Repair Systems — 03 — Facade & External Envelope
Subsill drainage systems
Technical product reference for subsill drainage systems used in window sill remediation on Australian Class 2 strata buildings. Covers aluminium subsill drainage channels, weep hole inserts and drainage tube systems — used to capture and discharge water that penetrates the window frame system before it enters the building structure.
What are subsill drainage systems for windows?
Subsill drainage systems collect and redirect water that has penetrated through or around the window frame sill joint, preventing it from tracking back into the wall cavity or into the building interior. They are a secondary drainage measure — not a substitute for a correctly installed primary perimeter sealant — and are used where the volume or persistence of sill-level water ingress requires a managed drainage path.
Product Reference
3 products — subsill drainage systems — scroll to view all
Extruded aluminium subsill drainage channel
Extruded aluminium subsill drainage channel — window sill
System Description
Extruded aluminium subsill drainage channels are installed at the underside of the window frame sill, between the sill and the wall opening, to capture water that penetrates through or around the frame sill and redirect it to the exterior through weep outlets at the base of the channel. The channel prevents water that has passed through the first line of defence from tracking back into the wall cavity or into the building interior. Channel profiles vary by manufacturer and supplier — some are purpose-designed with integral sill dams, weep outlet positions and sealant recesses; others are simple U-channel sections. Weep outlets must be drilled or punched at regular centres (typically 150–200mm) and must remain clear of blockage. The channel is set in a bed of compatible sealant and fixed with 316 stainless fasteners. All joints at ends, corners and at the frame seat must be sealed to prevent bypass. Anodised or powder-coated finish. Confirm channel dimensions suit the specific window sill width and frame geometry. TODO: owner confirm — confirm proprietary channel availability and preferred supplier before specifying.
Technical Properties
- Purpose-designed extruded aluminium channel — installed under window frame sill
- Incorporates weep outlets at channel base — confirm outlet centres and clear from blockage
- Anodised or powder-coated finish — confirm colour match with frame or facade
Limitations
- Weep outlets must remain clear — debris blockage will cause water ponding in the channel and bypass to the interior
- Not suitable for coastal locations without stainless 316 fixings and anodised (not powder-coat) finish
- Channel must be correctly sized for the sill width — undersized channel will not capture all water from the frame
PROCUREMENT SOURCES
Confirm channel dimensions, coastal suitability and weep outlet spacing before ordering.
Plastic / stainless weep hole inserts and drainage inserts
Weep hole inserts — plastic or stainless — window sill drainage
System Description
Weep hole inserts are small purpose-designed drainage devices installed in the mortar bed joint at the base of the window opening, in the sill course brickwork, or in the subsill zone to allow water that collects at the sill or in the cavity to escape to the exterior. Without weep hole inserts, mortar joints are often blocked by mortar squeeze-out during construction, preventing water from draining. Plastic weep hole inserts (typically HDPE or PVC) are suitable for inland and protected locations. Stainless 316 mesh or tube weep inserts should be specified for coastal locations. Weep hole inserts are commonly specified at 1 per half-brick course spacing (approximately 450mm centres) in the bed joint at the base of the window opening. They must be kept clear of mortar and debris — check and clean during maintenance inspections. Weep hole inserts are a simple and cost-effective solution for improving drainage at the window sill zone and are often used alongside a subsill drainage channel in windows with persistent water ingress problems.
Technical Properties
- Purpose-designed weep inserts installed in mortar bed joint at sill course
- Plastic (HDPE/PVC) for inland locations — 316 stainless mesh or tube for coastal
- Spacing typically 450mm centres or 1 per half-brick module — confirm with project requirements
Limitations
- Must be kept clear of mortar and debris — block weeps are a common maintenance failure
- Plastic inserts not suitable for coastal locations — specify 316 stainless mesh type for coastal
- Weep inserts drain water at the sill course — they do not prevent water penetrating through the perimeter joint
PROCUREMENT SOURCES
Confirm channel dimensions, coastal suitability and weep outlet spacing before ordering.
Drainage tube / sock drainage system at subsill
Drainage tube / sock system — subsill window drainage
System Description
Slotted drainage tubes (perforated flexible tube wrapped in geotextile sock) are used in subsill drainage applications where a conventional aluminium channel cannot be installed due to access constraints or where the drainage needs to collect water from a wider area than a simple channel allows. The tube is installed at the base of the window reveal or at the sill zone and connected to a discharge point that exits to the exterior through a weep hole or drainage outlet. The geotextile sock prevents fine debris from entering the tube and blocking the drainage path. This type of drainage system is more commonly used in new-build cavity wall construction but can be adapted for remedial applications where access exists to the sill zone during window replacement or reveal opening. TODO: owner confirm — confirm applicability and preferred product for this application in Australian remedial practice before specifying.
Technical Properties
- Slotted flexible drainage tube with geotextile sock — collects and redirects water from subsill zone
- Connected to exterior weep hole or drainage outlet — confirms clear discharge path
- Geotextile sock prevents fine debris from blocking the tube
Limitations
- Requires access to the subsill zone for installation — typically during window replacement or reveal opening
- Discharge point must be clear and accessible — blocked discharge will cause water backup into the wall
- Less common in Australian remedial practice for windows — confirm applicability with design team
PROCUREMENT SOURCES
Confirm channel dimensions, coastal suitability and weep outlet spacing before ordering.
System Comparison
Side-by-side comparison of subsill drainage systems. Confirm channel dimensions and weep outlet spacing before ordering.
| Product system | Type | Material | Coastal | Weep | Install | Primary use |
|---|---|---|---|---|---|---|
| Extruded aluminium subsill drainage channel | Channel | Aluminium extrusion | Inland (anodised for coastal) | Drilled outlets | At frame base | Primary subsill drainage collection — most common remedial specification for window sill water ingress |
| Weep hole inserts — plastic or stainless | Weep inserts | Plastic / SS 316 | SS type for coastal | Integral insert | Mortar bed joint | Drain collected water at sill course brickwork — simple and cost-effective retrofit for blocked weep holes |
| Drainage tube / sock system | Drainage tube | PVC / geotextile | Confirm | To discharge point | Concealed at sill | Cavity or concealed drainage where conventional channel cannot be installed — confirm AU applicability |
Critical notes for subsill drainage specification and installation:
- Subsill drainage is a managed water ingress strategy — it does not eliminate the need to repair the primary perimeter sealant or head flashing; treat the source defect first
- Drainage channels and weep holes must be installed with a positive fall to the outlet — blocked or flat drainage channels trap water and accelerate substrate deterioration
- Weep holes must be kept clear of sealant, render and paint — a common installation error is sealing over weep holes during render or coating application
- Aluminium subsill channels in coastal environments must be anodised or powder-coated — bare aluminium corrodes in salt-laden air and will stain the facade
- All drainage channel fixings must be stainless 316 — galvanised or carbon steel fasteners corrode and stain the facade below the window sill
- Install drainage channels before perimeter sealant application — the drainage channel typically forms part of the sill profile that the sealant seals against
Disclaimer
This page provides general technical information only. Subsill drainage system selection, sizing and installation details must be confirmed by a qualified remedial practitioner against existing window frame construction and site conditions. Drainage systems manage water ingress — they do not substitute for correct primary waterproofing.