay undone until spring, because once the ground is saturated, working around the foundation is messy and some tasks, like exterior caulking, cannot be done on wet surfaces. Here is the fall checklist in order of priority.

1. Clean and Inspect Gutters

Detached ADUs often sit under or near mature trees. In San Jose, that means live oak leaves, redwood needles, and eucalyptus bark. A clogged gutter sends water over the edge and down the wall, soaking the siding and any window trim below. Gutter sizing matters too. The standard 5-inch K-style gutter handles a typical Santa Clara County storm fine, but if your ADU roof is steep or your downspouts are few, overflow happens fast. Check the gutter pitch toward each downspout. Standing water in a gutter after the rain stops means the pitch is off or the hangers have sagged.

Clean the gutters twice in fall: once in late September, then again after the last leaf drop in November. If your ADU sits under a valley oak, you may need a third pass in December. A gutter that overflows during a heavy storm is the single most common cause of water intrusion we see on ADUs in San Jose.

2. Inspect Exterior Caulk and Flashing

Walk the entire perimeter of the ADU. Look at every joint: around windows, doors, utility penetrations, where siding meets foundation, and where roof meets wall. Press on the caulk with your thumb. If it is hard, cracked, or pulling away from the surface, it has reached the end of its life. Silicone caulk in San Jose’s dry heat typically lasts five to seven years before it loses adhesion. Polyurethane sealant lasts a bit longer. Look at the flashing over windows and doors. Bent, rusted, or missing flashing means water is getting a direct path behind the siding.

This is the one task you do not skip. A $12 tube of sealant can prevent a $4,000 drywall and framing repair. If you are not sure what you are looking at, take photos and get a professional opinion before the first storm.

3. Check Crawlspace Vents and Access Panels

If your ADU sits on a raised foundation with a crawlspace, the vents are there to let moisture escape. Make sure they are not blocked by leaves, dirt, or insect nests. Latch screens should be intact to keep rodents out. Crawlspace venting is not optional in San Jose: the wet winters make enclosed crawlspaces a breeding ground for mold and wood rot. If your access panel is loose or the hardware is corroded, tighten or replace it now. A closed crawlspace that cannot breathe is a warranty problem waiting to happen.

Many garage conversions in San Jose include slab-on-grade foundations rather than raised floors, but if you have a crawlspace, fall is the time to get under there before the ground stays wet for months.

4. Test Sump Pumps and Drainage

If your ADU has any active drainage, a sump pump, or a French drain, run it before the rains. Fill the sump basin with water and confirm the pump kicks on and discharges away from the foundation. Walk the perimeter and check that the soil grade slopes away from the slab. Two inches of fall over the first four feet is the standard. If the ground has settled toward the ADU, you will see water pooling against the foundation after the first storm.

Winter: Water Intrusion and Ventilation Checks

Winter in San Jose is not harsh by national standards, but it is the season when every deferred maintenance item shows up. Rain events come in waves. You can go three weeks with clear skies and then get two inches in 48 hours. That is the test. Here is what to check during and after each major storm.

During the Rain

Walk through the ADU during a heavy storm. Listen. The sound of dripping inside a wall is different from rain on the roof. Check the windowsills for water. Feel the carpet or flooring near exterior doors. Look at the ceiling around light fixtures and vents. If you see any water, mark the spot with painter’s tape and photograph it. Do not wait for multiple storms to confirm the leak is real. One confirmed leak is enough to act on.

After the Storm

Walk the exterior. Look for standing water against the foundation, clogged downspouts, or splashback from the roofline onto walls. Check the crawlspace for standing water or damp insulation within 48 hours of a big rain. Crawlspace moisture that lingers for weeks is the leading cause of wood rot and mold in detached structures.

Ventilation matters in winter too. ADUs with full kitchens and showers generate interior moisture. In winter, when windows stay closed, that humidity has nowhere to go. Run the bathroom exhaust fan during and for 20 minutes after every shower. Use the kitchen range hood when cooking. If you notice condensation on windows every morning, the ADU needs better ventilation or a humidistat-controlled exhaust fan. That moisture, left alone, grows mold inside walls and voids manufacturer warranties on drywall and insulation.

Spring: Foundation, Drainage, and Post-Rain Assessment

By March, the rains are tapering but the ground is fully saturated. This is the best time of year to inspect the foundation. The soil has gone through its annual soak cycle, and any movement that is going to happen has mostly happened. Spring inspection gives you a baseline before the dry season bakes everything out and hides the evidence.

Foundation Walk

Walk the full perimeter of the slab. Bring a flashlight and a level. Look for new cracks. Mark existing cracks with a pencil line and date so you can track movement over the next twelve months. The next section of this guide covers how to tell a cosmetic crack from a settlement problem.

Drainage Check

Look at the soil around the foundation. If you see eroded channels where water ran, that water is running toward the ADU from somewhere. Fix the grade or add a swale now while the ground is still wet and easy to work. Downspout extensions should discharge at least four feet from the slab. In San Jose, where the clay-heavy soils in areas like East San Jose hold water longer, that four feet matters. Water that dumps at the corner of the slab will find its way under over a wet winter.

Exterior Inspection

Spring is the second caulk inspection, after the rains have done their damage. Look for new gaps, cracked sealant, or paint bubbling. Bubbling paint means water got behind the paint film during the winter. Scrape it and check what is underneath. Minor bubbling is cosmetic. Flaking with soft wood behind it is rot. Deal with rot in spring, before the long dry season lets the damage spread undercover.

Summer: Mini-Split Care and Dry-Season Tasks

Summer is the easy season in San Jose, but it has its own workload. The dry heat that makes July pleasant is the same heat that bakes sealant, dries out AC drain lines, and turns dry grass into a fire hazard around outdoor units. Here is the summer checklist.

Mini-Split Filter Maintenance

Most detached ADUs in San Jose use ductless mini-split heat pumps. They are efficient, quiet, and easy to maintain. The filter interval matters: clean the filters every 30 days during heavy cooling season, or every 60 days if the ADU is used lightly. San Jose’s dry summers mean more dust in the air, and that dust loads mini-split filters faster than coastal climates. A clogged filter forces the unit to work harder, which shortens compressor life and raises electric bills.

To clean the filter, lift the front panel of the indoor unit, slide the filter out, rinse it with water, and let it dry completely before reinstalling. Do not use soap. Do not run the unit without a filter. Ten minutes, once a month, done.

Condensate Line Check

Mini-splits pull moisture out of the air during cooling. That water drains through a condensate line to the exterior. In San Jose’s dry summer, algae and mineral buildup can clog the line even when you run the unit daily. Check the exterior condensate outlet once a month. If no water drips during a cooling cycle on a humid day, the line may be blocked. A blocked line backs up into the indoor unit and drips down the wall, damaging drywall and paint.

Pour a cup of white vinegar down the condensate line once a season. It dissolves mineral scale and algae without damaging the pipe. If you have a condensate pump, verify it still activates and discharges properly.

Exterior Unit Clearance

San Jose summers turn grass dry and flammable. Keep the area around the outdoor unit clear of dry grass, weeds, and any stored material within two feet of the unit. The condenser needs airflow across the coil. Stuffed against a fence with a dead grass clump in front of it, it overheats and the compressor works harder. Clear the area with a string trimmer. Cut back any shrubs that have grown over the unit during spring.

If you own a home built on a room addition in San Jose with a conventional central HVAC system, the same clearance and filter rules apply. Change or clean the filter every 60 to 90 days depending on use and dust load. San Jose dust is fine and gets everywhere.

Exterior Sealant Touch-Up

Late summer is the right time to re-caulk exterior joints. The surfaces are dry, and the sealant cures best in warm weather. Do not wait until fall. By October, dew and morning fog extend cure times, and you are racing the first storm. Late August through mid-September is the re-caulking window in San Jose.

Which Maintenance Tasks Protect Your Warranty (and Which Void It)

ADU warranties, whether from the builder or the modular manufacturer, are contracts with conditions. The company agrees to cover defects for a defined period. The homeowner agrees to maintain the structure. Skip the maintenance, and a legitimate warranty claim turns into a denied claim. Here is what actually matters.

Tasks That Protect Your Warranty

• Document everything. Keep a written log or a phone note with dates, photos, and what you did. If a wall crack appears a year later and the manufacturer asks about maintenance history, your log is the answer.
• Clean gutters and downspouts on schedule. Water damage caused by a clogged gutter is routinely denied as a homeowner-maintenance failure.
• Maintain ventilation. Modular ADU manufacturers like Cover, Dvele, and Mighty Buildings include ventilation requirements in their warranty documents. Run exhaust fans, keep crawlspace vents clear, and address condensation quickly.
• Service mini-splits on schedule. Keep records of filter cleaning and any professional service. A compressor failure on a unit that has never had a filter change will not be covered.
• Address small leaks immediately. A small leak that you ignored for six months is often excluded. A small leak you caught and reported in week one is covered.

Tasks That Can Void Your Warranty

• DIY structural modifications. Cutting into a load-bearing wall, adding a window, or altering the roofline without an engineer’s sign-off voids the structural warranty on most ADUs.
• DIY electrical panel work. Opening the electrical panel or running new circuits without a licensed electrician is a code violation in San Jose and voids the electrical warranty. This is also a genuine safety hazard.
• Sealing the crawlspace. Some homeowners see a damp crawlspace and decide to seal the vents closed. That traps moisture and voids any warranty covering the subfloor or framing. Crawlspace venting exists for a reason.
• Using unapproved roof materials or coatings. Recoating an ADU roof with a product the manufacturer does not list as approved can void the roof warranty.
• Ignoring the maintenance schedule in the warranty document. Some warranties have explicit intervals for HVAC service, gutter cleaning, or sealant inspection. Read the document once, mark the intervals on a calendar, and keep the receipts.

The Garrison Standard that Fielding ADU San Jose builds to includes a written 365-day workmanship guarantee. If something fails inside that year, we return and make it right at no charge. That guarantee is in plain writing before work starts, not buried in fine print after you sign. But even a one-year guarantee has the same homeowner-maintenance requirement. Keep the logs, do the seasonal tasks, and the guarantee protects you the way it was written to.

Inspecting Your ADU Foundation: Cosmetic Crack vs. Settlement

Concrete slabs crack. That statement worries people, but it is the truth. San Jose’s clay soils expand when wet and shrink when dry, and that seasonal movement puts stress on every slab in the county. Most cracks are cosmetic and harmless. A few are signs of real settlement. Here is how to tell them apart without a structural engineer.

Step 1: Measure the Crack Width

1. Use a ruler or a crack gauge if you have one.
2. If the crack is hairline, less than 1/16 inch wide, it is almost certainly cosmetic. Note it, date it, and check it again in six months.
3. If the crack is 1/8 inch or wider, mark both ends with a pencil and take a photo with a quarter for scale.
4. Re-measure in 30 days. If the crack has grown, you have active movement. If it is stable, it is likely a shrinkage crack from the original cure.

Step 2: Check the Pattern

• Vertical or diagonal cracks in the slab near corners or pipe penetrations are usually shrinkage cracks from the concrete curing. They are common and rarely structural.
• Horizontal cracks in a foundation wall are more serious. They indicate soil pressure pushing the wall inward. This is uncommon on slab-on-grade ADUs but possible where a raised foundation wall exists.
• Cracks wider at the top than the bottom may indicate one side of the slab is dropping. That is settlement.
• Cracks that run through tiles, drywall, and the foundation in a straight line suggest the whole structure is moving as a unit. That is a problem.

Step 3: Check the Door and Window Frames

If doors that closed fine last year now stick or swing open on their own, the frame has racked. That is structural movement, not just a door hinge problem. In an ADU, the most common sign of foundation settlement is a bedroom or bathroom door that starts catching at the top corner. Check all exterior and interior doors quarterly. It takes two minutes and tells you more about foundation movement than any single crack measurement.

Step 4: Look at the Perimeter Grade

Stand at each corner of the ADU and sight along the foundation edge. If the slab edge dips or rises, you have differential settlement. A settled slab corner often shows a stair-step crack in the stucco or siding above it. If you see that pattern, get a professional evaluation before it worsens.

When to Act

A hairline crack in a slab that stays hairline through two seasons of wet and dry is nothing to lose sleep over. A crack that grows, a door that starts sticking, or a gap that opens between the slab and the wall plate needs a foundation specialist. Do not inject anything into the crack yourself. Foam injection and epoxy repair have specific applications, and using the wrong product makes a professional fix harder later.

Common Mistakes to Avoid

• Waiting until the first storm to clean the gutters. By the time rain is falling, you are working on a wet roof or a ladder in the wind. Clean gutters in September and October, not November. San Jose’s first big storm arrives fast.
• Skipping the crawlspace for a full year. Water can pool under a raised ADU in one wet winter and sit there for months. Check the crawlspace in fall before the rains and again in early spring after they taper.
• Using interior caulk outdoors. Interior latex caulk breaks down in one San Jose summer. Exterior joints need polyurethane or silicone sealant rated for UV and temperature swings. Read the label before you buy.
• Blocking crawlspace vents to keep animals out with solid covers. The vents must breathe. Use hardware cloth or vent screens, not plywood or plastic.
• Running a mini-split for a full cooling season without touching the filter. A dirty filter is the most common cause of mini-split failure we see in San Jose ADUs. Clean it every 30 days during cooling season.
• Ignoring a sticking door. A door that starts catching is often the first visible sign of foundation movement. Adjusting the hinge solves the symptom. It does not solve the settlement underneath.
• Doing electrical or gas work without a licensed contractor. In San Jose, unpermitted electrical work on an ADU is a code violation, a safety hazard, and a warranty killer. Some things are not DIY.

When to Call a Professional

Some seasonal ADU maintenance is genuinely do-it-yourself: cleaning gutters, changing filters, checking caulk, marking cracks. Other items call for a licensed professional. Call an electrician if you see scorch marks on outlets, flickering that is not the bulb, or a breaker that trips repeatedly. Call a plumber for a sewer smell, slow drains that do not clear with a snake, or water staining that reappears after fixing an exterior leak. Call a structural engineer if a foundation crack grows across two consecutive seasons or if doors begin sticking that never did before. And if water is running into your living space, call before the drywall soaks through. Fielding ADU San Jose offers free estimates in San Jose, and we answer the phone with a live person at (669) 201-3984. If you are not sure whether a problem is cosmetic or structural, a quick call costs you nothing.

Frequently Asked Questions

The Bottom Line

San Jose’s ADUs sit through a specific and repetitive weather cycle: six months of dry heat, six months of rain. The structures that get into trouble are the ones whose owners wait for visible damage to appear. The structures that last are the ones whose owners do the small, scheduled tasks, season by season. Clean the gutters in fall. Run the exhaust fans in winter. Check the foundation in spring. Service the mini-split in summer. Keep a log of all of it. That is the entire discipline, and it takes less than ten hours a year. For anything beyond your comfort level, call a professional at the first sign of trouble. Fielding ADU San Jose is here for the work you do not want to do yourself, priced in writing before anything starts.

Written by Reed Calloway, Owner at Fielding ADU San Jose, serving San Jose since 2006.