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Wet Basement Solutions and Prevention

If you own a home with a basement, the arrival of the rainy season can fill you with dread wondering if this year you will end up with a wet basement (again).

Wet basements can either be a moderate nuisance if you don't use that space for much or a catastrophe if you've just spent tens of thousands remodeling it into living space. Even if you don't use your basement, long-term water issues can cause structural damage to your home.

Wet basement solution and prevention

There are many causes of wet basements including cracks in the foundation, leaking windows and doors, cracks in the floor, failing siding, land sloping toward the foundation, plus bad gutters and downspouts. Fortunately, there are solutions for each of these.

Seattle has a lot of older homes and many of them have basements with varying degrees of waterproofing. The Pacific NW also gets a lot of rain which is more than happy to expose your home's weaknesses. In fact, I tell buyers that winter is the best time to buy a home because rain will expose a home's moisture issues that remain hidden, or at least, a lot harder to find, in the summer months.

This article specifically looks at issues resulting from water getting in from the outside as opposed to moisture vapor from for example badly vented clothes dryers and poor ventilation occurring inside the home.

Signs that your home has a wet basement.

Sometimes your nose will tell you (musty smell), sometimes your eyes (wet walls and floors), and sometimes your bare feet (damp carpet) will be your first indication that you have an excess moisture problem. If you have had water issues in the past, then a moldy smell will probably be the first sign that it's back as moisture reactivates old mold.

If your basement is unfinished or still has an exposed foundation (usually poured concrete), check for the following signs: (if not you might have to open up the walls for a good look).

  1. Obvious cracks in the foundation with moisture around the cracks.
  2. Water stains on the walls: these can come from leaking windows, moisture getting in at the junction of the siding and the foundation, or water getting in higher up behind the home's siding, tricking down to the top of the foundation wall, and then trickling down the inside of the foundation.
  3. Can you see any efflorescence? This is a chalky white salt residue resulting from moisture slowly migrating through your porous concrete foundation. The water dissolves calcium salts within the concrete which then evaporates on the inside surface of your basement walls. Compared to active water leaks, this is more of a glacial process and it can be hard to tell if it's from old waterproofing issues or is an ongoing, current problem.
  4. Patches of mold growth: besides your nose telling you there might be mold, can you see any? When humidity levels start getting above 65% it creates the perfect environment for mold growth. Mold can be a major health concern so controlling it is important.  Control the moisture and you can control the mold. Note that mold growth can result from poor internal ventilation and high humidity levels and is not always due to external water issues. 
  5. Leaking window frames: These can fail over time and start rotting. The interior wood framing may show signs of rot and water stains plus there will be water streaks on the foundation wall below the window.
  6. Damp floors can indicate water coming up through the slab or where the slab meets the foundation walls. It's easier to see if the basement is unfinished, otherwise, you might need to pull up the floor covering.

Wet basement solutions.

The solution will depend on what is causing the water intrusion, so determining where the water is coming from is the critical first step.  There's no point in spending money on the wrong fix. In addition, make sure to hire licensed contractors who know what they are doing.

Gutters and downspouts.

Failing gutters and downspouts can cause wet basementsMany a damp basement is caused by inadequate gutters and downspouts whose function is to capture rainwater off the roof. One inch of rain can dump 1,250 gallons of water onto a 2,000 square foot home. Your home needs to be able to divert all that water away from your home, particularly the foundation.

Bad gutters and downspouts are probably the most common cause of damp basements and I see it all the time when I'm looking at homes with buyers or attending home inspections. Fortunately, it is also the easiest and least expensive fix.

For gutters:

  • This may sound a little obvious, but does your home have any missing gutters (or downspouts)? 
  • Make sure they are not full of leaves and other debris otherwise they will just overflow when it rains. Where the gutter connects to the downpour tends to get blocked.
  • Are they solidly connected to the home and not detached from the facia board? Gutters need to sit right under the roof to catch the rainwater.

For downspouts:

  • The downspout can also get plugged up with leaves and other debris. Run a garden hose down the downspout to flush out any blocked material and free it up.
  • The most common issue (and easiest fix) is that the bottom of the gutter is not connected to anything and just dumps rainwater right beside the foundation. All that water needs to be diverted away from the home.
  • If the bottom of the drain pipe is connected to a "storm drain", is that drain actually working or just plugged up with decades of muck? Run a garden hose into the drain and see if it drains or just overflows.  Note that for older Seattle homes, storm drains just drain into sewer lines.
  • Those small little splash pads that are about 2 ft long are completely useless, especially when the property is sloping toward the foundation. That water will just pool beside the home.

Adding a French drain.

Assuming your gutters and downspouts are functioning probably the next most important prevention method is to install a French drain around the perimeter of your home or at least on the sides of the home that get the most water. In the Seattle area most of the bad weather comes from the south and this side of the home tends to be the most vulnerable to water intrusion.

A French drain is a simple wet basement solution and an effective way to capture rainwater and divert it away from your home. It basically consists of a perforated PVC pipe buried beside your home's foundation that drains water under gravity to a low point on your property.

French drains are not particularly expensive and it's also a job that you could do yourself if you are okay with some manual labor and digging. Just watch a few YouTube videos and it should tell you all you need to know.

The most important thing is to allow for at least a quarter-inch slope per foot to make sure the water drains away to the low spot. The cost of paying a contractor to install a drain is between $4,000 to $8,000 depending on the length of the drain and if they need to get through the concrete slab.

During the Gulf War, I won't if people were calling them Freedom Drains?

When your property slopes toward your foundation...

If the land your home sits on slopes towards your home, after a downpour, all that groundwater is going to race downhill toward your foundation trying to force its way into your basement.

You have two options. First, you can hire a landscaper to regrade your property so it doesn't slope toward your home. If your whole property is on a slope this may not be a feasible solution.

The alternative is just to add a French drain (see above) where water tends to pool on your property and run the drain to the low point in your property.  At the end of the day, you are just better off getting a French drain around the foundation of your home

Add a Rain Garden or Cisterns.

Seattle Rain Wise programA rain garden is a system where all the rainwater falling on your roof is captured and diverted away from the home and emptied into specially designed and highly absorbent landscaping.

For certain neighborhoods in Seattle,  if your home meets certain requirements, the city will pay for some or all of the installation. You can also get a cistern to capture all that rain and use it for watering your garden.

I had one installed in my Ballard home about 10 years ago and I didn't pay a penny. And it looks a lot better than the sad collection of red lava stones that used to sit there!

For more information, visit the Rain Wise Program. So, why would the city pay for these? Because it helps prevent sewer systems from getting overwhelmed after big storms and reduces pollution of Puget Sound waters.

 Even if the city is not willing to pay for one, it's still worth considering adding a rain garden as a way of protecting your home from rainwater. Plus they improve your home's curb appeal and help protest the Puget Sound.

Notes that a rain garden will not take care of groundwater issues. It just captures all the rain hitting your roof and bypasses the need for storm drains.

Adding Interior footer drains

If you have water getting into your basement at the junction of the concrete floor and the foundation wall, you might want to consider adding an interior footing drainage system. This is essentially the same as a French drain but runs around the interior perimeter of your basement. 

Installation of a footer drainage system will cost between about $8,000 and $15,00 depending on the size of your basement.

Note that these drains will not fix problems with water getting into the home via the foundation's walls. That needs to be addressed separately

Seal the foundation and repair foundation cracks.

Although a French drain will capture a lot of the surface water, if your foundation has cracks or allows water to permeate under hydrostatic pressure, then you'll need to take additional steps and add exterior waterproofing. Concrete is a porous material by nature and some foundations are more porous than others.

A contractor will dig all the soil away, down to theDimpled membrane drainage mat waterproofing home foundation bottom of the foundation. They will then seal the exterior foundation wall with rolled-on waterproof material. A dimpled membrane (drainage mat) is then added which allows any water to wick away from the walls.

If you are going to go this route, and haven't added one already, you should add a French drain while that area is opened.

Fixing the building envelope is not a cheap fix but if you are having water seeping through your foundation due to cracks or porous concrete then it's something you have to take care of sooner or later. The cost to do this treatment is approximately $10,000, make sure you use a contractor with a good reputation ask for references and make sure that they provide a dry basement warranty for their work.

Adding a sump pump.

For homes with a high water table or underground springs, adding a sump pump might be a good idea. These pumps are installed at the lowest point in the basement. The pump sits below the surface of your basement floor in a pit the size of a medium garbage can. 

After heavy rains, when the water table rises, the pump will activate and start expelling water to the outside of the home preventing it from getting into your basement. It will only activate when it is triggered by a float and the pit partially fills with water. In other words, the pump only runs when actually needed. 

It is critical to have a battery backup for a sump pump in case your power goes. And when does power go out? During storms when there's lots of rain!

Let Tom from This Old House show you how it works...


If water can get in behind your home's siding and has no way of getting back out, that water can potentially drip all the way down to the top of the foundation wall in the basement and then trickle down into your basement living area.

In the Seattle area, the most common homes with brick-and-mortar exteriors are Tudors and ramblers (ranch). Over time, micro-cracks can form in the bricks themselves and between the mortar and the brick. Rain combined with wind can be push water behind the brickwork, particularly on the south-facing side which gets the brunt of the Seattle storms.

Failing siding on homes and wet basements

For brick homes with water issues, the most common fix is tuck-pointing. A masonry contractor will remove about an inch of the existing mortar joints (cement) between the bricks and then replace it with fresh mortar. 

A good tuck-pointing will last 30 years or more. The cost will depend on the number of walls but is going to run you between $10,000 and $30,000.

The worst-case scenario is that the outer brick layer will have to be dismantled and rebuilt. The brick exterior is anchored to the interior wood stud framing via small straps. In old homes, these straps can fail over time and you will start to see bulging and settling in the brickwork if you look along the length of the exterior.

In this case, even if you tuck point the wall, new cracks will appear over time and allow water in as the wall slowly moves and settles. Long term, it is better to just bite the bullet and have the wall rebuilt which will guarantee you decades of peace of mind. And yes, this is definitely not a cheap solution and will cost between $20,000 and $30,000 or more potentially more depending on the size of the job.

Leaky windows and doors.

Basement doors and windows are other sources of water intrusion. If the basement has a door to the outside it is not uncommon for the water to get in under the door if there is no drain immediately outside, or the drain is plugged up.

Old windows and window frames can become leaky over time especially on the side of the home that gets all the rain and wind during storms. 

The good news is that relatively speaking, these are usually not expensive fixes and it might just be a case of adding new caulking around the windows and unplugging the drain outside the door. Plus it's really easy to tell if they are leaking because they're accessible and easy to see.


You're probably thinking, why don't I just get a dehumidifier which will remove the excess moisture and humidity from my basement. However, this is only treating the symptom and not the causes. Plus dehumidifiers won't do much when water is flowing through your basement.

If your humidifier is plugged in 24 hours a day then you have an obvious moisture issue. You are basically sticking your head in the sand hoping it will go away. It's the equivalent of spraying some Fabreze to take care of that moldy smell.

Here are some additional useful resources on wet basements and basements in general.

Potential Causes of Foundation Damage from Paul Sian who looks at the different causes of this issue and whether your insurance policy will cover needed repairs.

How Mold Can Impact the Sale of Your Home from Danny Margagliano who looks at the causes of mold in homes and what impact it can have when you try to sell a home with mold.

Will Updating Your Basement Add Value to Your Home? Bill Gassett looks at the different types of basement remodel layouts and what impact they will have on the overall value of your home.

Things to Repair Before Selling Your Home from Luke Skar so you can avoid a failed buyer's inspection. These include the home's drainage system and foundation. 

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