Fibre Reinforced Facade Encyclopedia: Design and detailing
An open joint is a gap between adjacent rainscreen panels that is deliberately left unsealed. It carries no sealant and no gasket at the face, and it is designed on the assumption that water, air and light will pass through it. What stops the water is not the joint but what sits behind it: a baffle, a labyrinth formed in the panel edges, or the drained cavity itself. Open joints are the normal joint type for moulded panels in glass fibre reinforced concrete (GRC, GFRC), fibre reinforced polymer, terracotta and stone.
An open joint is a designed leak. The wall is kept dry by the cavity and the weather barrier behind it, so the joint is free to be an architectural line rather than a seal.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Joint width | 8 to 20 mm typical | Set by panel size, thermal and moisture movement, and tolerance |
| Sealant | None at the face | Movement is accommodated by the gap, not by a bead |
| Behind the joint | Baffle, labyrinth edge profile, or open cavity | The water control line, not the joint itself |
| Baffle material | EPDM, silicone or a UV stable polymer strip | Located in a groove or on the rail, drained at its base |
| Labyrinth | Return and rebate formed in the panel edge | Available on moulded panels because the edge is in the mould |
| Weather barrier | UV stable and dark coloured | Visible through the joint and exposed to direct light |
| Cavity | Drained and ventilated, 25 mm clear minimum | Open joints ventilate the cavity heavily |
| Fire | Joint and cavity are a combined path | Cavity barriers required; whole system assessed |
What an open joint accepts
A sealed joint tries to stop water at the outer face and fails when the sealant fails, which it eventually does. An open joint makes no such attempt. Rain driven against the facade passes through the gap under wind pressure, runs in by capillary action along the panel edge and is carried in by air movement. All of that is expected. The water lands in the cavity, where the pressure drops, runs down the back of the panel or the face of the weather barrier, and leaves at the next flashing.
The design consequence is that the joint stops being a weathering component. It becomes a movement gap and an architectural line, and it can be sized and positioned for those reasons alone. Panel edges can be returned or profiled without regard to a sealant bond face, panels can be replaced individually, and the facade carries no sealant replacement liability across every metre of joint.
The baffle and the labyrinth
A baffle is a strip of EPDM, silicone or a UV stable polymer set in a groove in the panel edge or clipped to the rail behind the joint, standing across the gap a little way back from the face. Air passes around it, water driven at it runs down its face, and it is drained at its base onto a flashing. It is not a seal; it is a splash barrier set where the wind pressure has already dropped.
A labyrinth does the same thing with geometry instead of a component. The panel edges are formed with a return and a rebate so the path from outside to cavity turns through two or more right angles. Water driven at the joint cannot follow the turns. Moulded panels have a real advantage here: the edge profile is formed in the mould at no extra cost per panel, so a labyrinth edge is a drawing decision rather than a fabrication operation.
Sightlines and what is visible behind the joint
A joint you can see through is a joint you can see into. From directly in front you see a shadow line; from an oblique angle you see the cavity, the rails, the brackets, the fixings and the weather barrier. This is the part of open joint design most often missed, and it is the difference between a facade that reads as crisp and one that looks unfinished.
- The weather barrier must be UV stable. A joint 12 mm wide admits direct sunlight to a strip of membrane behind it for the life of the building, and a standard building wrap will chalk, embrittle and tear there long before the facade design life is reached
- It must be dark, preferably black. A light coloured membrane behind an open joint glows, and the joint reads as a white line instead of a shadow
- Rails, brackets and fixings within the sightline are powder coated or anodised dark, or set back out of view, and the panel edge return is deepened where they cannot be
- Check the sightline on the mock-up, from the viewing distances and angles the facade will actually be seen from, not on a bench
Fire, and the limits of an open joint
An open joint puts the cavity in direct communication with the outside along every joint line, so the cavity behind an open jointed facade is well ventilated and an efficient flue. Nothing about the joint type changes the requirements: cavity barriers at floor lines and around openings, reaction to fire classification for the panel, insulation and weather barrier, and on Type A and Type B buildings in Australia the Deemed-to-Satisfy requirement for non-combustible external wall construction, with any alternative assessed as a full system under AS 5113 and Verification Method C1V3 (check the edition adopted in your jurisdiction).
The other limits are practical. Open joints suit thick panels with edges that can be returned, and suit them less as the panel gets thinner. They pass more air into the cavity, so they need a well drained deep cavity. They put light onto the membrane, which raises its specification. And they leave the joint visible from inside wherever the facade passes a window reveal, which needs a closer or a return.
Standards and references
- CWCT Standard for Systemised Building Envelopes and CWCT technical notes on open jointed rainscreen design
- AS 4284 Testing of building facades: static and cyclic water penetration testing of the system as designed
- AS/NZS 1170.2 wind actions, for the pressures the joint and baffle are designed against
- NZBC Clause E2 and Acceptable Solution E2/AS1 (New Zealand drained and ventilated cavity provisions)
- NCC 2022 Volume One external wall provisions, AS 1530.1 and AS 5113 (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
Does an open joint leak?
Yes, by design. Water passing the joint is caught by the drained and ventilated cavity behind it and discharged at the next flashing. The weather barrier on the wall behind is the waterproof line.
How wide should an open joint be?
Commonly 8 to 20 mm, set by panel size, thermal and moisture movement and setting out tolerance, then adjusted to suit the elevation.
What is a baffle?
A strip of EPDM, silicone or UV stable polymer set behind the joint, standing across the gap. Water driven at the joint drains down its face. It is a splash barrier, not a seal, and it must be drained at its base.
Why must the membrane behind an open joint be black and UV stable?
It is permanently exposed to sunlight through the joint and visible from oblique angles. A standard wrap degrades in that strip, and a light coloured one makes the joint read as a bright line instead of a shadow.
Designing an open jointed facade in moulded panels? ShapeShell-RC and RT panels are formed with returned and labyrinth edges so the joint reads as a shadow line, not a gap. See ShapeShell panel details
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.