Fibre Reinforced Facade Encyclopedia: Design and detailing
Joint design is the detailing of the gap between two cladding panels, and of the gap where cladding meets a different element. The joint has to do four things simultaneously: allow the panels to move, control water, look deliberate, and remain buildable within the deviations actually achieved on site. Why a facade must move is covered under movement joints, and the deviations the joint has to survive under tolerances. This page is about the joint itself.
A 10 mm sealed joint filled with a sealant rated at plus or minus 25 per cent movement accommodates 2.5 mm. If the panel either side moves 4 mm, the joint is not marginal; it is wrong.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Nominal panel to panel joint | 10 to 20 mm | Below 10 mm both sealant and tolerance become difficult |
| Minimum sealed joint width | 4 times the movement to be accommodated | For a sealant of plus or minus 25% capability |
| Sealant depth to width ratio | 1:2 for elastomeric sealants | A 10 mm joint takes a 5 mm deep bead |
| Minimum sealant bite | 6 mm to each face, typical | Onto a sound, clean, prepared substrate |
| Backer rod compression | 25% to 30% of rod diameter | Closed cell or bi-cellular; sets depth and acts as bond breaker |
| Sealant movement capability | plus or minus 25% to 50% | ISO 11600 class; silicones sit at the upper end |
| Open joint width | 8 to 20 mm | With a baffle, labyrinth or overlap behind |
| Cavity behind an open joint | 25 to 50 mm minimum | So that the drained and ventilated cavity can function |
| Visible joint width variation | about 3 mm | The point at which variation in a run becomes noticeable |
Sealed joints
A sealed joint relies on a gun-applied elastomeric sealant bonded to the two panel faces and nothing else. Three-sided adhesion, where the sealant also bonds to the back of the joint, is the classic failure: the bead cannot stretch across its width and tears. The backer rod prevents it. A closed cell or bi-cellular rod compressed to 25 to 30 per cent of its diameter holds the bead at the right depth and acts as a bond breaker at the same time.
Bead geometry matters as much as the product. An hourglass profile about half as deep as it is wide lets the sealant stretch; a deep square plug of the same sealant fails at a fraction of the movement. Bite onto each face should be at least about 6 mm, onto a substrate cleaned and primed to the sealant manufacturer’s instruction. On porous cementitious faces the primer is not optional and the sealant choice is not free: low modulus neutral cure silicones are usual, and plasticiser migration from unsuitable sealants stains panel edges permanently.
Open joints
An open joint has no sealant. The gap is left open and the water that passes through it is managed in the cavity behind by a baffle, a labyrinth, an overlap or a rear gutter, with a weather barrier over the insulation doing the actual sealing. The principle belongs to rainscreen; the detail is here. What the joint must do is break the direct path: an open gap with a flat membrane 40 mm behind it and nothing else will let wind-driven rain reach the membrane in quantity, so a baffle strip, a returned panel edge or a stepped overlap is standard.
Three consequences get overlooked. The eye sees into the joint, so whatever is behind is a visible surface and must be dark, neat and consistent, usually a black-faced membrane or a painted rail. The membrane is exposed to ultraviolet light through the joint, so it has to be UV-stable rather than a general purpose wrap. And a horizontal open joint drains onto whatever is below it, so rails, brackets and barriers behind must shed rather than collect water. Width is a visual decision as well as a technical one: too narrow reads as a failed seal, too wide becomes a shadow line that changes the panel module. Eight to twenty millimetres covers most cases.
Getting the width right
The nominal joint width is the sum of three things: the movement the joint must accommodate, the deviation it has to absorb from tolerance, and a minimum practical width for the chosen method. Calculate the first, obtain the second from the tolerance analysis, and take the larger of that sum and the practical minimum. A sealed joint has a practical minimum around 10 mm, because a narrower one cannot be gunned cleanly or tooled consistently.
Consistency is a separate requirement from width. The eye picks up about 3 mm of variation in a run, and picks up a taper far more readily than a uniform difference, so a joint running from 12 mm to 20 mm across an elevation is the first thing anyone notices. Distribute deviation across many joints rather than letting it accumulate at the last one.
Corners, abutments and terminations
Corners are the hardest joints on any facade. A mitred external corner looks best and demands the tightest tolerance of any detail on the job, because two panel edges have to meet on a line with nothing to conceal the difference. A returned corner, where one panel wraps and the other butts into it, is far more forgiving and hides the deviation in a single joint. An expressed corner, where both panels stop short of the arris and a shadow gap or a separate corner trim occupies the junction, is the most forgiving of all and is the right answer on freeform and long-run facades.
The other conditions to detail explicitly are the head, the sill, window and door reveals, the abutment with a different cladding material, the soffit to wall junction, and the point where a structural movement joint passes through the facade. That last one is absolute: nothing continuous may bridge it, copings, flashings, rails and corner panels included.
Compatibility and the things that fail
- Sealant to substrate: primer and surface preparation per the sealant manufacturer, tested on the actual panel material and finish.
- Sealant to sealant: never apply a new sealant over a different cured one without a compatibility check; silicone in particular will not bond to most other cured sealants.
- Sealant to coating: test against the actual coating, including anti-graffiti treatments, which frequently prevent adhesion.
- Backer rod: closed cell or bi-cellular, correctly sized, never open cell in an exposed joint, and never punctured during installation.
- Baffles and membranes behind open joints: UV-stable, mechanically retained, and detailed to shed water rather than hold it.
- Movement crossing: check that no flashing, coping, rail or trim bridges a joint that was designed to move.
Standards and references
- ISO 11600 Building construction: jointing products, classification and requirements for sealants
- ASTM C920 Standard Specification for Elastomeric Joint Sealants
- ASTM C1193 Standard Guide for Use of Joint Sealants
- ASTM C1248 Standard Test Method for Staining of Porous Substrate by Joint Sealants
- BS 6093 Design of joints and jointing in building construction
- AS 4284 Testing of building facades, including water penetration at joints
- AS/NZS 4200.1 and AS/NZS 4200.2 Pliable building membranes and underlays, behind open joints
- GRCA Practical Design Guide for GRC, joint widths and sealant selection
Related terms
Questions specifiers ask
How wide should a cladding joint be?
Typically 10 to 20 mm between panels. Size it as the movement to be accommodated plus the tolerance to be absorbed, then take the larger of that figure and the practical minimum for the method, which is around 10 mm for a sealed joint.
What is the correct sealant depth?
About half the joint width for elastomeric sealants, giving a 1:2 depth to width ratio, with at least about 6 mm of bite onto each face. A deep square plug of sealant fails at a fraction of the movement an hourglass bead will take.
Why is a backer rod necessary?
It sets the bead depth and acts as a bond breaker so the sealant adheres only to the two panel faces. Without it the sealant bonds to the back of the joint as well, cannot stretch across its width and tears.
What is the difference between an open joint and a sealed joint?
A sealed joint is a barrier and the sealant keeps water out. An open joint lets water through deliberately and manages it in the cavity behind, with a baffle or overlap breaking the direct path and a weather barrier doing the sealing.
How should an external corner be detailed?
A mitre looks best but requires the tightest tolerance on the job. A returned corner hides deviation in one joint. An expressed corner with a shadow gap or separate trim is the most forgiving, and is usually right on long runs and freeform surfaces.
We supply joint details, sealant compatibility guidance and open joint arrangements for ShapeShell-RC and ShapeShell-RT at design stage. Talk to our engineering team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.