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TRIM, EXTERIOR CHOICES, INSTALLATION
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WINDOWS & DOORS
Building wall flashing materials, choices, installation details & specifications: this article provides details of foundation tops, building walls, exterior trim, wall corners, and roof-wall intersection flashing to prevent leaks & water damage. We describe the different flashing materials that can be used, how and where flashing should be installed at various building wall intersections and corners, at the intersection of a roof and building wall, at other locations, and how to seal these locations against leaks or insect damage.
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This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
Also see FLASHING ROOF-WALL SNAFU for examples of roof-wall abutment flashing foul-ups to avoid and for a discussion of single-piece roof-wall flashing at sloping roofs; see FLASHING MEMBRANES PEEL & STICK. For details about roof valleys see FLASHING, ASPHALT SHINGLE VALLEYS. If you are constructing a deck, see Deck Flashing at Building. If you are installing building siding, see FLASHING SIDING DETAILS.
Wall flashings are required at openings, corners, intersections, and wherever a roof terminates into a wall.
While peel-and-stick tapes have replaced these flashings at many details, metal flashings are still preferred for many standard details and applications where the flashing is visible or needs to hold a shape or serve as a drip edge.
Our photo of foundation top flashing (left) shows a couple of problems: the foundation extended past the building wall, forming a shelf that invites leaks as rain runs down the building wall.
The metal flashing placed on wall top slopes back towards the building, directing water inside the structure. Details like this risk rot and insect damage to the structure.
Maybe, in this case, the flashing extends "upwards" behind the building siding, forming a zee-shape that might reduce this risk. Without further inspecting we don't know.
Choose metal flashings that are compatible with the adjoining building materials and are at least as durable as the siding and roofing materials where they are to be placed. (See “Galvanic Corrosion” for information on metal compatibility.)
Most residential wall flashing today is made from light-gauge aluminum coil stock. Aluminum is inexpensive, easy to bend, and holds paint well. However, it tends to oxidize and pit in salty or polluted air and, if unpainted, will corrode from contact with masonry due to the lime and acids. Aluminum cannot be soldered. If using aluminum, use at least .029-inch coil stock, preferably anodized or pre finished, which is much more resistant to corrosion.
When the budget allows, copper is a good choice. Copper flashings come in two types: soft and harder cold-rolled. Soft copper is very malleable and useful for molding into irregular shapes. The harder cold rolled material is a better choice for most applications, because it is stronger and more durable.
Copper flashings solder easily and offer good corrosion resistance, even in polluted air and in contact with masonry. Over time, all unpainted copper will oxidize and develop a green patina that protects the underlying copper. While most people find the patina attractive, the runoff of the green oxidation can stain siding or trim.
Some experts also caution against using copper or lead-coated copper in contact with redwood or red cedar or its runoff. Over time, the copper surface will be etched by the acidic wood runoff. Although actual failures of copper flashings are rare, they have been reported in areas of the Northeast after 10 to 20 years of service. Acid rain, combined with exposure to runoff from red cedar or other corrosive materials, is suspected as the cause.
This is a sheet of copper with a lead coating on each side. Where staining of building components from runoff is a potential problem, lead-coated copper may be used, which has a less noticeable gray runoff. Also, without the lead coating, copper flashing will react with galvanized steel.
For special flashing applications where a high degree of malleability is required, lead is an option. In addition to being easily bent and molded, lead is very resistant to corrosion. Lead is relatively soft, however; so it should not be used where it will be bumped or walked on. Also, it is best if left unattached on one side; if rigidly fastened on all sides, it can tear from fatigue due to thermal movement.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Window and door flashings are discussed extensively separately at WINDOW FLASHING & SEALING Guide
Details about how to find and recognize insect damage on buildings and details about various methods to avoid termite attack on buildings or other wood destroying insect damage to buildings are found at INSECT INFESTATION / DAMAGE.
Below we discuss metal termite shields, one of those methods. Also see TERMITE SHIELDS vs TERMITICIDE for a discussion of termite shields and insect attack on buildings using foam board foundation insulation.
Metal termite shields are widely used atop foundations in the southern United States and in tropical climates as a physical barrier to termites. They sit directly on top of foundation walls, piers, and other supports before the first piece of wood is installed (see Figure 1-3 at left).
At one time termite shields were thought to block the entry of subterranean termites, the most widespread and destructive wood-boring insect in the United States. However, subterranean termites, which nest in the soil, will exploit the tiniest gaps in termite shields or other barriers to reach the wooden portions of a house and will build tunnels along exposed foundation walls and around termite shields if necessary.
Although the shields do not stop termites, they slow down their progress and force them to build their tunnels in the open where they can be easily seen during inspections.
To work at all, the termite shield must have tightly sealed joints and be sealed around foundation bolts and other penetrations. Joints can be either soldered or mechanically interlocking. If the barrier is unsealed, termites will find any small gaps and render the effort worthless.
Below our termite mud tube photos show that a termite shield appears to have been installed along most but not all of the building foundation top. Or was it? We don't know if this is wall flashing that leaves sills exposed just under the wall edge, or whether the flashing extends across the foundation to the interior (as recommended).
But our second termite photo (below right) shows a termite mud tube ascending the same foundation wall and passing under the termite shield. The shield makes it more difficult, but not impossible, for termites to attack a building.
In general, termite shields should be a minimum of 6 inches above grade and extend out 2 inches on either side of the foundation at a 45 degree angle. In addition to making termite infestations visible, they also form a capillary break between the foundation and sill. Areas where a termite shield cannot be used, for example, where a concrete stairway abuts a foundation wall, are at high risk for termite entry.
In termite-prone regions, the only reliable way to prevent termite damage is to use treated wood in critical locations and treat the surrounding soil with termiticide.
See TERMITE SHIELDS vs TERMITICIDE for a detailed discussion of termite shields and insect attack on buildings using foam board foundation insulation.
See INSECT INFESTATION / DAMAGE for termite and other wood destroying insect information in depth, including termite inspection case reports, field photos and advice. Readers concerned about termite damage associated with foam, fiberglass, or other building insulation materials should also see Insects & Foam Insulation.
Definition of water table trim:
What is "water table trim"?
On many traditional homes, a wide board called the “water table” is installed along the foundation and supports the first piece of siding.
The water table should extend about an inch over the foundation and be capped on top with either a preformed metal drip cap or a custom-bent flashing installed under the sheathing wrap.
Cut a slit in the sheathing wrap along the entire length of the water table and slip the upper leg of the flashing under the wrap (see Figure 1-4 at left).
It is critical to protect against leaks and water buildup at deck ledgers, since decay in this part of a building can lead to structural failure of the deck.
At a minimum, install a cap flashing that tucks under the sheathing wrap and goes over the ledger (see Figure 4-8 at left).
Adding a second flashing, either peel-and-stick membrane or aluminum-coil stock, between the sheathing and ledger, as shown, is a worthwhile backup should any water get over, around, or through punctures in the cap flashing.
Since pressure-treated wood can be corrosive to unfinished aluminum, use coated-aluminum or galvanized steel flashing.
Also see details at Deck Flashing at Building.
Corner boards are prone to leakage due to shrinkage of materials and wind exposure.
For simple, effective backup protection, add a spline of asphalt felt paper at outside corners so that it extends 6 inches beyond the corner boards. Inside corners also benefit from a spline (Figure 1-5 at left).
With this type of backup protection and with the end grain of the siding well sealed, it is unnecessary to caulk the siding joints at inside and outside corners.
Leaving a small gap and not caulking these joints allows any water that penetrates to dry to the exterior.
Eventually caulk joints will fail anyway, allowing water to leak in but inhibiting drying.
Integrate all step flashings with the sidewall-sheathing wrap by slipping the upper legs of the step flashing under the sheathing wrap (Figure 1-6 at left).
Where snow buildup is anticipated, add a band of peel-and-stick membrane over the step flashing but under the sheathing wrap, as shown. Where the step flashing terminates along a sidewall, a preformed or custom-bent kick-out flashing is the best way to divert the water away from the siding.
Splashback Damage Protection for buildings
In wall areas subjected to splashback, snow buildup, or high moisture from other sources, rubberized asphalt membranes in widths up to 36 inches can be used to protect the wall sheathing and structure.
Water damage from splashback is common in wall sections located under the eaves of a roof with no gutters. Walls above decks or flat roofs are also prone to moisture damage from splashback or snow buildup.
In all cases, make sure to detail the flashing membrane so that it tucks under the sheathing wrap above and over the step flashing or cap flashing below. If installed along the foundation, the membrane should cover the joint where the sill meets the foundation.
The importance of roof-wall flashing and sound counter flashing is apparent in our photograph (left). The absence of a gutter on the upper roof and the rain and roof runoff splash-up wear on the building wall is quite appareant. If sound counterflashing had not been provided when this Beacon NY church was rebuilt in 1944, serious leaks would have appeared in the building interior. - Ed.
NRCA (Berg) describes the basic specifications for roof-wall abutment step flashing, based on the method described in the NRCA Roofing and Waterproofing Manual, as follows: [paraphrasing]
For more details about splashback or splash-up water damage to exterior walls, see RAIN SPLASH-UP SIDING DAMAGE.
Also see FLASHING ROOF-WALL SNAFU for examples of roof-wall abutment flashing foul-ups to avoid, and see FLASHING MEMBRANES PEEL & STICK. If you are constructing a deck, see Deck Flashing at Building.
- Adapted with permission from Best Practices Guide to Residential Construction or select a topic from the More Reading links shown below.
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