The Problem:

Have you ever seen an enclosed balcony, deck or elevated exterior walkway with a concrete topping slab or other surface that was leaking or had subsequent structural concerns due to the water entry?  Unfortunately, these problems are all too common, but can be easily avoided if some very simple measures are taken in the design and/or construction process.

What would someone call a deck, balcony or elevated walkway that did have a waterproofing membrane installed, but did not have the ability to drain water from the membrane?  We here at Professional Investigative Engineers have a very scientific term we use to describe this type of construction, “a bathtub”.

Obviously the proper application of the waterproofing membrane (whether it be sheet applied or fluid applied) and its flashings are of the utmost importance to ensure that water does not infiltrate into the supporting structure.  However, there is another key factor that is frequently overlooked for long-term performance of a deck, balcony, or elevated walkway, with a topping slab or wearing surface, and that is the drainage of water from the system.  The focus of this publication is not the waterproofing membrane(s) associated with decks, balconies or elevated exterior walkways but rather the drainage of water from these elements.

The Industry:

Why is it that no one disputes the membrane surface of a low-slope roof must drain, however the same or similar membrane surface under a concrete topping slab or other wearing surface installed over a balcony, deck or elevated walkway is not as important?  The answer is simple: a lack of understanding, knowledge and even common sense.

There are a few industry recognized organizations such as the NRCA (National Roofing Contractors Association) for roofing and waterproofing, and the SWRI (Sealant, Waterproofing & Restoration Institute) for below-grade waterproofing and plaza decks that publish a wealth and abundance of knowledge on how to properly construct a roof, waterproofing or plaza deck system. However, finding publications on decks, balconies and elevated exterior walkways is a whole different animal.

Unfortunately, to the best of our knowledge no one or single organization has been developed to provide industry wide standards on the proper design and construction of decks, balconies and elevated walkways.  Why not you might ask?  Well, the answer is pretty simple.  The materials being used to waterproof these decks, balconies and walkways are the same materials used in the roofing and waterproofing industry.  From a shear marketing and profit standpoint, these manufacturers focus their energies on the larger and more profitable piece of the construction market that they specifically service.  Hence, the roofing and larger scale waterproofing sector.

Balconies, decks and elevated walkways with a concrete topping slab or wearing surface are systematically similar to a plaza deck.  In such, the proper design and construction of such systems should incorporate not only a waterproofing membrane, but also a composite drainage media and source or element for drainage at the membrane level.

The Solution:

So first we tackle the beast of slope!  As one of my instructors (Colonel Thomas Dion, author of Land Development for Civil Engineers) at The Citadel in Charleston, South Carolina would say:  “Get rid of the water bubba!”  Whether you are a civil engineer designing a grading and drainage plan or you are an architect/roof consultant designing a roofing system, the number one rule is to remove the water.  Unless of course your designing a pond, pool or “bathtub”. 

Therefore, prudent construction would include an accommodation for drainage both at the top of the wearing surface (concrete topping slab, tile, etc.) and also below the wearing surface at the top of the waterproofing membrane.   The sloping of the wearing surface should be such that it adequately drains all water too either an internal drain system, thru-wall scupper or properly detailed perimeter edge flashing.   The substrate or deck below the waterproofing membrane should therefore also be sloped to drain and have a space or media provided to accommodate the drainage of water beneath the wearing surface or topping material.  All too often, the deck sheathing or waterproofing membrane is not sloped and the topping or wearing surface is placed directly over the membrane with no means for drainage between the two elements.  The drainage at the waterproofing membrane level must also be controlled via an internal drain system, thru-wall scupper or perimeter edge flashing detail. 

We all know water travels down hill (i.e. gravity flow).  We also know that if there is no means for drainage, that water will collect or pond on the surface or substrate resulting in a number of problematic conditions whether that be in a grading/drainage, roofing or waterproofing scenario.  Hence this ponding or standing water will result in a greater likelihood for leaks and the resultant problems that can follow thereafter.  Products and materials developed specifically to handle these types of drainage conditions have been available for quite some time in the industry.  Polyethylene composite drainage mats, dual level drains and perimeter edge systems have been used in below-grade waterproofing for years.  While reasonable designers and builders have implemented such materials and practices into their construction, there will always be those who do not and therefore the problems will continue.

When the wearing or pedestrian traffic-bearing surface is comprised of a concrete paver system, the water is meant to pass through the joints in the pavers to the waterproofing membrane below.  In these cases, the paver wearing surface will probably not be sloped, but there must be positive slope and a means for drainage at the membrane level.  This means for drainage is often achieved in paver type systems due to the fact that the pavers are placed on pedestals that are elevated above the waterproofing membrane.  The added benefit to the installation of a concrete paver wearing surface is that access to the waterproofing membrane for inspections, maintenance and repairs is relatively easy and non-destructive, as it only requires the lifting and removal of the paver(s) which can be easily reinstalled upon completion.  In cases where a monolithic concrete topping slab or tile wearing surface is present, expensive demolition and full or partial replacement of the topping slab or wearing surface is required to facilitate the same inspection, maintenance and repair work. 

Final Thoughts:

Failure to consider how a deck or balcony will accommodate drainage of water both at its wearing surface and the waterproofing membrane level is a design and construction consideration that must be given more thought by both the designer and the contractor/builder. 

Furthermore, additional considerations should be given to the utilization of concrete pavers or other such wearing surfaces that can be easily removed and replaced should access to the waterproofing beneath a balcony, deck or elevated walkway become desired or necessary for inspection, maintenance or repair.  How can a designer or contractor expect an owner to inspect and maintain something that is not accessible!  If it cannot be accessed, it must be assumed that it was built with not only the correct materials, but with the proper construction techniques necessary to last the entire life expectancy of the structure.

Copyright 2005 Professional Investigative Engineers. All rights reserved.