Feature
by Steve Saarman
In most multifamily residential buildings, exterior stairs are part of the ingress or egress exiting strategies. If they are structurally compromised by wood decay, stairs may become life threatening to the inhabitants. Therefore, keeping them “healthy” and sound is a priority. These stairs are often exposed to the elements and, therefore, the numerous connection points between horizontal landings are prone to water infiltration and, subsequently, wood decay if left unattended. Controlling the free flow of water, either across the landings or down the vertical surfaces with their bolted connection to the stair stringer is crucial to mitigate water intrusion. Numerous flashing techniques and “best practices” have evolved to solve these issues. The information presented will be a bit technical, but stopping wood decay is all about executing details in a watertight manner. The concepts are simple, but, if not understood and executed properly, the damage to the structure can be considerable.
Though there are many different types of deck-to-stair conditions, a very common situation is a wood-framed deck and landing below, connected by a set of stairs with either one or two stringers. Stringers are the “backbones” of stairs and hold them up. The deck and landing are constructed in two ways: one way allows water to pass through a spaced decking surface and the other way has a solid surface where water must run off in some direction. With the first option, building code now requires that all exposed wood-framing members be constructed of pressure-treated material to proactively deter wood decay. A word of caution: all brackets, hangers, nails, bolts, etc., must be, at a minimum, hot-dipped galvanized or stainless steel—not electroplated.
The more complicated second case is the solid surface. Let’s consider the upper deck surface, exposed to the elements, and the first step down on to the stairs. The deck top surface must be sloped, minimally a quarter-inch per foot away from the walls toward the outside edge to avoid ponding. If the general deck slope doesn’t slope toward the stairs, a transition in the slope direction intentionally takes place at the corner of the deck where stairs are often connected. The deck now slopes toward the stairs, so the top edge of the deck (before one steps down to the first stair thread) and the top stair thread are constructed almost parallel, making the distance between the two horizontal surfaces consistent. The code requires no more than three-eighths-inch variation in the fixed rise between all threads in a flight of stairs. So, water now will run toward the stairs and flow over the very important structural connections of the stair assembly. Therefore, solid waterproofing techniques must be employed.
There are, in most deck to stair assemblies, two very vulnerable points where water can enter behind the siding and into the structure. In this article, I will focus on these two points. There are so many variations in stair-to-deck or landing configurations that describing all variations is impossible to detail at this time, but the basic principles will still apply to all.
The Inside Corner
Let’s first discuss the inside corner of the leading edge of a concrete-covered deck as it transitions to the stairs. This situation is complicated because the waterproof membrane protecting the wood-framed deck will sit below a slab of concrete, which will be the ultimate walking or wearing surface. Install small “weep” holes in the deck edge metal for membrane level drainage. This way, the waterproof membrane and its waterproofing connection can’t be damaged by normal wear and tear of occupant use. Another variation would be to have the waterproof membrane system lay on top of the deck surface, rather than below. This situation requires considerably more diligent maintenance attention, as the membrane is now the wearing surface. Any break in the surface (dropping an object, movers), heavy wear, traffic spots or lack of consistent maintenance (UV degradation) can cause a localized waterproofing failure that allows water into the structure. That is why a wood deck with a properly installed waterproofing membrane system and a concrete or tile wearing surface above is the best in terms of long-term performance and minimal maintenance attention. The same basic flashing details apply to both types.
The first step in making a waterproof assembly is properly installing the deck-to-wall “L” flashing. On a sloped substrate, the “L” flashing is intended to span the transition between the horizontal surface of the deck and the wall. The theory behind a sloped substrate is to use gravity to move water as quickly as possible over the membrane and let it exit. Drainage mats are used on top of waterproof membranes to provide a greater space for water to flow unimpeded. This “L” flashing is intended to span the transition between the horizontal deck surface and the vertical wall. It also provides consistent vertical backing for the membrane to turn up upon.
This is the first place installation can go wrong. A standard “L” flashing is made with four-inch horizontal and six-inch vertical legs. The horizontal leg must be installed with fasteners into solid wood backing, not just the plywood surface. Stock sheet metal generally comes in 10-foot lengths, which, when joined, must be lapped at least six inches with three rows of parallel caulk beads, or it may be soldered. Corners, both inside and outside, are best prefabricated, with soldered joints, before installation.
The wall “L” flashing will run along the wall and abut the leading open edge of the deck that extends out from the building. Theoretically, it must now change shape, because the deck has turned away from the wall at 90 degrees, so the six-inch vertical leg must now turn down. This can leave an “open corner” or break in the flashing system at this transitional inside corner. A special saddle flashing must now integrate both “L” flashing orientations, as well as protect the 12-inch vertical drop off the edge of the deck at its inside corner-to-wall transition. This vertical outside deck edge to wall must also be watertight. It is often a “best practice” to extend this vertical flashing a half inch below the bottom of the deck surface, so water won’t track back across its surface. This new soldered “saddle” flashing must now be integrated “shingle fashion” with the building paper (Jumbo Tex 60 Minute is very good) and flexible membrane sheet goods (Flashband, Polyken, Vicor). The flashing that has just been installed is meant to be integrated into the wall’s primary waterproofing system, composed of building paper and flashings. If properly installed, water coming behind the siding should never have a chance to penetrate into the wall cavity and structural framing.
The “L” Flashing
Now let’s discuss the “L” deck-to-wall flashing that abutted the deck edge and turned 90 degrees. On the deck, the six-inch vertical up leg now turns down and its shape slightly changes. (For concrete decks only, this flashing is actually “T” shaped, so the upper arm acts as a form for the concrete when poured.) The bottom edge is “hemmed” or folded over itself, so the raw cut steel is not exposed to water (potential rust), and it also adds strength to that edge. The last half inch with the hemmed edge is now bent outward to form a drip edge. This is specifically designed so water coming down its surface that has flowed off the deck surface will be “kicked out” and free fall away from the surface, rather than continue to track down the surface. Remember, the objective of all waterproofing is to keep water moving so it will not accumulate and have a chance to enter any part of the structure and cause damage.
This drip edge sits right above the bolts that connect the structural stringers of the stairs to the deck edge. These bolts run right through the deck framing and are often bolted on the inside. Believe it or not, in time, water will seep down the length of these bolts, saturate the wood framing and cause wood decay of a vital structural wood-framing member. The best practice is to pre-bore the boltholes and fill them with caulk before inserting the bolts. Secondly, space the flashing for the drip edge one inch out away from the deck edge, so falling water misses the bolt heads.
Get Proactive
So, now that all these flashing systems have been installed and “best practices” techniques have been implemented, there are still some proactive measures yet to be employed to control the surface water on top of the concrete or tile surface and further restrict water from entering the siding system and deck waterproofing system. Again, let’s look at that vulnerable inside deck corner where the stairs attach. All the flashings and waterproofing have taken place on one plane. When the siding is installed, it will generally stick out three-quarters of an inch. This can act as a dam or a scoop to direct surface water at that exact inside corner behind the siding. The system should be 100% watertight, but with workmanship error, what is ever 100%? Why add stress or surge to a system? Here’s a basic waterproofing principle: keep as much water flowing away from the building and its entering systems as possible. Install a “diverter flashing” on the deck surface at the three-quarter-inch transition with the siding to keep water from entering. Caulk can work, but it doesn’t last forever, must be maintained and should only be considered a Band-Aid.
The next “best practice” technique for concrete or tile decks is to have a continuous caulk joint installed over backer rods around the perimeter. Concrete’s daily amount of expansion and contraction is different than wood’s. Therefore, an uncaulked perimeter joint will open and close daily, depending on the temperature differences. A sealant joint installed there will properly handle this differential expansion and contraction between dissimilar materials. Its function is to restrict as much water as possible from touching the membrane below, as well as handle thermal movement. Next, don’t defer maintenance. Keep all painted surfaces well maintained and caulked. Remember, paint is the first “line of defense” for your building.
A very good way to see how the building’s existing conditions function on a deck-to-stair or landing situation is to turn water on from a hose at the high point of a deck next to the stairs. Observe the water flow patterns. If the water runs towards vulnerable points as described, be aware of and check for discoloration and water stain marks. Often, the darker the color of the stain marks, the more severe the decay. Being observant is the best proactive tool to keep a building healthy and ready to serve you for decades.
The opinions expressed in this article are those of the author and do not necessarily reflect the viewpoint of SFAA or SF Apartment Magazine. Steve Saarman is with Saarman Construction, Ltd. Copyright © 2007 by SF Apartment Magazine. All rights reserved.