Water damage with no leak? Here’s how to handle hydrostatic pressure

Moisture problems can happen to anyone, even if there's no obvious leak. 

New Avenue designer Cindy Lan is currently working with a Bay Area family to convert a garage into a habitable space. (Cindy is an expert in garage conversions—read her notes about permitting accessory building conversions here.)

Fortunately, the slab from the garage is completely usable. That’s great news for the owners, who stand to save several thousand dollars by re-using the old foundation. However, Cindy has discovered that they’ll need to make some modifications to the slab (and possibly the landscape around it) to avoid water damage in their new ADU.

Water damage: It’s not just about leaks.

Everyone knows that water damage is bad news. Mold, mildew, and dry rot (all of which need moisture to grow) can seriously damage your home and cause health problems.

Unfortunately, sources of potential moisture aren’t always obvious. It isn’t just broken pipes and leaky roofs that cause water damage. Hydrostatic pressure can also be a problem.

That’s why it’s important that all construction sites—whether you’re using an existing foundation or a new one—be evaluated by a professional who understands moisture and drainage issues.

What is hydrostatic pressure? What can be done about it?

Hydrostatic pressure comes from water pressing against a building. It can force water into a porous or damaged foundation, causing serious erosion and leaks. Many people imagine hydrostatic pressure coming from an underground stream or runoff flowing down a hill. While those are common causes of water damage, hydrostatic pressure can also come from saturated soil around a building’s foundation.

Concrete acts like a sponge, absorbing water and water vapor. Once water gets into your concrete foundation, it will evaporate into your basement, garage, or home. This is what gives garages and basements that musty smell you’ve probably experienced.

Of course, this kind of moisture problem can happen anywhere: garages, basements, hillside homes with retaining walls, and any  home on a slab foundation.

It's especially a problem with garage-to-ADU conversions. When building or remodeling, it's important to prevent water damage, both for your health and the longevity of your home. Your project team can help you choose the right strategies to prevent water damage.

Here are some options:

Seal the slab with a waterproof barrier.

Waterproofing can be added inside and outside a home. A waterproof barrier such as Drylok or RedGard (both available at home improvement stores) can be painted onto the slab. These products are toxic and may be useful in many cases, but they’re not always the best solution.

A vapor-barrier underlayment with laminate flooring is a good choice for topping off a slab where moisture is a concern.

Be aware that when there’s a hydrostatic pressure issue, a waterproof barrier alone won’t solve the problem. It’s best to address the source of the water.

Look for ways to improve drainage.

Too much water around the foundation spells disaster, so it’s important that the site be well drained.

In some cases, grading may be the problem. The ground and any other surfaces around a building should slope away from the building. But soil settles over time, so older accessory buildings may not have an ideal grade leading away from them. In some cases, the landscape can be renewed to improve the drainage.

The best-case scenario is a French drain around the perimeter that is deeper underground than the bottom of the slab. The French drain should flow downhill and away from the building. If you can’t go downhill to daylight, you may need a sump pump inside a dry well.  (This is the same type of sump pump you’d have in a basement or crawlspace; it can go into a small well and pump water out into the street.) While adding a drain (and possibly a pump) will increase your expenses, it will save your new ADU in the long run.

Finally, consider how the foliage in your yard affects the drainage. The right landscaping can reduce erosion and help keep your lot free of unwanted flooding.

Good idea: Building a new home or remodeling an existing one. Bad idea: Not checking for moisture issues.

If you’re going to replace or convert an old garage, it’s worthwhile to consult an expert who can help you prevent water damage.

To talk to Cindy or another New Avenue pro, click here and schedule a call. We’ll answer all your questions and help you explore the possibilities for your property.