Water inside the channels of a multiwall polycarbonate sheet needs a different investigation from droplets on the greenhouse-facing surface. First establish where the water is. Then trace how it entered and whether the installed sheet and edge profiles allow it to leave. Running silicone around every visible gap can conceal the source or obstruct the drainage route.
This guide concerns hollow, multiwall sheets. Solid polycarbonate, corrugated single-skin roofing and sealed glass units have different details. Obtain the instructions for the actual sheet and glazing system before removing a panel or changing an edge treatment.
Locate the water before buying tape
View the affected section from both sides. Surface droplets can usually be wiped from an accessible face; moisture within the cells remains behind it. Photograph the channel ends, glazing bars and lower profile as well as the wet patch. Record whether the problem follows rainfall, irrigation or an overnight temperature change.
A single wet strip below a joint suggests a different investigation from a light haze across many panels. These patterns are clues, not proof. A roof leak can travel along a member before appearing somewhere else. Compare one affected panel with a nearby dry panel of the same construction, keeping orientation and exposure in mind.
Make a small inspection sketch with four labels: upper channel end, lower channel end, sheet face and frame joint. This prevents a conversation about “sealing the panel” from combining several separate jobs.
The upper and lower edges have different jobs
For its SUNLITE multiwall product, Palram's US guidance specifies solid aluminium tape at the top and ventilating breather tape at the bottom. This is a product instruction, not a reason to substitute household tape or copy the detail onto every glazing product. Follow the complete regional instructions, including the matching end profiles.
The practical distinction is between excluding unwanted entry and preserving the intended outlet. Examine the lower profile for debris, an incorrectly fitted cap or a bead of sealant across its drainage openings. Check the upper edge for damaged tape or a joint that allows rain to enter. Do not drill new drain holes until the supplier confirms where they belong.
Channel orientation also matters. The SUNLITE Plus greenhouse installation guide provides product-specific sheet and fixing details. Compare the installed arrangement with its drawings rather than assuming that a rectangular sheet can be rotated without changing drainage or support requirements.
A perimeter seal is not a replacement for an edge detail
Three functions need to remain separate: sealing a weather joint, treating an open channel end and retaining the sheet against its supports. A bead of sealant at the frame may address a joint while leaving the channel ends untreated. It may also glue together parts designed to accommodate movement.
Before selecting a repair, ask the supplier to identify the compatible sealant, tape and profile by product name. Include photographs and the sheet identification. “Suitable for outdoor use” is not enough information to establish compatibility with polycarbonate or its coatings.
Do not copy screw spacing, hole size or expansion clearance from a different panel thickness. The same greenhouse guide explains that fixings must accommodate thermal movement. Those details belong to a complete glazing system; an improvised extra screw can introduce a new problem while appearing to secure the old one.
Dry and clean only by an approved method
If access requires removing a sheet, plan the work for suitable weather and use the support and access method required by the structure. Never stand on the glazing. Mark the panel orientation before removal so the designated exterior face and channel direction are preserved during reinstallation.
Ask how the manufacturer permits trapped moisture and debris to be removed. Do not direct a heat gun, solvent or aggressive pressure jet into the cells on the assumption that faster drying is better. Cleaning a visible patch will not prevent recurrence if the entry path remains open or the lower edge stays blocked.
Separate cosmetic staining from physical deterioration. A supplier may need to assess persistent internal contamination, cracks, damaged coatings or a sheet that no longer fits its support system. Replacement decisions should use that assessment, not a universal age limit.
Confirm that the repair addressed the right route
After the approved repair, repeat the photographs from the same positions. Recheck after the next relevant event: rain for a suspected weather joint, normal irrigation for splash entry, or an overnight temperature cycle for condensation. Record what happened rather than expecting every trace of haze to disappear immediately.
If water returns, revisit the entry-and-exit sketch before adding another layer of sealant. Include the adjacent roof joint and gutter in the investigation. For the separate task of choosing a covering system, see CFGET's polycarbonate greenhouse overview.