Why Podium Deck / Buried Roof Waterproofing Systems Leak

We inspect leaking podium deck & buried roof waterproofing systems

I thought that this topic would make for an interesting post.

The examples all involve apartment buildings with basement car parks beneath, where that basement extends beyond the line of the structures over, with this creating external decks subject to rainfall.

Even in basement car parks where a degree of seepage may be considered to be acceptable (BS8102 grade 1a environment), penetration through a soffit can be an issue since water moving over or through concrete (cracks etc.) leaches free lime, which is damaging to paintwork and so the space may not be fit for purpose as a result.

Obviously in a higher grade environment where no penetration is acceptable, as expert waterproofing specialists we know this is definitely an issue.  

So what causes deck waterproofing systems to leak

In a deck scenario, the objective should be to exclude water or in other words a physical barrier should be provided to block  water out, so that this does not penetrate into the space beneath.

As with any barrier system, this must be perfect (100% free of defects) to exclude water in the event of hydrostatic pressure coming to bear.

Hydrostatic pressure results where water stands in a vertical column and the weight of that water exerts said pressure.  Even a small head (vertical depth) of water can be enough to force water through a minuscule defect.

Because it is not necessarily reasonable to assume that any structural waterproofing will be installed perfectly / free of defects, the lowest risk approach is to seek to avoid any hydrostatic pressure coming to bear upon the installed waterproofing / tanking system.

This is where designers tend to fall down when considering decks

Unlike a flat roof, decks tend to be accessible/traffic-able and therefore typically include a build-up over which can vary from green roof/vegetation to macadam or paving, with the latter including everything from block, stone or concrete paving, laid over grit-sand, lean/dry mix mortar/concrete, or podium/pedestal systems.

It is this build-up which tends to collect water, causing this to pressure on and penetrate past the installed waterproofing measures.

So what causes this?

If we take the scenario described where we have an apartment block with basement car park beneath, where the car park extends beyond the line of the building over therefore creating the exposed deck, if we think of that deck as a rainfall catchment area with an ‘impermeable’ base, what will happen to rain falling upon it, in the event that say, the finish is block paving over grit-sand?

That’s right, while some of the water might flow at surface/finishes level, some water will always drain down through the joints in the paving to access the bedding material beneath.  No form of paving/stone/tile etc. is considered a waterproofing system or barrier in its own right and in time water will always access the bedding layer.

Once water is at this level, where does it go?  In many cases decks are essentially ‘hemmed in’ on all sides, in that you have the external wall of the building, then commonly a parapet/wall detail running around the rest of the deck perimeter.

This can act to create a sort of ‘bowl’ for lack of a better description, which when filled with water, forms a reservoir of standing water, with this applying hydrostatic pressure to the waterproofing system, exactly what we don’t want.

Commonly when looking at such issues, you can form trial holes in the build-up to expose the structural deck, and when you open it up, it’s wet but not necessarily a hole full of water.  Wait twenty minutes – more or less with this being dependent on the permeability of the bedding – and often this hole will subsequently start to fill with water.

Leaking buried roof waterproofing systems and surface water drainage

But what about the surface water drainage system?  Well this is often the problem, in that surface water drains are designed/installed at top of finishes level, rather than at deck level.  The assumption is obviously that water will flow along at finishes level and then into these drains, and most of it will, but again, some water will always drain down through the finishes to a lower level.

If there is no drainage at deck level (finished surface level drainage only), then once water is below top of finishes level, it can no longer access those drains, and can only stand and pressure.  Water does not flow up-hill!

Even in some cases where I’ve seen inlet gullies which accept water from deck and finishes level, bedding material laid directly over the deck structure is impermeable to the degree that water takes time to move through it, and substantial rainfall causes pressure and penetration anyway.

Water leaking into buildings adjoining decks & buried roofs

Penetration can occur into the basement space beneath, or equally there may be issues whereby water standing upon the deck pressures laterally on the external walls of the structures built over the deck.  This can result in penetration past upstand details, with water then accessing the internal floor construction within the apartments.

In some cases it is more likely that penetration will occur through the external walls (despite tanking upstands) because these are typically masonry – although not always – which is permeable, whereas a substantial reinforced concrete deck might be relatively impermeable in its own right.

How to prevent water leaking into buildings adjoining decks & buried roofs

The key is to seek to form a perfect barrier to water as far as possible, then integrate effective drainage within your design so that water is continuously removed and is therefore not allowed to stand and pressure on the barrier / tanking, or the external wall of any structure formed over the deck, to any great degree.

If there is no or limited pressure, then risk is substantially reduced because the installed measures essentially do not have to fight against this.

We typically achieve this by forming substrates to suitable falls, over which an applied waterproofing barrier is installed. To provide the drainage we then lay a double cuspated geodrainage membrane. This is a double dimpled membrane which creates a drainage space both above and below the membrane, with a geotextile filter fabric welded to the face of the membrane (upper face), so that bedding material laid over does not access and block the drainage cavity provided by the membrane.

The double cuspated aspect of the geodrainage membrane is important. If you just use a standard geodrainage membrane, the sort of product employed externally on retaining walls, then these typically include a dimple to one face but may be structured so that the rear face is flat.

The implication of this is that when laid over a substrate to falls and weighted by bedding material laid over, the flat underside of the membrane can act as a bund and trap water over the barrier, which again is what we are seeking to prevent, and I know that this detail has caused issues. I do not think that all manufacturers recognise this risk..

In any case, get the drainage right and you have a much greater chance of avoiding issue!  How to get the deck drainage right?  Use an experienced deck waterproofing designer that understands the issues.

‍