Flashing at Bottom of Exterior Walls

Flashing at the bottom of exterior walls needs to be integrated into a continuous drainage plane on the exterior wall. The goal is to create a wall system that can manage the water that will inevitably get behind any cladding system.

Principles of a Water Managed Wall System

Water leaking through exterior cladding assemblies is a common cause of construction defects and call-backs.

Flashing at Bottom of Exterior Walls
Figure 1 – Common callback: This damage was caused by a leak at the base of the wall cladding that did not include proper through-wall flashing.

Typically, these wall leaks are caused by missing flashings or poorly executed flashing details at the following:

  • roof-to-wall intersections
  • penetrations
  • windows and doors
  • transitions between different claddings when more than one cladding is used at the bottoms of exterior walls where cladding systems interface with the foundation.

The key to eliminating costly leaks is more than just applying a piece of flashing. You need to create a functional wall assembly that:

  • Deflects water away from sheathing, framing and floor-to-foundation intersections.
  • Drains water as fast and freely as possible to prevent water from being trapped and absorbed by building components.
  • Dries. Select building materials that allow the wall to dry out when it gets wet, and avoid materials that have the ability to trap moisture (BSC 2007).

Continuous Drainage Plane

Drainable wall assemblies integrate a weather-resistive barrier and flashing to properly drain water away from building components. The specific materials and installation methods depend on the cladding used on the building. House wrap and flashing must be properly integrated to create a continuous drainage plane. Note in Figure 2 that the flashing at the bottom of the wall is just one of a number of wall details that work in concert with each other to ensure water drains down and away from the building (Straube 2007).


Proper installation of flashing for drainage
Figure 2 – From the roof to the foundation, wall flashings are integrated into a continuous drainage plane to divert water.

Installation Sequence

A continuous drainage plane is typically created using a weather-resistive barrier (WRB), such as building paper or housewrap. To work effectively, the weather-resistive barrier must be properly “shingled.” Shingling is a term that describes the installation sequence that will allow water to drain away from the wall.

Proper shingling of drainage plane materials
Figure 3 – The layers of the drainage plane are installed from the bottom up in overlapping layers that provide proper shingling, so water drains down and away from the building. 

Figure 3 shows how the layers of the weather-resistive barrier must overlap to divert water out and away from the structure. In the sequence of construction, the flashing would need to be installed first. The bottom layer of building paper would be installed next, overlapping the vertical leg of the flashing. The top layer of building paper would be installed last (BSC 2007).

Rain screen wall systems also provide an air space between the wrap and the cladding. This air space helps reduce water problems in several ways.

  1. The larger space can create faster, unobstructed drainage.
  2. The space creates a capillary break to prevent water from wicking into building materials.
  3. The space allows for additional air movement to facilitate drying.
  4. The space can provide pressure moderation and help reduce pressure differentials that can help draw water into openings in the cladding.
  5. Helps prevent water vapor from the cladding from being driven into the wall assembly.

The air space varies depending on the cladding material. Wall venting behind brick and stone veneers is especially important. Under the right conditions, energy from the sun can push vapor through wet brick with the force of a steam boiler. A ventilation gap behind brick helps to dissipate this vapor before it is injected into the framed wall cavity. The gap should be 1 or more inches behind brick or stone veneer, 3/4 inch behind stucco, and 1/16 inch behind lap siding (Wrinkled house wrap will provide this 1/16-inch gap.)

How to Construct a Drainable Wall Assembly with Wood or Fiber-Cement Sided Walls

  1. Base flashing. Start by installing a metal or vinyl base flashing first. This piece should be applied directly to the wall sheathing using roofing nails. When more than one length is needed to span across the bottom of a wall, overlap pieces at least 8 inches. The flashing must include a drip-edge to direct any water that runs down the drainage plane away from the bottom of the wall.
  2. Weather-resistive barrier. Next, a weather-resistive barrier, such as building paper or house wrap, is applied to the wall, overlapping the base flashing. The example pictured in Figure 4 is a “drain wrap,” which has a texture to it to facilitate drainage after the siding is applied over it. You don’t need a drain wrap to create a drainable wall assembly, but the texture drains water faster than conventional smooth house wraps. There are several types of drain wrap: one type has a wrinkled surface; one type has plastic bumps that create a space between the wrap and the cladding  (Smegal and Lstiburek 2012).
  3. 8-inch minimum between siding and grade. To prevent splash back from the roofline soaking the base of the wall, the bottom edge of the base flashing should be at least 8 inches above grade. This is a design detail that must be worked out before the foundation is poured. The grade must be sloped on all sides of the building so the water continues to drain away from the building. A 5{3fd454b43fbfd7aa8a551fa339f02ecfdee40c34b6b5df10527629ca3647b822} slope is recommended. This is a 5-inch drop per 100 feet, or about 5/8-inch per foot  (Straube 2007).
Wood/fiber cement drainable wall assembly
Figure 4 – This drainable wall assembly for a wood or fiber cement clad wall uses wrinkled house wrap for the drainage plane and air gap integrated with flashing at the base of the wall. 

How to Construct a Drainable Wall Assembly with Stucco Clad Walls

Stucco is porous and will always crack. This is true even when installed perfectly. It is brittle, and cracking is in its nature. However, while these shortfalls will lead to water getting through the surface, the water can be managed. The key is to create a good drainage plane behind the stucco that drains to a weep system, allowing the water to flow out and away from the building.

  1. Self-adhesive flashing. When the weep screed attaches to plywood, OSB, or another wood-based framing material, a self-adhesive flashing membrane is needed first to protect the wood from water draining through the perforated screed (EPA 2015).
  2. Weep screed. Next, a weep screed is installed. The bottom edge of this piece of flashing should extend onto the foundation. The size of the screed will depend on the stucco thickness, with traditional three-coat stucco requiring a deeper profile than the screed needed for a one coat system. On an exterior insulation and finish (EIF) system, this flashing piece is called a weep channel, and it holds the bottom edge of the foam. It is perforated and works in principle just like a weep screed (EPA 2015).
  3. Two-layer weather resistive barrier. Most building codes require two-layers of building paper (or another type of weather-resistive barrier) beneath the stucco. The first layer (installed against the sheathing) serves as the drainage plane and protects the wall sheathing from water. The second layer works as a bond breaker. Wet stucco tends to adhere to building paper and house wrap. Water absorbed by the stucco will wick right through this bond breaking layer, but will drain down the wall between the two layers. Both layers should overlap the top edge of the weep screed (Lstiburek 2003).
  4. Lath. On wood framing, wire lath must be installed with the long dimension running perpendicular to the wall studs. Best practice calls for securing lath with furring nails, which place the lath in the center of the scratch coat (Note: Using paper-backed lath over a single-layer of house wrap or building paper will work as well as a two-layer weather resistive barrier and will save time) (DeKorne 2006).
Stucco drainable wall assembly
Figure 5 – This drainable wall assembly for a stucco-clad wall incorporates two layers of building paper over flashing and a weep screed behind the lath and stucco.

How to Construct a Drainable Wall Assembly with a Rain Screen Air Gap for Wood or Fiber-Cement Siding

Best practice with any wood or fiber-cement siding calls for building a rain screen assembly. This technique uses all the same water management details of a drainable wall assembly, but adds an air space behind the siding. This space creates a capillary break, which prevents water that wicks through the siding from being absorbed into the water-resistive barrier. The air space also allows air to flow freely behind the siding, which increases the wall’s ability to dry.

  1. Base flashing. The wall system begins with flashing at the bottom of the wall, which collects water that drains down the wall and kicks it out at the base. Metal or PVC flashing can be used. It should have a drip edge that hangs below the sill plate, with the bottom edge at least 8 inches above grade (EPA 2015).
  2. Weather-resistive barrier. After the flashing, a weather-resistive barrier (WRB), such as building paper or a house wrap is installed to create a drainage plane. The first course of the WRB needs to overlap the vertical leg of the base flashing. Subsequent courses of the WRB must overlap the course below (EPA 2015).
  3. Furring. The air space is created by installing furring strips (vertical pieces of wood or plastic) spaced every 16-24 inches over the weather-resistive barrier. These need to be at least 3/8 inches thick. They can be ripped out of plywood or purchased precut from a lumberyard, and nailed every 12-16 inches with 5d galvanized nails (Hanley-Wood 2007).
  4. Insect screen. To block the entry of insects, install nylon screening. Tuck the screening behind the furring strips, wrap it around the ends of the strips, and pin it in place with the siding starter strip. Nylon is recommended, but other types of screening can be used, as long as it is an open mesh that will not block the free flow of air. Metal will rust and is not recommended (EPA 2015).
Wood/fiber-cement rain-screen assembly
Figure 6 – Wood furring strips provide an air gap and rain screen in a wood or fiber-cement-clad wall assembly. 

How to Construct a Drainable Wall Assembly with a Rain Screen Air Gap for Brick Veneer Siding

Properly detailed brick veneer functions like a rain screen. The key is making sure there is an unobstructed path for water to flow out at the bottom of the wall. This path starts with the airspace behind the brick. Water freely flows down through this space, gets collected by through-wall flashing at the bottom of the wall, and drains out through weep holes.

brick veneer rain screen assembly
Figure 7 – Mortar mesh provides an air gap and rain screen behind brick veneer.
  1. Through-wall flashing. The first step to detailing the base of a brick-veneer wall is to install a through-wall flashing, which extends down the sheathing and out across the support ledge (often a concrete extension of the foundation, but may also be formed with angle-iron).
    Through-wall flashing can be made with metal, but is usually formed with a flexible rubber-based membrane, such as the one shown in Figure 8 (Hanley-Wood 2007).
    Flexible rubber through-wall flashing
    Figure 8 – A flexible rubber through-wall flashing overlaps the bottom of the sheathing and works with the weather-resistant barrier to guide any water that gets past the brick down and out of the wall.
  2. Weather-resistive barrier. Next, a weather-resistive barrier, such as building paper, must be applied over the framed wall assembly. The courses of the weather-resistive barrier must be applied “shingle fashion,” with the lowest course applied first, overlapping the vertical leg of the through-wall flashing. The upper courses are installed afterwards, overlapping each lower course (EPA 2011).
  3. Air space. The brick must be installed with a 1-inch air space behind it to allow water that seeps into the brick to freely drain. The illustration above shows a mortar mesh installed in the air space at the base of wall. This helps to stop mortar droppings from clogging the weep holes (EPA 2011).
  4. Weep holes. Water that collects on the through-wall flashing must be able to drain out through weep holes, which are formed by an “open” head joint (as shown in the illustration above), or with rope inserted into the  mortar joint (as shown in the photo below). In this illustration the open head joints have been filled with an open-weave mesh. This is a good idea for keeping mud wasps from building their nests in the weep holes, and to prevent other obstructions from blocking the free flow of water (EPA 2012).
    Weep holes: Rope inserted in the head joist between bricks will allow water to weep out of the base of the wall assembly
    Figure 9 – Rope is inserted in the head joist between the bricks in the lowest course of bricks to allow water to weep out of the base of the wall assembly. 


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