A pool on a terrace: structural load and waterproofing

Putting a pool on a terrace is two separate problems, and the order matters more than it seems: first you have to know whether the slab will hold it, and only then does it make sense to talk about how it gets waterproofed. Reversing the order is the expensive mistake here, because it produces detailed quotations for work that may turn out to be impossible.

And there is a third thing, which is the one almost nobody says: the load of a pool is not live load. It is dead load, permanent, and that changes who has to check what.

The arithmetic that ends the conversation

Let us start with the number, because in half the cases the number ends the discussion with nothing else needed.

NSR-10 gives the density of water in its table of material masses: fresh water, 1,000 kg/m³ (and sea water, 1,030). And its dead load chapter requires densities to be multiplied by 9.8 m/s² to obtain weight. One metre of water therefore weighs 9.8 kN/m². A depth of 1.20 m weighs 11.76 kN/m², which is about 1,200 kilogrammes per square metre.

Now, what to compare it with. The minimum design live load NSR-10 requires for the private rooms of a dwelling and their corridors is 1.8 kN/m² (180 kgf/m²).

The water alone in a pool 1.20 m deep weighs six and a half times the design live load of the bedroom next door. Add the shell and the finishes and it passes nine times. And it is not a load that is sometimes there and sometimes not: it is always there.

The terrace table in circulation has been superseded

This point deserves a section of its own because it affects anybody who consults the code themselves, and because the error propagates from files that look official.

Live loads for roofs, flat roofs and terraces are in table B.4.2.1-2 of NSR-10. The Title B PDFs circulating online carry the original 2010 version, which has four rows and settles terraces with a cross-reference. That table was replaced by Decreto 092 de 2011, which says literally "Table B.4.2.1-2 shall read as follows" and substitutes one of six cases.

The difference is not cosmetic: in the table now in force the load on a terrace depends on what it gives access to and on what the planning permission and the propiedad horizontal by-laws say.

Why it matters that this is dead load

A live load comes and goes: people walk into a room and out again. A pool does not. Its forty tonnes of water are there on Monday and on Sunday, in summer and in winter, and that has two consequences that often get overlooked.

The first is long-term deflection: concrete under permanent load goes on deforming for years. A slab that "holds" can hold and still end up with a sag that breaks the finishes and throws precisely the edge of the pool out of level.

The second is more serious and is the one hardly ever mentioned in quotations: seismic mass. The force an earthquake imposes on a building is calculated from its mass. Forty new tonnes on a high slab do not load that slab alone: they load the columns below, the shear walls and the foundations, and they do it amplified by height. The problem with a pool on a terrace is not only on the terrace.

There is also a clause that always applies and is worth knowing because it kills the desk survey: in an existing building, dead loads must be assessed on the basis of field observation and measurement, and may never be taken as less than those of Title B. Assuming the weight of what is already built will not do: somebody has to go and measure it.

The chapter almost nobody cites: C.23

When Title B deals with uplift on buried slabs, it closes the clause with a sentence pointing somewhere else: "The same consideration must be given to the design of tanks and pools. See chapter C.23".

Chapter C.23 is entitled "Tanks and environmental engineering concrete structures" and its scope says, in these words, that it covers "tanks and watertight compartments such as pools and basins forming part of the equipment of buildings". It is not a chapter for treatment works: it is the chapter for your pool.

What it requires, and what rarely appears in a pool contract:

• —Minimum concrete strength. The chapter sets a minimum f′c of 28 MPa for the structures it covers. That is a good deal more than the default on small works.
• —Low permeability. Title C classifies concrete exposure into categories, and the one for a structure in contact with water where low permeability is required is class P1. C.23 itself adds a category for exposure to corrosive chemicals, which is exactly what is inside a chlorinated pool.
• —The watertightness test goes on the drawings. The chapter requires drawings and specifications to include the requirements for testing impermeability and watertightness before the surrounding backfill is placed. It is not optional good practice: it is compulsory drawing content.
• —An empty shell also loads. It requires uplift on empty tanks to be considered and ballast provided against flotation failure. On a terrace this rarely applies, but it does apply if part of the shell is buried, or with a pool in a basement or semi-basement.

The chapter is based on an American Concrete Institute document, but this has to be said carefully because it too gets cited wrongly: NSR-10 does not refer you to the American standard, it incorporates it translated and adapted. What is compulsory in Colombia is Chapter C.23, not the original document.

Who signs what

This admits of no nuance and should be clear before engaging anyone. The statute governing earthquake-resistant construction allocates the signatures:

What is needed before anything is drawn is a structural assessment of the existing slab, signed by a civil engineer, answering one of three things: it holds, it holds if strengthened, or it does not hold. An architect cannot sign that, and neither can a pool contractor. If you are offered a start on site without that document, what you are being offered is to carry the whole risk yourself.

If the terrace is in propiedad horizontal

There is a confusion here that costs litigation. Even where the terrace is for the exclusive use of your flat, the slab holding it up is essential common property. The terrace is yours to use; the structure carrying it is not.

That means two things are needed at once, and they are cumulative: authorisation from the general meeting, because common property is being altered, and planning permission in the appropriate category, which here is structural alteration. Neither replaces the other, and the sensible order is to ask the general meeting first: paying curaduría fees for something you are then not allowed to do is money lost. The detail is in altering a flat in propiedad horizontal and in do I need permission to build a pool?.

The second problem: it is a roof and a container at once

Once the structure is settled, the problem that produces the lawsuits arrives. A pool on a terrace is two incompatible things at the same point: a roof, whose job is to keep water out, and a container, whose job is to keep water in.

And it has a cruel feature: a leak does not show up where it is. Water escaping the shell does not appear on the terrace; it appears on the ceiling of the flat below, and by then it has travelled through the slab and wherever else it found a way. Diagnosing the origin is a problem in itself, and it is the same problem described in roof leaks.

From which follow three design rules:

1. 1Two independent systems, not one. The waterproofing of the shell is one thing and that of the roof is another, with its own drainage and its own falls. The shell being watertight does not excuse the roof from being watertight too: the day the shell fails, the second barrier is what decides whether the damage is a repair or a lawsuit.
2. 2Every penetration is a future leak. The drain, the overflow, the supply, the return and the light all pass through the waterproofing. Each penetration is a point of failure, and the number of them is a design decision: they can be grouped, they can come out through the side rather than the base, and the underwater light can be done without.
3. 3The shell kept separate from the slab. Wherever possible the shell should bear on the structure without being the structure, with its own waterproof membrane beneath it and a void or drainage layer that collects whatever gets through and takes it to a visible point. A leak that drips where you can see it is maintenance; a leak you cannot see is a demolition.

Two details that get forgotten and cost money

Emptying. A terrace pool has to be emptied at some point, and thirty or forty cubic metres do not fit down the rainwater pipe of a terrace, which was sized for the rain falling on those square metres. A dedicated drainage point is needed, with enough capacity and with a lawful destination: Resolución 929 classes pool water as a non-domestic discharge, because it carries chlorine, algaecides and pH correctors, and refers its management to environmental legislation.

Balustrades and glass. The design load for a balustrade is not where almost everyone looks for it: it is not in Title K but in Title B, which sets a horizontal thrust of 1.00 kN/m in general and 0.40 kN/m in single-family housing, applied at the top. And Title K, which does govern the material, requires safety glass in spaces where the activity creates high risk, expressly naming pools, wet areas and spas.

What I did not find, and I say so

Three declarations, because an article that says what it could not confirm is worth more than one that fills the gap:

• —Title J of NSR-10, on fire protection, does not mention pools once. Nor does Title H, on geotechnical studies. The pool’s connection to Title H arrives through Title B, not through H itself.
• —No Colombian rule sets a specific "pool load". What exists is the density of water, the obligation to compute it as dead load, and Chapter C.23. The load comes from the arithmetic, not from a table.
• —Minvivienda does not currently publish the Titles of NSR-10 as downloadable PDFs on its own site. The official copies I consulted — bearing the Comisión Asesora Permanente cover — are hosted on another government domain. The amending decrees do come from official sources. I note it because anyone wanting to verify this for themselves will run into the same thing.

When the slab will not take it

Strengthening is not always the answer. There are four ways out and they are worth looking at in this order, because they run from cheapest to dearest:

1. 1Reduce the depth of water. The load is proportional to depth. A pool 0.60 m deep to cool off in weighs half what one of 1.20 m weighs and is still a pool.
2. 2Swap the pool for a spa, which concentrates less water in total even though it concentrates more per square metre. What that involves is in a built-in or a portable spa.
3. 3Its own structure. Take the weight down to new columns reaching the foundations instead of resting it on the slab. It is expensive and usually invasive for the floors below, but it solves the problem at root.
4. 4Strengthen the existing structure, which is the option most often proposed and hardly ever the cheapest, because strengthening rarely stops at the slab: it climbs the columns and goes down to the foundations. The general approach is in structural strengthening and NSR-10.

Where this comes from

Whoever leads this line of work in the studio brings fifteen years of designing hydraulic systems and bodies of water — pools, spas and waterfalls with their networks, filtration plants and plant rooms — and the studio does building diagnostics, which is the other half of this article.

The two halves always meet in the same place. The terrace pool that gives trouble is not usually the one that fell down: it is the one dripping onto the flat below, five years later, when nobody has the drawings any more and the contractor no longer exists. Everything that prevents that is decided before the first concrete pour.