Building Envelope – Water Damage from Outside

Water Damage

Water are extremely deadly to buildings. Their presence can cause mold and rot to organic building components. If we spot the damage early, it can be repaired relatively inexpensive. However, if we cannot identify the signs, it can become extremely expensive when the damage spread to the structural building. Then we need to do structural repairs. We do not want water in our building but water can change form under different temperatures. This makes water ingress prevention extremely difficult.

Water from outside

In order for water penetration to happen, 3 factors are required.

1. Water has to be present at the exterior.
2. An Opening for water to move through.
3. A driving force (Wind, Gravity, or capillary action.)

Sometimes we may have inexperienced homeowner pressure washing their building and force water into their wall assembly, but that can be avoided with education. The majority of water comes from rain, and we cannot escape from that.

Sloped Elements

A building without a roof is a swimming pool. The roof of a building is the first thing that comes in contact with the rain. Its main purpose is to shed water away from interior space. A sloped surface is the most efficient way to do so.
To touch on horizontal elements for a bit. It is true that most contemporary buildings have flat roofs.I can guarantee 99.99% of deck over living space are going to leak. They are not recommended.
If you ask someone to draw a roof. Most people will give you a triangle. Roofs are much more than that. They are a series of layers working together as a system to protect the building and itself.
A roof includes

1. Water shedding layer
2. Ventilation space
3. Structural Support
4. Another layer of ventilation space in case of unevaporated water
5. Thermal Insulation
6. Aesthetically pleasing interior finishes

We will discuss water shedding layer and the ventilation spaces. 

The water shedding layer is basically your roof shingles. There is a selection of materials like Asphalt shingles, wood shingles and shakes, slates, metal roofing, clay roof tiles, and concrete roof tiles. Notice they are all shingle/tile type of materials. The best practice for roofing is to layer them and have the layer above protecting the one below, all the way to the top. When water flows to the edge of one shingle, it gets on top of the next shingle, and never in between the shingle and substrate.
The first ventilation layer is right beneath the water shedding layer. If small amount of moisture were to get through the first layer, they are given a chance to air out. This air space is made possible by “purlins”, similar to the “strapping” in rainscreen system.

The second ventilation layer is also called “the unheated attic space”. Building code requires 1 in 300 ventilation for attic spaces. This means for every 300 square feet of attic space, calculated horizontally, you need to provide 1 square foot of ventilation to the outside. Having this clause in the building bylaw tells us that the BC government is aware that water will get into the attic space.
That triangle people draw as their interpretation of a roof has a horizontal line at the bottom. The 2 sloped line represent the roof rafter with water shedding layer above. That horizontal line represent our ceiling (looking from inside of the house), and ceiling joists, also the insulation in between structure. If there is water in the unheated attic space, they are likely to be sitting on top of the insulation. If the ventilation is adequate, it should be able to remove the water as moisture. However, if the ventilation is somehow blocked, or if too much water gets into the unheated attic space, then mould may start forming.

During my time in the construction industry, I have seen a lot of homeowners converting their unheated attic space into livable space. Whether for rental or self use, they usually do that to create more value to their home. When they do the conversion, the venting is definitely gone. Usually a layer of drywall is added to the underside of rafter (sloped element of truss). This not only takes away the 2nd drainage layer, but also divert water towards wall structure. Some owners may even alter engineered truss support. This can be extremely dangerous.

Horizontal Elements

Deck over living space

Flat roof, is what most people call this type of roof structure. Essentially, people do flat roof to increase usable space. I have mentioned earlier that 99.99% of this type of roof will leak, and that statement is backed by numerous examples of failed projects over decades. Of course having extra space to use in your home is ideal, but you have got to think of the implication under building envelope.

First of all, water shedding roof materials don’t work in flat roof. Gravity does not work when the floor have minimal height difference. Homeowners have to rely on the 2% minimum slope to shed water, which is extremely ineffective. If for any reason such as building material expansion and contraction, or structural sagging over time, you are going to find a little puddle sitting on the flat roof.

Water puddle may not seem much, but when we are talking about building envelope, you can consider them with the same regard as termites. When you see a puddle, that means the intended water shedding strategy has already failed.

As for the waterproofing membrane, flat roof homeowners are limited to use liquid applied bituminous membrane, or torched on bituminous membrane. These materials are applied when they are heated liquid, and dries to elastic plastic. They are susceptible to ultraviolet aging, meaning the UV light from the sun can make them brittle and crack. The common way to protect the membrane from the sun is to put a layer of gravel on top. This is not ideal if you want your roof garden to be accessible. Many buildings adept to create drainage layer above the waterproofing membrane with durable materials such as concrete tiles above. When built correctly, this can be an effective detail. This increases the construction cost, and many homeowners are not willing to go that extra mile because single detached house roof garden has relatively low traffic compared to apartment buildings.

Compared to the sloped roof with un-habitable attic space, flat roof is also missing the 2 layers of ventilation. There is a way to detail deck so moisture gets vented through the wall nearby. Unfortunately roof deck’s horizontal area is too large for that method to be effective.

Sun Deck and Balcony

What are the main differences between a sun deck, and a balcony? Think of Romeo and Juliet. That half circle horizontal protruding element with beautiful guardrail where Juliet is standing in, is a true balcony. There is no structural support below, thus the depth of balconies are generally limited. Don’t even consider cantilever your floor joist to increase the depth of the balcony. Horizontal element (Floor joist) penetrating vertical element (Wall joist) is going to expose the interior floor assembly to weather damage.

Decks on the other hand has structural support below, usually in the form of 6” x 6” wooden posts. There is a fundamental difference between a Sun deck and a Roof deck. Obviously, one is also acting as a roof. The implication is that a sun deck has no living space below. The excess water/moisture is free to leave the structure from below. There is also a lot more opportunities for the structure to dry when it is not raining.

Both balcony and deck has to attach to the wall. They are both considered as horizontal protrusion elements in a building. They increase complexity of a simple straight wall. For obvious reasons, a correctly constructed horizontal protrusion is supposed to be sloped away from the building face by a minimum of 2% slope.

Transition between vertical element and horizontal protrusion elements

Transitions are weak points in a system. They should be over designed, namely the waterproofing membrane. A vertical waterproofing membrane should continue without major penetration. When horizontal protrusion elements are being attached by bolts, caulking and tapes should be used to seal the penetration. Another layer of waterproofing membrane should be installed tightly against the original vertical waterproofing membrane, and then carry above the horizontal protruding element. This way, even if the balcony leaked, the vertical waterproofing membrane can still protect the wall structure.

Ventilation space for houses are easier to achieve. Normally the underside of the decks can be exposed, because there’s nothing beneath the deck. This makes ventilation a lot easier. It is a lot harder to ventilate decks for a wood frame apartment building. The decks in apartment buildings are often stacked. The underside of the decks are often the lower unit’s ceiling where developers put exhaust vents. The horizontal protrusion cannot vent through the bottom. To make things worse, a lot of developers would use vinyl decking to create a more appealing illusion for the buyer. This basically means your deck is sealed from top to bottom. You cannot detect any problem in a newly constructed deck like this. Unfortunately vinyl decking gets brittle and fragile over time. A sealed system means the water cannot escape once it gets into the structure.

A wall section detail on paper shows the waterproofing membrane as a thick vertical dashed line. That is one dimension. When it turns onto the horizontal protrusion element, it becomes an L-shaped dash line, which is two dimensional. However, building science requires 3 dimensional visualization skills as the designer need to realize there is a termination of the balcony. I know this is hard to visualize, but this is were most of the problems take place. Anyone can waterproof a two dimensional corner, but you need a skilled contractor to correctly waterproof a 3 dimensional corner with an exterior corner meets an interior corner.

Another important aspect to watch out for is the guardrail attachment. In both houses and apartment buildings, guardrail attachment contribute to a large amount of water ingress into horizontal elements. The conventional way of installing guardrails are to bolt them down above the waterproofing membrane. The bolts will penetrate through the waterproofing membrane, and the bolts are sealed and taped to eliminate the hole created. This has been proven to fail. Over time, gravity and moisture still find their way through the bolt. A new mandatory way of installing guardrails is the “Side mount” method where guardrails are mounted on the side of the horizontal protrusion. It is ideal if the top layer of the deck can create a bit of a lip as an overhang for the penetration. Even without the lip, having the penetration facing the side is already going to be far more effective than the “Top Mount” method.

Sloped Ceiling

There is a type of roof construction that is a hybrid between sloped attic roof and flat roof. The exterior of the roof is sloped, and so is the interior ceiling space. The benefit is having an exterior sloped roof to shed water away, and having more usable interior head room. This type of roof can install purlins to create that first layer of ventilation, but it does not have an attic, so the opportunity for venting is limited. The British Columbia Building Code recognize the challenges for providing the same amount of insulation as an attic roof, so they have reduced the insulation requirements for sloped ceiling roof.

Vertical Elements

Vertical elements are basically your walls around your building. Their main purpose was to support the roof above. Over the years we have incorporated a complex amalgamation of building systems inside. There are many ways to construct a wall which we will discuss on our Wall Construction article. In this article, we are going to focus on their performance under nature.
Thanks to gravity, everything fall downward. Vertical planes have relatively less standing water. They are likely to receive weather damage. Our exterior walls are not perfectly smooth, so there may still be residue due to friction. Windows and doors are also part of vertical elements. Their transition location creates weak point for weather penetration. The construction industry has learned to adopt weather protection elements to reduce risk of water ingress.

First Layer of Defense for Water Damage – Overhang

Rain water doesn’t fall straight down. They are always affected by wind. We call this “Wind Driven Rain”, which means the rain usually go downward and sideways. This also means that whenever it rains, there are always 2 sides of the building that is being watered. If there is no overhang, rainwater will soak your cladding. Don’t forget, wind driven rain also include wind that will pressurize the exterior to push to rainwater into any openings in your wall structure. To greatly reduce the risk of water ingress, we try to eliminate the majority of the rain by creating an overhang. As long as we are not dealing a rainy day with constant strong wind coming from only 1 direction for a long period of time, the typical overhang of 1’-9” (including gutter) is good for most situations. If we are, no amount of overhang is going to stop the rain from hitting the cladding. Think of this as using an umbrella on a rainy day. Your feet my get wet, but your body should remain dry.

Second Layer of Defense for Water Damage – Cladding

When rainwater hits the cladding, majority of the water will flow into the ground into the perimeter drain. For impermeable cladding, small rain droplets are being held by friction and surface tension stays in the wall. Permeable cladding materials will absorb the water, to be evaporated afterward.

There are many cladding choices. We would get into them in detail in the cladding material article. For building envelope discussion purpose, lets just say siding-type cladding are more effective for water shedding. They are also better at drying because of the natural cavity created by siding overlap. Over the years, there are creative use of siding materials such as putting them vertically, or diagonally. These are not effective practices because water can soak through the joint of materials easily.

There are two major cladding systems, Face Seal and Rain Screen. Face Seal System rely on a perfect outer seal to prevent water from getting in. It has been proven a failed method of construction, and have created the leaky condo era in the construction industry. We are still talking about it because they still exist in older buildings. It still cause home-owners tens of thousands of dollars in Building Envelope Repair. Rain Screen system expect water to get through the cladding, and incorporate a built in air gap behind the cladding for water to flow out and/or evaporate. This air gap is created by putting thin wood strips of “strapping”.

The overhang and cladding combined will get rid of 99% of the water.

Last Layer of Defense for Water Damage – Air Barrier

We have already talked a lot about air barriers and pressure differences on our Building Envelope Overview article, so we are just going to lightly touch upon this topic. Air barriers is the last layer of defense in terms of Water damage. It is supposed to be airtight. You can think of it as putting plastic sheets around your wood structure, so moisture doesn’t get to it. However, plastic sheets don’t come in large size to cover the entire building, so we need to tape around the joints. When there is no moisture around, everything is fine. When there are moistures nearby with a negative indoor pressure, we rely on the quality of tape, and the workmanship of the contractor.

If water was able to get in, it is basically trapped inside your wall. I’ll explain why. The water got in because an external force asserts enough pressure at a leaking point. Unfortunately it is extremely unlikely for when the pressure difference is reversed, that exact leaking point is still the weakest part of the air barrier. The reversed pressure difference is more likely to be balanced by an open window. Because of this, that invisible crack that allowed a bit of water to get in, becomes a one-way passage for water to enter. Since the entire building is supposed to be airtight, heating and evaporation would only relocate moisture within the wall assembly.

Below Grade – Water against Foundation Wall.

Foundation wall under ground are always surrounded by damp dirt. Part of the reason is our exterior cladding shed water away from the building and the water goes into the ground next to our building. Water can move through the reinforced concrete foundation wall. They may move slow, but eventually moisture will go through. This process can cause damage to the structure of the building in the long term. As a preventive measure, the construction industry adopts moisture diversion strategy to move moisture away from the foundation wall. The common method for this is to apply bituminous waterproofing membrane as a separation between dirt and concrete.

The bituminous waterproofing membrane are usually applied in its liquid form. The liquid hardens as it cools and become a layer of elastic plastic. Due to various reasons, bituminous waterproofing may crack and allow water to enter the concrete. Over the years, the technology evolved to incorporate a layer of drainage mat outside the waterproofing membrane. Similarly to the cladding system above, this divert majority of water away before getting in contact with waterproofing layer.

The foundation footings are also made of concrete, so the same method as foundation wall water shedding system is also being used. The bottom of the footing does not require drainage mat because water below the foundation wall usually don’t flow upward. Also, if you put a layer of drainage mat, they are only going to be squished by the weight of building anyway. There is a layer of waterproofing membrane between concrete footing and the dirt though.

Moving Water Away From Building

What happens to the water that is drained away from the building face? They are not going to be left pooling around the building. Building Science do not leave anything to chance. If we leave the water around the building, it’s going to leak eventually. Gravel are used as backfill to create larger drainage path. The path of travel between gravel is less restrictive than the drainage mat, so the water will automatically flow from drainage mat towards gravel. The path then leads water to a perforated pipe near the concrete footing.

The perforated pipes are called “Drain Tile”. The technology of drain tile have also gone through a few variations due to technology advancement. Sometimes you can still see clay drain tile in older buildings, but perforated PVC drain tiles are the most common material now. A properly done drain tile should be located in close proximity to the perimeter footing. Also the bottom of the drain tile should be at the same height elevation as the bottom of the footing.

This perimeter drain tile not only take away water that are shed away by our building, but also surface water that may be flowing towards the building for any reason. Grass and natural ground can dissipate water into the soil. Some homeowners decide to pave their backyard. The water that does not go into the ground is called “Rain water run-off”. The rain water run off only has 3 places to go. It either flow towards city street, your either side neighbour, or towards your house. The first 2 options can get you into a legal dispute. The 3rd option increase the perimeter drain tile’s workload. When it cannot handle the load, there is going to be water against your building.

On a last note, rain water diverted by perimeter drain tile end up in storm sewer system. Similarly to your backyard, when city build more impermeable surfaces like roads and sidewalks, it also increases the load of storm sewer system at a much larger scale. This means that the storm sewer system can back up. It is better to have some grass on your site as part of the strategy to protect your building.

Layered system and construction philosophy

As some of you may have already figured out. The current strategy to prevent water damage to our building is a risk reduction system. It is not a waterproof system, but rather a system to shed water in layers. The different layers may have failed, however, there won’t be water damage to the building without the water present. Under extreme circumstances, the water is still going to get in. Everything is based on cost versus effectiveness. Since extreme conditions don’t happen every day, it does not make sense to construct buildings to be waterproof at exponential cost. This kind of design philosophy is good enough for typical construction.