When shrink-wrapping sites makes sense

Open up a roof or strip a cladding and you have made a deliberate decision to expose the structure and everything inside it to the weather. The question is what you put back over it while the work runs. Temporary encapsulation — heat-shrunk reinforced polyethylene tensioned over a scaffold — has moved from a specialist option to a routine line on reroof, recladding and remediation jobs, and it is worth knowing when it earns its place and what it asks of the scaffold underneath.

The case for wrapping is strongest wherever the building’s envelope is open for more than a day or two. Reroofs and recladding are the obvious ones: with the existing skin off, the framing and interior are exposed, and a wrap keeps rain off so interior trades can keep working rather than waiting on a dry window. Weathertightness remediation is the same logic at higher stakes — the whole point is to dry the building out and rebuild it dry, and that is undone if it takes on water mid-repair. The other strong cases are hazardous-material containment, heritage and leaky-building repair where the structure can’t be left open, and emergency cover after storm or fire damage.

Why wrap, not tarp

The driver is moisture. BRANZ has been clear for years (in Dealing with construction moisture in new buildings (Build 587, July 2015) that water trapped in framing during the build leads to decay, mould and finish failures, and that keeping the structure dry is cheaper than drying it out later. A tarp slows that; a wrap stops it. What shrink-wrap does that a tarp doesn’t is stay put; heat-tensioned film doesn’t rip, flap or pull loose, so the building under it stays dry and the crew keeps working through weather that would otherwise stop them.

A sealed envelope also buys a controlled work environment. It contains dust and debris, holds dropped material inside the scaffold line, and shields the public. For asbestos and lead-paint work that containment isn’t a bonus, it’s the job. WorkSafe’s asbestos guidance treats enclosures, sealed polyethylene sheeting and negative-pressure control as core controls, and removal of any friable asbestos requires a Class A licence. Encapsulating the work zone is how a removalist keeps fibres in.

The scaffold becomes a sail

The trade-off is structural, and it is the part most often underestimated. A bare scaffold lets wind pass through it. Once you sheet a scaffold you have built a sail, and the structure underneath has to carry every newton of it. WorkSafe’s Scaffolding in New Zealand Good Practice Guidelines (November 2016) put wind loads on screens and sheeting squarely in the designer’s live-load calculation, note that environmental loadings are complex, and direct scaffolders to seek advice from a chartered professional engineer where those loads haven’t been verified. The relevant standards are the AS/NZS 1576 scaffolding series and AS/NZS 1170 for structural design actions. SARNZ’s good practice guidance, issued under its delegated authority from WorkSafe, carries the same message: a wrapped scaffold needs additional ties, anchors and bracing, and high wind on a sealed face can both lift the structure and create a vacuum behind it. Where wind or accumulated rain on the film exceeds what the build was designed for, the film comes off.

That makes the wrap a design decision, not a finishing touch. Plan the ties and the engineering with the scaffold, not after it. Build in access and ventilation so the sealed space stays workable. Keep ignition sources away from the film, and check with the provider on recycling, much of the polyethylene used here is recoverable rather than landfill.

For any job that opens the envelope for more than a few days, the calculation is straightforward: weigh the wrap and the engineering it demands against the cost of a wet structure and lost weather days. On most reroofs and remediations, the wrap is the smaller number, provided the scaffold under it has been built to carry the load.