Waterproofing Around Skylights and Penetrations: What Auckland Builders Get Wrong

Every hole cut into a waterproofing membrane is a potential leak waiting to happen. Skylights, vent pipes, drainage outlets, post fixings, electrical conduits, and any other penetration through a flat roof or waterproofed deck represent the weakest points in the system. When these details are done poorly, the consequence is water entering the building.

Why Penetrations Are the Biggest Problem in Waterproofing

A flat membrane surface, properly installed with correct falls, is inherently reliable. The membrane is continuous, factory-manufactured, and designed to be watertight. Problems arise when that continuous surface is interrupted — by a skylight, a pipe, a post, or any other object that passes through the membrane.

Each penetration requires the membrane to be cut, shaped, sealed, and terminated around a different material (metal, plastic, timber). These junctions are subject to thermal movement, UV degradation of sealants, physical stress, and exposure to Auckland's intense rainfall. It is not surprising that the vast majority of waterproofing failures we see across Auckland originate at penetrations, not in the field of the membrane.

Common Mistakes We See in Auckland

After decades of waterproofing work in Auckland, here are the most common penetration detailing mistakes we encounter:

The Correct Approach to Penetration Detailing

Proper penetration waterproofing follows these principles:

• Build a raised curb or upstand around the penetration (minimum 75mm, ideally 100mm+)
• Lap the membrane continuously up and over the curb with no gaps
• Use manufacturer-approved accessories (collars, boots, flashings) where available
• Allow for thermal movement with flexible details, not rigid sealant
• Apply sealant as a secondary measure, not the primary waterproofing
• Ensure drainage falls away from all penetrations so water does not pool against details
• Surface-mount fixings wherever possible rather than penetrating the membrane

Skylight Waterproofing: Getting It Right

Skylights are the largest and most complex penetration on most flat roofs. A correctly waterproofed skylight in Auckland requires: a raised timber or metal curb (100-150mm minimum height), membrane lapped continuously up all four sides of the curb, counter-flashing or capping over the membrane termination, a drip edge or gutter on the upslope side to divert water around the curb, and flexible sealant at the frame-to-curb junction as a secondary seal.

If your skylight is leaking, a re-seal with silicone is a temporary fix at best. The proper solution is to strip the membrane around the skylight, rebuild the curb if necessary, and re-detail with new membrane. This is specialist work that should be done by an LBP-licensed waterproofing specialist, not a general builder.

Pipe and Post Penetrations

Pipes (plumbing vents, gas lines, electrical conduits) and structural posts (balustrades, shade sails, pergolas) each require their own waterproof detail. For torch-on membrane systems, this typically involves a pre-formed collar heat-welded to the field membrane. For TPO systems, prefabricated pipe boots are hot-air welded to the membrane sheet.

Structural posts are best surface-mounted on plates that sit on top of the finished membrane, rather than penetrating through it. Where through-membrane posts are unavoidable, the detail must allow for post movement without breaking the waterproof seal.