Waterproofing Concepts and Effective Detailing
This article on waterproofing concepts is for building designers, architects, engineers,
project managers, construction foremen and students.
TABLE OF CONTENTS
- Free-board
- Distribution and drainage
- Containment
- Containment under pressure
- Containment reliant on sealant adhesion
- Containment under pressure reliant on sealant adhesion
- Concrete
Introduction
A construction foreman becomes the designer of any minor concealed detail whenever a typical common detail is not provided. A competent foremen has a comprehensive understanding of construction materials and methods. It is not uncommon to find the architect and the construction foreman collaborating to resolve a tricky detail.
Waterproofing is something designers 'need to get their head around'. Membranes, specialist coatings and flexible adhesive sealants are being relied upon way too often. I see failures in buildings where sealant failures have occurred over and over again. In some, if not most of these failures, sealants weren't needed at all. What was needed was better detailing.
The introduction of waterproofing or 'waterproofer' as a sub trade has not reduced the incidents of water penetration. The ability for a dwelling to withstand the elements, the egress of water, is a fundamental design issue which extends way beyond the influence of a tradesman who is license to apply a waterproof membrane.
Free-board
In summary free-board is simply the height provided in any sectional detail that elevates a dry area above a wet area. When assessing what is adequate free-board we need to know;
a) The depth of possible flooding or what the extreme weather expectations are in the specific environment
b) The performance of the drainage
c) Any potential water penetration due to wind or capillary action
Free-board is a term used in boating to describe the amount of the hull of a boat which is above the water line. In building the same term refers to the height of a step between a balcony or a path and the inside floor level. Free-board is our first waterproofing detail concern. If we can provide adequate free-board we can raise the internal areas of a dwelling above any threat of water egress. Free-board is a design principal and as so is applicable to waterproofing concerns.
Water will flow downwards unless it is being contained in which case it will rise in the container. In a location such as an external balcony it is possible during heavy rain for the balcony to flood should the storm water drain in the balcony become covered by fallen leaves and debris. Our design concern hear would be to provide adequate free-board to the internal floor area which is higher than any other turn ups in the balcony's external edging detail.
Even in a sheet roofing profile free-board is a design principal. The height of the undulations in the profile will determine how much water the roofing material can carry before the water flowing over the roof sheeting will cover the fixing holes on the higher undulations. The height of the profile will also determine the maximum span and the minimum pitch of the roof material.
Although the most obvious example of free-board is a house on a floodplain built on stilts, free-board is equally significant in the shower recess on a submerged submarine cruising the depths of the ocean. Free-board is a very reliable detail principle. We may be dealing with tens of metres or millimetres, what matters is having adequate free-board for that specific detail, in that specific location, in that specific environment.
I have seen time and again water penetration from a external balcony into a room below. When taking up some of the tiles and removing some of the wall we would often find that the waterproofing membrane was not turned far enough up inside the wall to contain the water. A perfectly good and expensive job rendered useless because an individual made a decision not to extend the membrane further up the wall. It is very easy to be wise in hindsight so I won't go as far as calling the idiot who did this names. It is very probable that he may have been working under misinformation regarding the the thickness of the tile bed, maybe there was not quite enough membrane, regardless, the end result is the same. Where there is only a single step the membrane should extend higher then the door threshold. Because the door threshold is the visible entry the membrane hidden in the wall should provide additional free-board as a design failsafe. Free-board can be the failsafe aspect of a waterproofing detail.
An other good example of inadequate free-board is with window sill details. It is common in building that when a concrete slabs extend through external brick work that there is a rebate in the edge of the concrete which effectively lowers the first brick course of the outter skin of bricks and provides one course of bricks as free-board between the two skins of bricks. This relieves the need to rely entirely on the performance of the DPC or tray flashings. This very effective detail gets into strife at the window sill because water is now being collected at a different level then in the brick cavities. We are talking here of only millimetres of water. We may only need twenty millimetres of free-board but there is also other factors to consider. firstly there are the porous materials that can corrupt the detail, Secondly here in these details capillary action can cause water to travel upwards. The detail concern is to extend the free-board above the capillary action.
Capillary action can undermine the effect of free-board if the free-board does not extend above the detail that is subject to capillary action.
Without adequate drainage there is no such thing as adequate free-board. When considering a sectional detail solution be aware of the fact that the specific environment that any sectional detail has to deal with extends beyond the edge of the drawing. It extends from the heavens to the ocean.
Free-board and extreme weather conditions is a complex issue because a lot of what is designed to withstand the onslaught of the elements can not cope with extreme weather conditions. Rain driven by very strong wind can cause water to travel one and a half metres horizontally through the top of a door rebate seven metres above the ground. For that matter extremely strong wind can blow a houses completely away. But there is a point and a contentious one at that. At what extreme does failure in the performance of a domestic residential design constitute a natural disaster and not a detail failure. I believe this to vary from one location to another and can be determined by the frequency of the weather occurrence.
One needs to consider what is the expected extreme of "normal" weather conditions in any specific environment.
2 Distribution and Drainage
In summary distribution and drainage is the ability of any waterproofing detail to shed water to a point where it can be channeled away. When assessing what is adequate distribution and drainage we need to know;
• What the extreme weather expectations are,
• How gravity is going to direct the distribution of the water
• How a prevailing wind is going to effect the distribution of the water
• How surface tension is going to effect the distribution of the water
• That the drainage system is adequate,
• Where the water is going to go should the drains become blocked,
• How are we going to distribute water contained or leaking because of blocked drainage.
When rain water falls on a surface it is either contained, distributed or drained away. Contained as in a pool, puddle or pond. Distributed as shed from a sloping surface and drained, as into a system of pipes, pits etc. Distribution and drainage is our second waterproofing detailing concern. These detailing concerns are enlisted to examine the effectiveness of a detail in order of performance reliability. Free-board for example is either adequate or inadequate. There is no bad workmanship performance concerns. Drainage on the other hand can be subject to failures due to workmanship or blockages hence the need for free-board. When a drain blocks the water behaves as in a container and distributes itself according to the surrounding levels and prevailing forces until a second drainage outlet comes into play. It may be a planed installation to allow water to escape from a ponding situation or it may simply be runoff into a garden. The water may be allowed to become fifty millimetres deep before it escapes or it may only flood to a depth ten millimetres. If one is very cleaver maybe there will be no flooding at all.
You need to be aware of where the water is going to go in the event of a blocked drain. Storm water can be very erosive in garden beds, play havoc with reactive soils and undermine foundations in extreme situations.
Distribution of water occurs on any given catchment area prior to the water running to a drain. The levels of the catchment area surrounding the drain are of concern and not only on the finish floor levels. On a balcony for example, the tiles are often laid on a porous tile bed. The levels of the substrate under the tile bed are just as concerning. When water is trapped under balcony tiles it can lead to a number of problems such as salts discharge at the edge of the balcony.
If storm water is not being contained it is either being caught and distributed or collected and drained away. Levels are very important as we don't want water to be collected where there is no drain or distributed away from a drain. Both on a finished tiled surface and under the porous tile bed.
On balconies decks and large external areas the drains and grates need to be of an adequate size, so as not to become covered or clogged easily. Roofs are also prone to water penetration because of the accumulation of leaves and debris in the gutters, valleys and down pipes. The market place now has a variety of gutters and other fittings to keep leaves and such out of the gutters. The responsibility for the designer is to know where the water is going should the gutters flood. A number of gutter cross sections have the front of the gutter somewhat higher than the side of the gutter which is attached to the fascia. In effect we have no free-board. Water may flood into the eaves. Not a good idea from a maintenance point of view. In the event of the gutters overflowing the surface tension of the overflowing water will cause the water to cling to the gutter and the water will flow between the gutter and the fascia if the gutter is spaced off the fascia.
Window sills have the same set of distribution and drainage principles applied on a smaller scale. On a sliding aluminum window we have a track which has the possibility of containing water. There are usually slots under the track to allow water to drain through to the outside. These slots can become clogged and water can flood the window sill. Usually there is another fin higher then the sliding window track to provide a few more millimetres of free-board. The window may leak at the juncture of the vertical and horizontal members. A properly installed sub sill flashing or section will catch any water leaking through these joints and shed the water to the outside.
The shedding or distribution of water is influenced by several different forces. Gravity will cause the water to flow downward. Surface tension will cause the water to cling to the surface it is running over and around. Wind will drive rain water back up a low incline. Pressure variations and capillary action can cause water to appear to defy gravity.
Turn ups, turn backs, turn downs and drip grooves are installed to control the way water is distributed. If you take a cup and saucer to the kitchen tap and run the water at a modest flow rate over the flat saucer you will see the surface tension causing the water to track back under the saucer. If you place the cup upside down under the water flow you will notice the water will drop off the rim. The water's surface tension is not strong enough to cause the water to flow up the inside of the cup.
3 Containment
In summary containment is the aspect of a waterproofing detail that holds water. When
assessing what is adequate containment we need to know;
• What is adequate free-board,
• That there is distribution and drainage,
• The quality and durability of the waterproofing material,
• The effect of movement between different building elements on the waterproof material,
• That the workmanship has been tested and proved effective.
Containment is what we need to do with storm water if we are unable rise above it or drain it away. Rising damp is the exception. Containment can be thought of as both in the positive, like a glass of water or in the negative such as a boat. It is important to understand that water can not be held back like one would hold back an angry crowd. As silly as this sounds, wrong thinking is the reason most waterproofing details are designed incorrectly.
Water is either contained or it isn't contained. There are no half measures. Whatever the containment we are relying on the performance of the waterproof materials and the workmanship of those who installed them. That is why I would prefer to rise above the water or drain it all away in preference to containing it.
Water is contained very reliably by a bath tubs, less reliably by below tile shower trays and even less reliably by DPC and brickwork tray flashings. Water is contained reliably by sheet metal roofing and less reliably by concrete. The ability for concrete to contain water is questionable as concrete can have flaws and varying compositions. More often then not concrete proves not to be waterproof in the real world.
Sheet metal roofing is designed to shed water, however in heavy rain it has to contain it as well, hence the undulations and the turn ups. Sheet metal roofing is a good example for understanding the three waterproofing principle we have discussed so far. The undulations and turn ups contain the water as well as provide free-board so the water will not flood in to the roof space under the cappings. The slope of the roof has to be adequate so the water is shed without leaving puddles on the roof. The turn downs at the edge of the roof have to be properly formed or surface tension will direct the water back along the underside of the roof sheet and into the eaves or roof space. The gutters also has to have adequate slop and free-board so as not to flood and of course the downpipe drainage has to be adequate so the water will drain away. As stated previously containment requires a waterproof material to form sides and a bottom. If one side is missing then we have containment and distribution. If the material is not waterproof we don't have containment we have distribution.
In installations such as a lift overrun pits or underground basements it is important to achieve a bottom and all sides containment as it is very probable that in these circumstances we are well below a drainage point. If we can drain the water away then we don't need to contain the water even in these locations.
Reliability of waterproof materials is always a concern. Primarily because durable materials are usually hard and likely to fracture should there be any movement between different building elements to which the material is applied and flexible materials which can accommodate movement are soft and susceptible to mechanical damage.
4 Containment under pressure
free-board, distribution and drainage and containment are the three principles employed for all intelligent waterproofing details. Containment under pressure is not a preferred detail if only for reasons of risk. It may be unavoidable in circumstances where there are no other options. We need then to ensure we employ reliable waterproofing materials and methods recommended by the waterproofing material manufactures as the reliability of the waterproofing depends solely on the waterproofing material and will only be effective for the life of the material.
Should a detail be required to hold water to a depth of more than twenty millimetres then it can be regarded as containing water under pressure. Although at twenty millimetres the head or pressure is still quite modest it increases very rapidly with additional depth. A number of complications can arise which can cause what seemed previously to be a successful waterproofing detail to fail and allow water to penetrate a residence. For example: An external elevated tiled concrete balcony has become flooded to a depth of fifty millimetres because of a blocked drain and water is now penetrating the ceiling of the room below. Investigations show there is a reliable waterproof membrane and adequate free-board. An opening provides runoff so the water will not rise above the the membrane upturns. We investigate and find a blockage in the pipes themselves and the head of water is causing the water to travel up under the membrane that is turned down into the storm water pipe and back down around the outside of the pipe where it passes through the concrete slab. Unfortunate, maybe, but the owners of the residence weren't impressed. They believed that the architect and the builder had let them down badly. Maybe in this situation there should have been a cast iron outlet with a membrane clamping ring. But even then it still could have failed if the seal was not intact.
Pressure or a head or depth of water will cause water beneath the water level to move upwards or in any other direction with force and velocity.
As the aforementioned illustrates containment under pressure is a detail more prone to failure than containment without pressure. If there is no head of water the water will not be driven in all directions with force and velocity.
In a simple detail in a small controlled area water can be contained under pressure very reliability. When the detail becomes complex, sprawling and messy the waterproofing can become very unreliable.
When designing or constructing a waterproofing detail it is important to appreciate that water is not an invasive force. Water is a liquid with very predictable properties. The forces we are dealing with when designing or constructing a waterproofing detail are gravity and wind. If we enlist gravity to remove the water through distribution and drainage then gravity is an ally. If on the other hand we are containing water under pressure then the forces of gravity are turned against us.
5. Containment reliant on sealant adhesion
Failed sealant adhesion is one the most common reason minor leaks occur in residences. Either in a metal to metal location or a glass to wood location. Polyurethane or Silicone sealant in their many forms are the most popular for use in building.
Even when used for their manufactured recommended applications failures occur for a number of reasons;
• The adhesive sealants has become old and the the adhesion has failed.
• The adhesive sealants is not properly adhered to the applied surfaces.
• The adhesive sealants is not compatible with the material* to which it is applied.
• Too great a movement has opened a joint and torn the adhesive sealants.
• To little adhesive sealants has been use and a joint has opened and torn the adhesive sealants.
Modern building techniques and materials has introduced a reliance on sealant adhesion which is now widely excepted as being a reliable method of accomplishing a waterproofing detail. Joints in glass or in aluminum and zincalume sheet roofing, and around penetrations for vent pipes are typical applications. When applying an adhesive sealant to metals or to any surface for that matter, it is important to thoroughly clean the surface of all water, oil or dust. Poor workmanship is often responsible for failures in the short term.
Sealants should always be restricted for use on materials that are specified suitable by the manufacturer of the sealant.
Although modern sealants are reliable and used with confidence, excessive use and dependence on sealants should be avoided as sealants may deteriorate and fail in time. The consequences of sealant failure should be considered in the designing of waterproofing details.
6. Containment under pressure reliant on sealant adhesion.
Adhesive sealants can be very reliable, even under pressure. The question is; How often and for how long ? This entirely depends on the adhesive sealant and the knowledge and care taken by the workman applying the adhesive sealant.
This is the place we end up if we haven't thought our waterproofing details through carefully.
When we have a detail that requires a waterproof membrane but because of the design we are only able to waterproof three sides and there is no way of shedding water on the fourth side we have no option but to seal the membrane to a dense material. The concern is that water can get under the membrane where it is sealed. Water proofing membrane manufactures have come to terms with this situation and provide both adhesive and mechanical fixing at these junctures. A problem can occur in the corner, in the nooks and crannies that can't be detailed like they are in the sales brochure.
Representatives form waterproofing membrane manufactures always seemed to me to be like insurance salesman selling concern and confidence. This blind faith thing is really a no go area with regards to waterproofing. If it is cheap it doesn't necessarily mean that it's a bargain. If it is expensive doesn't necessarily mean it will work. If it has a proven track record. Well you probably made a good choice. Guarantees always impress everyone but a twenty five year guarantee from a company that is only two years old means very little.
Some special adhesives for use with flexible membranes are able to contain water under pressure with adhesion. These are use specifically on concrete and are usually epoxies and are not flexible themselves. Some of the polyurethane adhesive sealants when used with adherence improving primers can also be moderately successful. If the pressure is to great all adhesives will fail.
Solvent adhesives have proven themselves exceptional in their use with PVC plumbing pipe work. Their application is however carried out with due care by apprenticeship trained tradesman.
There are also cement based materials known as water plugs or water stops which are used on concrete and or rock to seal water egress. These are special materials for specific purposes and have specific performance credentials. The manufactures of these materials will determine their applicability. As far as I am aware they are not usually used inside a domestic residence.
7. Concrete
I remember very vividly sitting in on a meeting with a client, architect and builder when I was a young foreman in my late twenties. The architect was a well known and respected professional of the time. My presence at this meeting which took place in the early evening in the architects quite massive and stately offices was purely to do with the fact that the job I was supervising for these respected professionals had a natural water course running through the site and more concerning, there was no waterproofing membrane specified for around the lift overrun pit. After waiting my turn I stated my concerns and my reason for being present. The architect turned to me. Took a moment to compose his thoughts and stated in a somewhat pompous tone that good concrete does not leak water.
Not long after the building was completed the pit began to fill with water. Numerous attempts to waterproof the pit were unsuccessful. Finally a pump was installed in the pit and I believe it is still running even to this day some thirty years on. Possibly not the same pump though.
Form defects, imperfections, honeycomb, shrinkage cracks and numerous penetrations all add up to one thing, concrete is not a reliable water container. The exception is a container specially preformed for the purpose of holding water. Concrete can be made to be waterproof, unfortunately concrete very rarely is.
For the purpose of discussing waterproofing principles I have adopted the attitude that in the absence of technical information stating otherwise concrete is simply a dense material that can be sealed to in certain locations and conditions. This depends on the impact of moisture in the concrete and the condition of the concrete.
In an elevated car park for example it may be an acceptable detail to rely on a flexible ribbon and epoxy adhesives to waterproof the control joints in the concrete floors should the concrete prove to be free of imperfections that would allow water to penetrate elsewhere. Although this detail can be regarded as being containment reliant on sealant adhesion, economics and the minor consequences of failures in the performance of the system of waterproofing make it a viable solution. Not so when applied to the roof of an apartment complex.
Because of the consequence of possible failures is much greater the concrete roof of a residential complex needs to have a membrane or waterproofed by other means. It is not uncommon to install metal deck roof over a concrete slab roof.
Concrete structures are very often affected by water penetration problems. Occasionally because concrete is sometimes waterproof and sometime not waterproof but more frequently because of poor formwork detailing.
A designer may have overlooked the action of the waters surface tension when running over the structure or workmen may have simply overlooked the detail stipulating drip groove locations. Vertical grooves and rebates are just as necessary as horizontal ones as water can track across a vertical surface in a downward diagonal direction creating the illusion of jumping a drip groove.
Suspended concrete balconies with or without a planter box, built over an habitable room are probably the most prone to failure in terms of water penetration. Beam and column ties can be a reason for water penetration. Sometimes a twenty millimetre hole can be left through a column or beam depending on the ties used. Should the hole be sloped ever so slightly downward to the inside of the structure, water will be able to enter via these holes, even when they are rendered over. Electrical conduits set in the concrete can be a potential water entry point. Either via the conduit or around the outside of it. Cold joints can also be a major water entry point even in vertical surfaces.
The obvious solution is to impose restrictions on where these joints can be so they occur under a waterproof membrane or in a controlled detailed location. Where concrete adjoins brickwork or block work there must be drip grooves, cavity bridging slip joints with water controlling folds, rebates in the concrete and expansion and contraction control joints. Reglet are a very effective way of installing flashings to distribute water to the outside of the structure as long as we have the foresight to install the reglet at the formwork stage.
Problems in detailing concrete structures that eventuate in water penetration can be a direct result of the building design and construct procedure as well as design inexperience. Should the architect not diligently review the engineers details and should the engineer disregard the architects minor detailing. The stage in most certainly set for ensuing problems. The form worker in his need to have only one master will disregard the architectural details, referring to them for dimensions only and work almost entirely from the engineers details. Should the engineers details be lacking in water control detailing the whole water penetration concern may end up at the discretion of one of the employees of the formwork subcontractor simply by reason of default. This is a very common occurrence. Some very silly mistakes are set in concrete. A good building foreman should be in control of these little indiscretions but more often then not he gets to busy to second guess everything every tradesman does. He may be of a mind set to blindly follow whatever is shown on the details. The building foreman needs to get the thing built quickly or he might not have a job next year. If they wanted a drip groove they should have drawn a drip groove. Not a good attitude, granted. But it happens more often then we would like to imagine.
I couldn't help notice over the years that the easy stuff gets detailed while the contents of the to hard basket never gets drawn at all. Where one simple traditional detail solution intersects another completely incompatible simple traditional detailing solution. This juncture which usually draws the attention of the building foreman way to late in the construction time frame for him to alter either of the two details. It may end up as variation to the building contract or it might simply get built wrong. Architects need to detail the intersections of differing sectional profiles.