Keeping Water Out: 7 Design Techniques to Control Water Movement

Roofs, exterior walls, and foundation assemblies are constantly working to protect your home against a barrage of natural forces. But it’s not surprising that water infiltration is one of the biggest culprits of building failures in a home. This is why the exterior design is one of the most essential aspects of the homebuilding process to get right.

The good news is that there are a few basic strategies you can use to help prevent water intrusion. The most effective thing you can do, though, is to be proactive about the waterproofing design during the design phase before even breaking ground. That way, you can easily incorporate those water control strategies into the overall design for little to no additional cost.

When it comes to designing and building homes, the best way to control water movement is to create an overall objective to reduce the number of gaps/openings, divert water from those openings, and neutralize the forces acting on the water.

In this post, we’re going to talk about the 7 design techniques for keeping water away from the building: shed, overhang, drip, overlap, drain + weep, capillary break, and labyrinth. Then, stick around to the end, and you’ll learn a helpful exercise we perform on all our design projects to ensure a watertight design.

But first, what does it mean to control water movement?

To control water movement at a building’s enclosure means determining how the building will deal with rainwater, surface runoff, and percolation. We’re not going to talk about specific water collection devices like gutters, drains, scuppers, and scuppers in this post, but rather design techniques to keep water away.

The goal is to get as much water away from the building enclosure as possible and provide effective drainage for any remaining moisture that may penetrate and/or accumulate.

For a simple house, the basic strategies for controlling water are depicted here:

[Graphic courtesy Whole Building Design Guide]

The overly simplified solution for managing water is to shed it from the roof and away from the building. Roof overhangs help deflect rain and minimize the amount of water striking the wall surfaces. A below-grade drainage system removes percolating water that cannot be managed by the site grading.

There is obviously more to it than this. Houses are complex. There are various roof conditions, intersections, bump-outs, openings, material and height transitions, and grade changes to understand and to figure out how to prevent water from infiltrating.

Three things are needed for water to penetrate through a building assembly:

an opening – It can be a crack around a window, door or penetration, a gap at the roof, or a joint between two pieces of cladding.
a presence of water – rain, melting snow, ice, frost, groundwater, and surface runoff
a force to push the water through the opening – These forces can act on water:

- Gravity: water naturally falls downwards
- Surface tension: this causes water to cling to the underside of a surface
- Capillary action: water can also move upwards in cracks or holes with smooth sides which can act as a capillary and by mechanisms of surface tension, draw in water
- Momentum: wind-blown rain possesses momentum that can move it through an opening
- Pressure differential: water will move from areas of higher pressure to areas of lower pressure

Short of having fewer window/door openings and penetrations, the way to reduce openings in the building assembly is to design and construct the building so that we eliminate and reduce the small gaps that result from poor construction methods and poor design detailing. Sealants and gaskets can help do this, however, a building skin shouldn’t rely on these alone. Sealants and gaskets tend to leak over time.

Knowing that we will inevitably have openings in our building, the best thing we can do is to design the structure to divert water away from openings and neutralize the forces that would otherwise move water through the assembly.

That’s where these 7 design techniques come into play. Let’s look at them one by one:

1. Shed

The first strategy to keep water away from the building is to shed the water. Shedding water simply means sloping a horizontal surface to drain water away from the building. This works for almost all exterior building elements like roofs, canopies, window and door sills, and grading. If the material is particularly porous (ex: grass and earth) a steeper slope is better for faster removal of water.

Best Practice Tip: With few exceptions, slope exterior horizontal surfaces a minimum of 1/4″ per foot or 2% slope away from the building.

Even so-called”flat” modern roofs when detailed correctly shed water to drains, gutters, scuppers, or the roof edge at the code minimum 1/4″ per foot. You may not be able to perceive it, but they do in fact slope. See the below detail of a “flat” roof detail we’ve used on a recent project.

2. Overhang

Water shedding from a surface can be diverted from the building envelope by creating an overhang. With an overhang, water is forced to fall away from the face of the wall assembly and onto the ground (again, away from the building).

The roof-to-wall transition is one of the most important details of building construction so an overhang is a great technique to keep water away at that critical juncture.

At entries, canopy overhangs not only divert water but also provide immediate protection when you’re opening the door.

In areas that see a lot of rain or snowfall, roof overhangs serve double and triple duty. They divert water from the building, keep groundwater away from the foundation, and provide sun shading.

Best Practice Tip: Overhangs are great water diverters. Just make sure the roof overhang, canopy, or building overhang is shedding water away from the building or channeling it to a collection device. The last thing you want is a flat or backward sloping overhang with nowhere for the water to go.

3. Drip

Depending on the material and speed of the water runoff, surface tension may cause water to cling to the underside of the material. To prevent the water from being drawn into the assembly, a break in the underside surface is usually enough to relieve surface tension and force the water to drip down the face of the wall instead of into the wall assembly.

Z-flashing profiles above windows and doors are good examples of the drip technique. The z-profile pushes the water out away from the building while the downturn leg creates the drip to relieve surface tension. At roof soffits, even a small change in the height of the material on the underside of the overhang can be enough to prevent water from clinging to the surface. This is why there’s typically a material change or break in the material at this corner. Another example of a drip is at window sills. It’s not uncommon to see a small groove on the underside of the sill to force water to drip.

Best Practices Tip: Anytime you have an overhang, it’s good practice to include a drip. Avoid a continuous vertical to horizontal transition at the underside of the overhang. Instead, change the height of the soffit, switch materials, or create a drip.

4. Overlap

Another strategy to keep water from infiltrating a building assembly is to overlap materials in a shingling fashion on vertical and sloped surfaces.

“Shingling” materials is when you overlap the higher element over the lower element. Because of gravity, water will run down over the higher material, onto the overlapped area, then onto the lower material, and continue to drain away from the building. If the materials were incorrectly lapped (the bottom material over the upper material), water will run down in between the materials at the seam.

An obvious example of overlapping materials is roof shingles. Again, if the material is more porous, use a steeper slope. For example, an asphalt shingle roof typically has a minimum slope of 3:12, whereas a metal roof slope can be much shallower.

Best Practice Tip: When in doubt, install materials in a shingle fashion (lower piece first, then upper piece) unless the manufacturer recommends otherwise. Even when installing another sheet of Tyvek or peel-and-stick membrane, it’s always good practice to install the lower piece first. The upper piece then laps over the lower piece to properly drain water away from the building.

5. Drain + Weep

In some instances, we anticipate water penetrating the first layer of a building assembly and design the system to drain the water to the exterior without penetrating the rest of the layers of the assembly.

A double-wythe masonry wall and a rainscreen system are both examples of this “drain and weep” strategy. In these systems, water is assumed to penetrate the exterior cladding. Then, it drains down the wall along the water-resistant membrane (which is installed in a shingle-fashion). And finally, it weeps out via weeps in the cladding layer or over the z-profile base flashing (a drip).

Best Practice Tip: It’s good practice to assume that some water or vapor will penetrate the outermost wall cladding material. Once you accept this, you can design the wall assembly to rid itself of water by draining and weeping it to the exterior. We’ll save this rainscreen principle for another blog post.

6. Capillary Break

Capillary action is the process in which water is able to pull itself upward or across a narrow crack. Think of a paper towel in a cup of water. Water soaks in and climbs up the paper towel due to capillary action. It’s only when the force of gravity overpowers, does it stop climbing. Plants and trees couldn’t survive without capillary action. It’s what helps carry water from the soil into the roots and up into the plant.

Water molecules like to stay close together and are attracted to and stick to other surfaces. Water is sticky and water can be travel in unexpected ways, like defying gravity.

Capillary action in building construction is possible when water finds narrow cracks between materials.

Foundation details are notorious for needing capillary breaks. That’s because there are a lot of solid surfaces with small cracks that can wicking water.

To prevent capillary action, provide a capillary break so the water doesn’t have the opportunity to find the crack. Capillary breaks are materials that allow water to drain freely instead of finding and seeping through those small cracks in the wall or floor assembly. Gravel bases and vapor barriers under the slab are examples of capillary breaks.

Best Practice Tip: In below-grade applications where there are cold joints in the material make sure you provide capillary breaks. Joints between concrete pours and masonry blocks can let in a lot of water if they don’t have capillary breaks like drainage planes, dampproofing coatings, gravel separation, keyways, and vapor barriers under the slab.

7. Labyrinth

Capillary action is strengthed by momentum, like wind-driven rain. One strategy to help prevent raindrops from entering a building assembly on those windy days is to consider a labyrinth path of travel (anything other than a straight line). That way, if water droplets find their way into a joint, it’s hard for them to pass through the wall.

Siding profiles like tongue and groove siding and shiplap are good examples of siding joints that prevent water from passing through with its own momentum.

Best Practice Tip: Cladding materials vary in their profiles and installation techniques. To combat water infiltration due to driving rain, either consider a material with a non-square edge or be prepared to drain and weep the water behind the finish cladding layer.

Custom Homes Require Special Attention

Custom homes are particularly tricky when it comes to waterproofing details. Chances are, there will be unique conditions that the contractor hasn’t seen before. There is no doubt that custom details add a level of uniqueness to a project. They create special moments that highlight unique aspects of home, conceal unsightly areas, or reinforce a certain design principle. But they are also areas that need extra attention to ensure they are constructed to withstand the test of time.

At YR Architecture + Design, we love designing custom homes. We also know that designing and detailing custom homes takes more time and effort. There can be numerous unique conditions and atypical details on a custom project. It could be the result of material choices, wall components, interior to exterior transitions, grade changes, or important alignments.

For example, very rarely do we use off the shelf windows purchased from a big box store. Instead, many custom homes have larger window walls where the detail of how the cladding terminates at the window is unique.

All those unique areas need extra attention to ensure they won’t leak, degrade, or fail decades from now.

During the construction documents phase, we work through all the details of the project, especially the custom details. When it comes to exterior details, we pay particular attention to prevent water infiltration. On any given custom home project, we’ll have numerous drawings like this documenting various details of the project. Among other things, these drawings convey to the contractor how to create a watertight home:

This leads to sharing one of our go-to exercises to ensure a watertight design…

The “Pen Test”

One of our favorite quality control exercises during the construction document phase is the “pen test”. Before we finalize construction drawings, the pen test helps ensure there is a continuous water barrier around the house to help prevent water infiltration.

If we detailed the exterior of the house properly, it should be possible to trace the waterproof layers and design techniques mentioned in this post around a sectional view of the building without lifting the pen from the paper. The waterproof layers of the walls, roof, and foundation must form a continuous, six-sided box with no gaps.

You’ll have roof underlayment, roof coping, metal flashing and counterflashing, drips, weather-resistant barriers, window and door flashing, base flashing, sealant at gaps and cracks, gaskets at window glazing, sloped sills, waterproof membranes, and mastic dampproofing below grade.

As you can see, there’s a lot of components to a watertight design!

This process typically starts with a building section, like the one below. We perform the pen test and trace around the building to ensure we have an overall waterproofing strategy for the roof, walls, floors, and foundation. We don’t always note this on this overall section, but the pen test ensures we have thought about it everywhere. From there, we circle the critical areas that need more study and zoom in on these details. These areas include roof edges, base details, material transitions, and window/door details.

On these zoomed-in details, we perform the pen test and document everything necessary for a watertight design.

Here’s another look at those design techniques on the smaller details from the building section above.

It’s All in the Details

The details get much more attention than this, though. We research products, make alignment adjustments for various material intersections, and coordinate MEP+S systems (mechanical, electrical, plumbing, structural). As we work through the details over time, we layer information on them for the contractor.

Working through the details can sometimes take weeks or months to fully develop depending on the complexity. Sometimes we have to wait for a product representative, contractor, engineer, or owner to answer a question for us. Other times, we need to wait until we solve other details first. In the end, we probably spend 4-6 hours on each detail. It’s a lot of work, but it’s necessary to create a watertight design. And it’s these custom details that turn lackluster designs into special one-of-kind custom homes.

It is difficult to achieve this degree of quality control and integrity in our designs, but it’s an essential practice. This is especially true when we’re designing custom legacy homes that require a lot of design rigor and construction precision.

The pen test not only works great for water control but also for the insulation layer and the air barrier. We run through these exercises for each of those exterior design vulnerabilities before we finalize construction documents. Once you get in the habit of doing it a few times, it becomes second nature.

Finally, solving waterproofing details will help you make aesthetic decisions.

Getting the exterior details right is extremely important both in terms of performance, but also aesthetics.

The primary purpose of building assemblies is to prevent water intrusion. And it’s not surprising that these assemblies are the most expensive parts of a building. They are your ultimate line of defense against natural and man-made forces but they are also the layer that everyone sees – the visual representation of your home.

Once you understand these 7 design techniques you can use this framework to help make decisions about aesthetics.

Are you going to have shed roofs with box gutters? Do you want a rainscreen system with z-flashing profiles? Are you going to have shiplap siding? As you start to make educated decisions, you’ll begin to feel more confident in your ability to curate a material palette and rule book of details for your home’s exterior design, and you’ll be one step closer to a watertight design.

We’re here to help! If you have a unique property, are looking to build a new modern custom home, and would like our help, we encourage you to reach out.

When it comes to designing and building homes, the best way to control water movement is to create an overall objective to reduce the number of gaps/openings, divert water from those openings, and neutralize the forces acting on the water.