Quick answer: HPL expands and contracts with temperature and humidity. Panels must be fixed with oversized holes, one fixed point per panel, and open joints of the manufacturer’s stated width – typically around 8 to 10mm on exterior work. Rigid fixing is the main cause of bowing and cracked panels.
What This Guide Covers
- Why the Panel Has to Move
- The Three Rules
- How Joint Width Relates to Panel Size
- Where Site Teams Go Wrong
- Diagnosing an Existing Problem
- Frequently Asked Questions
Why the Panel Has to Move
A west-facing dark panel in India can go from a very high surface temperature at four in the afternoon to substantially cooler by midnight. It expands during the day and contracts at night, every single day, for decades. Humidity adds a second, slower cycle across the seasons.
That movement is small in millimetres and enormous in force. If the panel cannot move, the force has to go somewhere – and it goes into bowing outward between supports, elongating fixing holes, or cracking outward from a hole.
The Three Rules
- One fixed point per panel. Usually near the centre. This anchors the panel so it expands evenly outward in all directions rather than drifting.
- Every other hole oversized. The hole in the panel must be larger than the fastener shank so the panel can slide – a common approach is around 8 to 10mm for a 5mm rivet, following the fixing system specification.
- Open joints between panels at the manufacturer’s stated width, typically around 8 to 10mm on exterior work, wider for larger panels.
The panel is anchored once and free everywhere else. Every rigid fixing you add is a future crack.
How Joint Width Relates to Panel Size
Thermal movement scales with dimension. A small panel moves very little; a tall panel moves noticeably more across its length, and it needs a proportionally larger allowance.
| Situation | Effect on movement | Practical response |
|---|---|---|
| Large panels | More total movement across the dimension | Larger joints and oversized holes |
| Dark shades in direct sun | Wider temperature range | Do not reduce the stated joint |
| Fully exposed elevations | Both sun and rapid cooling | Follow the maximum stated allowance |
| Sheltered internal courtyards | Narrower range | Standard allowance still applies |
| Terraces and roof-level elements | Widest range of all | Never compromise the joint here |
Where Site Teams Go Wrong
- Tightening joints for looks. Someone decides a 10mm gap looks unfinished and closes it to 4mm. The facade bows within one summer.
- Over-tightening rivets, which clamps the panel and defeats the oversized hole entirely.
- Drilling holes to fastener size because it is quicker and feels more secure.
- Filling joints with sealant, which locks panels together and also blocks the cavity drainage.
- Screwing panels tight at all four corners, which is intuitive and completely wrong.
- Installing panels tight against a return wall with no allowance at the ends of a run.
Diagnosing an Existing Problem
- Panel bows outward in the middle – either it cannot move, or support spacing is too wide for the thickness.
- Crack radiating from a fixing hole – the hole was not oversized or the fastener was over-tightened.
- Joints uneven across the elevation – setting out was inconsistent, or panels have shifted because there is no fixed point.
- Panel edges touching at joints in summer – the joint was too narrow for the panel size from the start.
- Rippling across a run – the sub-frame is not in one plane, or members are flexing under wind load.
None of these are material defects, which is why panel warranties exclude installation errors. They are all preventable at zero extra cost by following the fixing guidance.
Key Takeaways
- HPL moves with temperature and humidity, and must be free to do so.
- One fixed point per panel; every other hole oversized.
- Open joints of the stated width, typically around 8 to 10mm outdoors.
- Never tighten joints for appearance or fill them with sealant.
- Bowing and hole cracks are installation issues, not material defects.
Frequently Asked Questions
Why are my HPL panels bowing?
Almost always because the panel cannot move. Fixing holes drilled to fastener size, over-tightened rivets, joints closed up for appearance, or support spacing too wide for the thickness are the usual causes.
What gap should be left between HPL panels?
Typically around 8 to 10mm on exterior work, following the manufacturer’s stated figure for the panel size in use. Larger panels need larger joints because thermal movement scales with dimension.
How large should the fixing hole be?
Larger than the fastener shank so the panel can slide – commonly around 8 to 10mm for a 5mm rivet – with one fixed point per panel where the hole matches the fastener.
Can I fill the joints with sealant?
No. Filling open joints locks panels together, prevents movement and blocks the cavity drainage that keeps the wall behind dry. The joint is meant to stay open.
Is panel bowing covered under warranty?
Bowing caused by rigid fixing, incorrect hole sizes or excessive support spacing is an installation issue rather than a material defect, and installation errors are excluded from panel warranties across the industry.
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