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Building components · Layers & Barriers

Waterproofing Layer

Explains waterproofing as a single continuous position whose behaviour is set at its junctions and terminations rather than in its field area.

Form:Continuous layerRoles:Moisture controlWeather control

Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.

Overview

What a waterproofing layer is

The waterproofing layer is the position in a build-up at which water is excluded outright — on a flat roof, beneath a terrace or balcony, around a basement, under a wet-room floor. Unlike a layer that sheds or drains, it is relied on where water is expected to sit on it, run across it or push against it.

It is a single plane, and a single plane is only as continuous as its weakest point. Almost everything that determines how it behaves is at its edges: where it turns up, where it is terminated, where a pipe or a fixing passes through it, where one sheet laps the next. The field area is rarely the part that decides the outcome.

Boundaries

How it differs from nearby elements

The distinctions that keep this entry from being a restatement of its neighbours.

  • It is not a waterproofing material. Bituminous sheet, single-ply membranes, liquid coatings, tanking slurries and bonded sheets are published material entries; the layer is the continuous plane they are required to form, and a single plane is commonly formed from more than one of them.
  • It is not a protection layer. The published protection layer entry states that it protects waterproofing and is not itself a barrier: it takes the backfill, ballast, traffic and subsequent work so that the barrier beneath is not damaged. The waterproofing layer is the barrier being protected, and the two are sequential rather than alternative.
  • It is not a damp-proof course or damp-proof membrane. Those are published materials that resist capillary and ground moisture at defined positions in a wall or floor; the waterproofing layer is the plane relied on where liquid water can stand on it or act against it, which is a different demand answered in a different way.

Terminology

Common names and aliases

These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.

  • tanking line
  • waterproofing plane
  • waterproofing line

Dependencies

What it depends on

What has to be right elsewhere for this element to do its job.

  • Continuity at every penetration, upstand, lap and termination, since these are the positions where the plane stops being uninterrupted.
  • What it is laid on and whether it is bonded to it, because a substrate that moves, remains wet or is poorly prepared acts on the layer for the whole of its life.
  • Whether water is intended to leave the surface or to stand on it, which is a drainage decision made elsewhere but borne here.
  • Being protected and covered in the right order, since the period between installing the layer and protecting it is when most damage occurs.

Considerations

Topics that genuinely apply here

Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.

Moisture

The layer is relied on absolutely rather than partially, which means a defect anywhere in it is a defect in the whole, regardless of how small the area affected appears.

Buildability

Penetrations made after the layer is complete — for fixings, plant, handrails, services — are the recurring way a finished plane is interrupted, and they are made by trades who were not present when it was laid.

Durability

Once covered by insulation, ballast, screed, paving or backfill, the layer cannot be seen, which is why its condition at the moment of covering is worth far more attention than its condition afterwards.

Maintenance & access

Finding a defect later means removing everything above it, and the point where water appears inside is commonly nowhere near the point where it entered.

Materials

Commonly formed from

Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.

Materials LibraryEPDM roofing membrane →Single-ply sheet forms the plane in large areas with relatively few joints.Materials LibraryTPO roofing membrane →Another widely encountered single-ply form of the same plane, with its own jointing method.Materials LibraryBuilt-up felt roofing →Multiple bonded layers form the plane, so continuity is built up rather than laid in one piece.Materials LibraryLiquid applied roof coating →Forms the plane without joints, which is why it is often met at complex geometry and around penetrations.Materials LibraryCementitious tanking slurry →Forms the plane against masonry and concrete below ground, bonded to the substrate rather than laid over it.

Assemblies

Building systems this element is met in

The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.

Control layerWarm Flat Roof →Sits above the insulation, which places the layer at the warm, dry side of the build-up but also at the exposed surface.Control layerInverted Roof →Sits beneath the insulation, which protects it thermally and physically but puts it below everything that would have to be lifted to reach it.Control layerSingle Ply Roof →The plane the assembly is named after, with its behaviour governed by seams, terminations and fixings.Control layerLiquid-Applied Roof Build-Up →Formed in place and continuous with its own upstands, which removes joints at the positions where joints are least wanted.Control layerBarrier Waterproofing →The barrier itself, as opposed to the drained or cavity approaches that manage water rather than exclude it.Control layerTrafficked Roof Deck →Lies beneath a surface that will be used, which makes protection and the route of later fixings the governing concerns.

Clarifications

Commonly misunderstood

  • That the layer is judged by which product forms it. The great majority of problems occur at junctions, terminations and penetrations, which are decided by detailing and workmanship rather than by the sheet.
  • That it can be inspected later if there is a problem. For most of its positions the layer is buried immediately after installation, and the only realistic inspection is the one carried out before it is covered.

Conversations

Questions for qualified professionals

Questions worth asking someone with the project in front of them. This page cannot answer them.

  • Where does the waterproofing plane run, and where does it terminate at every edge?
  • How is each penetration through the layer formed and sealed, and who is permitted to make new ones later?
  • What inspection takes place before the layer is covered, and how is it recorded?
  • What protection is provided over the layer, and how soon after installation is it placed?

Go deeper

Related Build Design Hub guides

Planning guidance behind the decisions this element involves.

Preparation

Related planning checklists

Owner-side preparation before the conversation where this element comes up.

What this entry does not do

  • This entry describes a position in a build-up. It states no product, no thickness, no upstand height, no lap dimension and no performance rating, and none can be inferred from it.
  • Waterproofing manufacturers set their own requirements for substrates, laps, terminations and protection, and those govern for a specific installation rather than any general description.

Envelope Layers & Barriers

Continuous positioned layers within a build-up — insulation, separation, protection, drainage and root barriers — named by the job they do rather than the product used.

Browse all layers & barriers entries →