Over the past few weeks, we’ve looked at different aspects of a freestanding building foundation. Although this is new construction, it is a good context to use as a point of comparison to understand the historic building foundations here in the inner city of Washington DC.
Some of the main components of the foundation waterproofing and drainage system include the following:
- Waterproofing membrane
- Pressure relief system
- Collection pipe
- Discharge
Waterproofing Membranes
There are two main categories or types of waterproofing membranes that can be used against the foundation wall of a building: liquid applied or sheet applied.
Liquid applied membranes have become increasingly popular and used more prolifically throughout the industry. Historically speaking though, applied systems were more common. In the earliest types of foundation sealing, damproofing was used, though even before applied waterproofing systems were used. Builders would often just coat the exterior foundation in tar, similar to modern fluid applied systems but the tar applications had a much lower resistance to shifting, cracking, expansion, and separation. Modern membranes generally have a high degree of elasticity and resilience. Often, when these materials can move to any size over 300% larger than the applied thickness, those materials are referred to as elastomeric.
In this particular case, the foundation is built with cast in place concrete. Concrete is a bit rare for foundations in the historic areas of Washington DC, but found still in DC at new construction which has been inserted between historic areas of the city. Most of the historic buildings in the city though are built with masonry foundations, mostly either historic common brick or stone foundations.
A waterproofing membrane can be built against the concrete foundation wall with two main options or categories of types. In Washington DC the foundation generally is not made of concrete, instead more often built with masonry, except for new construction that has been inserted into the older parts of the historic neighbors of the city.
Pressure Relief Systems
In addition to the waterproof membrane against the foundation wall, it’s also a good idea to have a pressure relief system installed to allow water to flow from the are beside the foundation, down to a collection drain. There are different types of pressure relief systems, in this particular case, since it is a deep foundation, they have used a dimple board. The dimple board essentially sets a permeable filter and an impermeable barrier together, against the foundation wall.
Collection Pipe
Working in concert with the pressure relief system, a perimeter foundation drain, typically a perforated pipe encased in washed gravel or stone, is used to collect and run the water which builds at the foundation, away from the foundation. This drain, positioned at the footing level, encompasses the entire foundation, collects the water channeled down by the dimple board. From here, the water is directed away from the foundation, either to daylight if the terrain permits gravity drainage or to a sump pump for active removal.
The image below shows where that pipe is stubbed up above the subgrade, and this stage, the pipe is just roughed in and partially buried but must be terminated later and connected.
The efficacy of a drainage system is often further enhanced by several additional components. Filter fabric often wraps the pipe, as shown here, or the gravel surrounding the perforated pipe, preventing a large degree of soil intrusion that could clog the system. The landscape grading around the foundation slopes away from the building, encouraging surface water to flow outward. Gutters and downspouts direct roof water away from the foundation area. Gutters, when installed at the eave of roof will collect and concentrate that water to run down a downspout from that point when the downspout carries that water to the ground level, that water then must be diverted away from the building even further for it to work properly to avoid dumping large loads and concentrations of water directly at the foundation. One way for this to work better is to run rain leaders underground which can either daylight at a lower area of ground, if there’s sufficient grade on site, or run the rain leaders to a discharge sump pump. In almost all cases, it’s better to have a site that allows for sufficient fall for positive water flow away from the building foundation.
While these exterior measures are the first line of defense, interior strategies are also important, especially for basements built to become living spaces. These may include interior collection pipes, a modern form of weeping tiles along the foundation’s inner perimeter (shown here), leading to a sump pump, a layer of gravel beneath the concrete slab floor with a vapor barrier, and possibly even interior waterproofing systems applied to the foundation walls before finishing.
The symbiotic relationship between these elements creates a multi-layered defense against water intrusion. Each component, from the dimple board to the perimeter drain, from the graded landscape to the interior sump pump, contributes to the overarching goal: a dry, stable foundation capable of supporting a comfortable living space.
Effective foundation drainage is not as simple as just about keeping water out; it’s about actively managing its flow, strategically guiding it away from the structure. It’s about relieving the pressure and also simultaneously providing waterproofing to keep the water out while it’s flowing and changing pressure values around the foundation. A foundation waterproof and drainage system has to work with both of these different functions happening simultaneously, waterproofing and drainage. The logic and coordination of the details in the design make this work properly.
The image below shows a picture of a polyethylene dimple board with integral filter fabric. The filter fabric allows water to be filtered, so water will enter, by gravity pressure, but heavy soils and debris will be trapped on the high pressure side of the membrane. Then, the other side of the assembly has a three-dimensional texture with dimples to allow water to flow within a small cavity around those dimples but also simultaneously provide a waterproof polyethylene barrier on the inner side against the foundation wall.
Discharge
For the system to work, in a complete cycle, the water, once it flows through a path of low resistance, it is then collected at the foundation and lastly must then discharge away from the building foundation. There are also two main methods for discharge away from the building foundation: Flow by gravity, or pumping. If the grade allows, if there’s a part of the building site that is lower than the foundation, within practical reason and configuration, a non perforated pipe can carry that water away from the building foundation, and out at that location, the foundation water may flow away, on the surface of the ground.
The other option, pumping, is much more common in Washington, DC. Generally, here in Washington DC building lots are relatively Limited in size and generally do not have the overall grade to allow water to flow by gravity alone away from the foundation. And these typical cases, a sump and crock may be installed to collect the water at the lowest point of the system. From that point, with a typical Electric pump, water can then be discharged out onto the surface of the ground. In most cases the surface of the ground will be higher than the sump pump, therefore necessitating the pumping of the water.
We can Help
Our company focuses on historic restoration more than modern building upkeep, maintenance, and construction, but our company understands both types of construction very well and a full picture well-rounded approach is needed in any niche in the construction industry. Although we focus on historic restoration, repointing, tuckpointing and historic brick repair, our company also has technical knowledge and competencies in the areas of modern and contemporary construction as well as we become one of the leaders in that area of the market today. Understanding both historic and modern or contemporary construction is useful because both aspects help understand the challenges and potential solutions for challenges in building science and construction.
We can help with a variety of historic masonry restoration needs and upkeep, from modest tuckpointing and or repointing to complicated and extensive historic masonry restoration. Infinity Design Solutions is a historic restoration specialist contractor specializing in both historic masonry restoration such as tuckpointing our repointing, and brick repair. If you have questions about the architectural details or facade of your historic building in Washington DC, reach out and say hello and if we can help we’ll be glad to assist you. You can email us or call us on the telephone at the following link: contact us here.
