Principles Of Rain Drainage In Architecture

Buildings are foremost a means of shelter. The essential structure was originally the roof—an assembly of wood, masonry, or metal that kept out natural elements. Modern technologies improve this effort. We use new roof materials and water shedding methods that are cheaper and easier to install.

With the scarcity of natural resources and a renewed emphasis on a healthy environment, we also seek renewable materials and reusable methods for rainwater. Rain from the roof can be reused as greywater for landscaping.

Perhaps more importantly, however, modern buildings isolate us from the outside world and natural events. Rain provides an opportunity for us to visually and audibly connect to the human experience. The way we treat rain and snow can enhance this connection.
 

Roof Design


(Martin Cathrae– flickr/creative commons license)

The purpose of a roof is to keep natural elements away from a building’s interior space. There are a few key things to consider. First, a designer needs to determine how much of the natural elements they want to keep away. They might want skylights, for example, to allow some of the sunlight inside. Or a trellised roof of a courtyard might allow rain and snow. Determine exactly what roles and functions it needs to perform.

Slope – Gravity is the main factor in shedding away precipitation. Water reaches the ground more easily on a steeply sloped roof. Buildings in heavy snow areas need especially steeply sloped roofs because snow sticks to the roof surface and takes time to melt. You will almost always see snow resorts and cabins with at least a 6:12 pitched roof.The minimum slope a roof can have is determined by local building codes. Usually it is around 2%. Even the flattest roof needs to have some amount of slope.
Overlap – Roof materials should overlap on a downward slope, so that the uppermost piece is outside of the underside piece. We see this done with roof tile configurations. The installer begins with a bottom row of tiles, and then adds rows of tiles upward until he reaches the top of the roof.This is important for anything that is exposed to the natural elements. Wall tiles need to be likewise overlapped with the uppermost piece outside. Same for flashing around chimneys and mechanical systems. The shallower the slope, the more they need to overlap. There will be capillary seep of water between the objects, so seal tightly and overlap generously. overlapping tiles roof

Vapor Barrier & Ventilation – A waterproof layer is necessary on the underside of the roof and wall assemblies. Moisture barriers are dictated by building codes, to separate materials that should get wet and materials that should not get wet. A considerable amount of moisture could build up from the interior due to temperature differentials, so these barriers are important. Condensation builds up at poorly insulated ceilings and walls.

Insulation and ventilation will prevent condensation of water. Consider the ventilation that you need for cold or warm roof assemblies. Traditional thatching is a warm roof with considerable insulation but little ventilation and no vapor envelope. Modern roofs always need some kind of ventilation to prevent water condensation from the air.

Parapet – Parapet walls often rise above the eave of roofs. These are convenient, because they hold back roof snow from falling down. It is important to adequately drain parapet walls, with downspouts placed more frequently than you would for eaves. As a rule of thumb, downspouts are placed every 40 ft of eaves, and every 20 ft of parapets. Consider the roof area above to determine how many downspouts to include. You can’t have too many.

It is good practice to provide overflow drains for parapets and roof drains, particularly if the roof is low-slope. These overflow drains might be scuppers, which are simply holes in the parapet and channels that pour out onto the landscape below. Naturally, you need to consider if there is a entrance, sidewalk, or deck below so that water does not pour onto people below.


(ell brown– flickr/creative commons license)

You also need to consider if the drains and downspouts are external or proceed down heated interior space, as freezing temperatures could clog drains. Drain sizing needs to be large enough to prevent clogs, as well as large enough to handle unusually large rainfalls.

Flashing – Avoid flat areas on the roof at all costs. Tight spaces, such as behind chimney flues, are places that are prone to leaks. Build up a cricket is areas such as this, sloped build-ups. Also surround these crickets and chimney flues with well-sealed metal flashing.

Flashing is an anti-corrosive metal piece that surrounds any penetration in the roof or any transition between materials, and it is sealed sufficiently to prevent leakage. Careful overlapping is also necessary with flashing. Consider that snow may build up in these tight areas and leech up the flashing. Research proper details for flashing and counter-flashing, as this is the primary culprit of roof leaking, and make sure it is appropriate for the roof material and design condition.


(DolfinDans– flickr/creative commons license)
Skylights – Skylights are great for daylighting, but they are also primary culprits of roof leaking. Any window that comes in contact with water can experience leaking, or moisture condensation due to temperature differentials. First, shelter the skylight as much as possible. Second, slope the skylight as much as possible. You want to avoid standing water and allow water to drain away from the glass as quickly as possible. Consider that snow on the roof may build up onto the glass and cause water seepage. Third, seal the material transitions. Fourth, provide adequate ventilation and window thickness to prevent moisture condensation.
(marimbajlamesa– flickr/creative commons license)

Gutters

 

Sizing – Roof gutters come in all shapes and sizes. Typically sheets of metal are bent on site into a simple rectangular or round shape. Custom metals or shapes can add to the aesthetics of the building. Gutters are typically 4 to 6 inches wide and tall, though larger roof areas and wetter climates require larger gutters. Lay the gutters on a slight slope so that water can pour easily to a nearby downspout. Sometimes people will build “internal gutters,” which extrude down into the roof a few feet up from the eave edge.

It is important to fold the backside of the metal gutter up the fascia and under the upper roof material. Dripping water could curl under and get under the roof material otherwise. The gutter also needs proper bracing, because 6 inches of water is considerably heavy. Gutters frequently get torn off from large rainstorms or snow.


(cogdogblog– flickr/creative commons license)
Gutter Screen – Gutter screens are a good way to keep the water  channel free of leaves, snow, and debris. In choosing a screen, consider the clinging nature of water. You don’t want water to just run over it to the other side. You also want snow and debris that accumulates atop to easily slide off or melt.

Another benefit of gutter screens is that they keep away pests. Of course, to achieve this you need the bottom of downspouts screened as well, and if any debris does get in there, it will clog up more quickly.

Avoid Gutters – Why use gutters at all? There are only two conditions that a roof needs to have gutters. Gutters help prevent too much precipitation from hitting the exterior walls, if the wall material is unsuitable for such weathering. Secondly, gutters prevent nuisance dripping above walkways, decks, and other pathways. But if neither of these conditions are the case, then just let the water run off the side.

Roof material should lap several inches over the fascia to avoid water curling underneath. It is a good idea to use some kind of deflector to push falling run-off outwards, as shown in the image to the right. It is also a good idea to have a trench drain in gravel below to handle the water after it drips.

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Wall Vents

 

Rainscreen – Exterior walls need to have overlapped materials that are robust enough to keep out all precipitation. But like the roof, there is also water vapor and condensation to consider. Moisture tends to collect between the exterior cladding and the wall insulation. To get rid of this insulation, there needs to be a capillary break at this location behind the cladding.

Allow thin vertical channels of airspace to run up the wall. Provide weep holes every so often allow ventilation of this airspace. Some suggest a ratio of 10:1 of airspace versus weep hole area. Provide a small flashing with a drip edge to let the condensed moisture drain out of this rainscreen space. Use a waterproof layer to prevent this moisture from damaging the sheathing or insulation.


(51% Studios Architecture– flickr/creative commons license)

Rain Event

 

Gargoyle – Ancient builders were connected to natural cycles and weather events. Nature’s effects were much more evident, as precipitation grew moss on stone walls and wore down wood roofs. In our effort for permanence, we avoid such weathering on building materials. But this comes at the sacrifice of our connectivity with nature.

Cathedral builders sculpted their scuppers into fantastical creatures called gargoyles. Gargoyles date back to a very ancient date, to Egyptian times, but their involvement in the dynamic function of shedding water is rather recent. It is an interesting idea that makes the art really come alive and gain relevance to nature and the building’s effort to provide shelter.

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Sound Of Rain On Roof – Few things are as soothing is the sound of rain falling on a roof. There is a primal reaction to this sound, a knowledge that you are safe and sheltered. Unfortunately, insulation in the ceiling prevents this sound from pervading buildings today. The gap between roof and ceiling in our cold roof designs lessen our reaction to external weather events.

This is just one more reason why warm roofs are generally better. Roof insulation is certainly important, as is ventilation, however we should do all we can to increase the audible reaction of rain on roofs. Or maybe we could get the sound of rain hitting well-sealed ribbon windows.

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Evocative Downspouts – If we cannot achieve a pleasant sound of rain on the roof, maybe we could get it from the downspout. A well-sized stream of drainage falling onto concrete is hard not to hear. Will that be too much of a nuisance? As designer, you need to decide how to achieve a desired noise level and which is more important, shelter from noise or connectivity to nature.

Imaginative drainage systems are also visually evocative. Use gargoyles. Or you could conjure mythological connections with waterfall streams and interesting landscaping. If temperatures reach freezing, maybe the transformation of ice and hanging icicles could enhance the visual experience, if considered carefully and with appropriate structure.


(s58y– flickr/creative commons license)

Reusing Rain Water

 

Impluvium – Roman buildings collected rain water and stored it for future use. Sloped roof surfaces directed rain water toward the central atrium. It dripped into the building, onto a porous regressed surface. Here, it filtered through gravel and sand into a cistern basin.

The drip of the water provided visual and audible grandeur from winter storms. The filtering gravel cleaned the water and made it suitable for human use. In summer months, evaporation of the water cooled the entire building. This compluvium water system was efficient, and it was an excellent use of this important natural resource.

Water Collection – Almost all buildings in America drain the rainwater from site into city gutters. Why? With the rising cost of utilities, water is a precious resource that literally falls on our doorstep, and we cannot afford to waste it.

Many cities have laws that either ban or penalize the collection of rainwater, which is stupid. So you need to research if any laws apply to you. If there are, determine how you can get around them. For example, is it legal for you to dump your downspouts onto the ground and then collect water from the ground? Or could you at least place a waterproof plastic layer under the lowest spot of your lawn and form a collection pool to naturally cool your house?


(mikecogh– flickr/creative commons license)

Rainwater is dirty, so you are going to need to filter it before storing it. Make sure to include an overflow outlet in your storage containers as well. Place screens in all of your inlets and outlets to prevent contamination. Water will need to be cleaned again before you use it again, especially if you plan to use it for drinking or bathing. I recommend just using it for watering your lawns. It also is handy in case there is an emergency and you need water, such as in a fire or if a disaster disrupts your utility connection. You could also pour it into a pool in the summertime, as the Romans did.

Use gravity to your advantage. Direct your downspouts from the roof, through a filter, into your cistern containers. Locate them near your water utility inlet. If your cistern outlet filter is sufficient and the amount of collected water is large, why not pump the water to sufficient pressure and include it directly into your utility inlet?
 

Site Landscaping

 

Site Drainage – After rain water sheds from the building, it needs to direct away. Perhaps the most common reason for water damage is water entering the building from the site. Maybe the foundation line is too low, or the ground around the building is sloping inward.

The soil around the building needs to slope outward. The concrete foundation needs to rise well above the ground line. Drainage pipes also need to surround the footings to remove any moisture that might collect in that area and endanger its stability. Also keep in mind how water might enter the site, redirect it away from the building. You want to keep water away from the building as much as possible, or contain and control the water you collect.


(boviate– flickr/creative commons license)

But once you get the water away from the building, then what? The entire site needs planning for water drainage. An entire city needs such planning. The film Grease famously portrayed the Los Angeles river as a concrete, man-made path that one could drive a car on. There needs to be a careful balance and consideration for natural water flows and human intervention. Too much man-made planning could be unsuitable for what nature wants, and then when a heavy storm rolls through, massive flooding will just wipe everything out.

Consider where water will go in an overflow situation. Are drainage pipe suitable for a heavy storm. If not, where will that excess water go?

And again, water is a precious resource, so it is good to use what you can. Collect water into attractive pools and wetlands next to your parking lot. Determine what you are allowed to do from your local laws and codes. Water was once a primary consideration for buildings and construction. Today, we take for granted that we can just connect a pipe to city utilities and get whatever we want. It is important to connect ourselves back to the primacy of nature and remember what is important in life. Some of the basic elements of nature—water, sunlight, earth, air—are missing from our living environments, and it sacrifices our health and happiness.
 
 
 

(featured images by Picdrome Public Domain Pictures on wikipedia/public domain, Jnzl’s Public Domain Photos, wilmtap, mlinksva, Internet Archive Book Images on flickr/public domain)