Pergola glass panels in a Marathahalli courtyard: thermal expansion gap and the seasonal water-pooling risk

A pergola glass panel fitted in January sits flush against its frame. By May, when Bangalore's ambient temperature climbs from 22°C to 37°C, that same panel has contracted into its frame by 2 to 3 millimetres—enough to trap monsoon water in the joint line when the rains arrive. The risk is not theoretical. On a recent Marathahalli courtyard commission, we discovered the architect's shop drawing had specified a 4mm uniform gap, which became a 1.5mm pooling line by June.

Thermal expansion in glass is a material constant, not a design choice. But the joint line that results from it is entirely within your specification control. This article walks through the calculation and the site protocol that prevents water ingress on pergolas across Bangalore's seasonal swing.

Why Bangalore's seasonal temperature range matters more than you think

Bangalore sits in a thermal band where winter lows touch 15–18°C and summer highs reach 37–39°C. That 20°C swing—larger than many architects assume—produces measurable linear expansion in 10mm or 12mm tempered glass.

Glass expands at a coefficient of approximately 9 × 10⁻⁶ per degree Celsius. A 1-metre-long panel experiences roughly 0.18mm of linear expansion per 10°C rise. Over the full seasonal range of 20°C, a 1.5-metre panel (typical pergola width) expands and contracts by approximately 2.7mm. A 2-metre panel moves closer to 3.6mm.

In winter, when you fit the panel, you have a known gap. By summer, that gap has closed. By the monsoon (June through September), when humidity peaks and Bangalore's hard water (TDS 200–300 ppm) carries mineral deposits, a closed or nearly-closed joint becomes a capillary trap.

The pooling mechanism: why water sits in the summer-closed joint

How the gap closes under thermal load

When you specify a 4mm joint gap in your shop drawing, you are typically doing so at the design temperature—usually ambient site conditions in the 25–28°C range. This is your baseline. As summer heat arrives, the glass expands into the frame, reducing the gap. The frame itself also expands, but at a different rate than glass (aluminium expands at 23 × 10⁻⁶ per °C, nearly 2.5 times faster than glass). The net result is that the glass panel moves inward relative to the frame opening.

By May and June, when monsoon humidity begins to rise, your 4mm gap may have narrowed to 1.5–2mm. This is no longer a drainage gap. It becomes a slot that capillary action can exploit.

Capillary action and mineral deposit risk in Bangalore's water

Bangalore's Cauvery water carries dissolved solids in the 200–300 ppm range. When water enters a narrow joint and evaporates, those solids remain. Over a monsoon season, this accumulation can cement the joint line and create a visible stain on both the glass face and the frame finish. More critically, water trapped in the joint can migrate into the frame's internal drainage path (if present) or, on frameless installations, can seep into the underlying structure.

The risk is highest on east- and west-facing pergolas, where afternoon sun heats the glass and accelerates evaporation, concentrating minerals further.

Calculating the correct gap: the formula for your shop drawing

Step 1: Establish your reference temperature

Choose a reference temperature at which you will measure and fit the panel. On most Bangalore projects, this is 26–28°C. Document this in your RCP and shop drawing. This becomes your baseline zero-point.

Step 2: Calculate seasonal expansion

Use the formula: ΔL = L₀ × α × ΔT, where:

• L₀ = panel length in millimetres
• α = linear expansion coefficient (9 × 10⁻⁶ for glass)
• ΔT = temperature change from reference to summer peak (assume 37°C as your worst-case summer ambient)

For a 1500mm panel at 26°C reference, expanding to 37°C:

ΔL = 1500 × 9 × 10⁻⁶ × (37 − 26) = 1500 × 9 × 10⁻⁶ × 11 = 0.149mm per panel edge.

For a 2000mm panel: ΔL = 2000 × 9 × 10⁻⁶ × 11 = 0.198mm.

These are per-edge movements. If the panel is free to move on both sides (which it should be in a proper pergola detail), add both sides: a 1500mm panel moves approximately 0.3mm total across its width; a 2000mm panel moves roughly 0.4mm.

Step 3: Set your minimum drainage gap

The joint must remain open enough to shed water, even at summer temperature. A 2mm gap is the practical minimum for a pergola in monsoon conditions. Below 2mm, surface tension and capillary forces dominate, and water will not run off cleanly.

If your calculated summer closure is 0.3mm, and you want to maintain 2mm minimum, your winter specification gap must be 2mm + 0.3mm = 2.3mm. Round to 2.5mm for site tolerance.

Step 4: Account for frame tolerance and as-built variation

Aluminium frames are typically tolerance-held to ±1mm over a 2-metre run. Your opening dimension at the time of fit may vary by 1–2mm from the architectural plan. Specify the gap as a range, not a single number: "Joint gap 2.5mm ±0.5mm, measured at reference temperature 26°C, minimum 2.0mm at summer ambient."

Shop drawing protocol: what to specify and where

Your shop drawing must include four elements:

1. Reference temperature and date of fit: "Gap measured and fitted 15 January 2024, ambient 26°C. Panel free to move with seasonal temperature change."
2. Joint detail at 1:5 scale: Show the gap dimension, the frame profile, and the direction of water flow (typically downward and outward).
3. Drainage path: If the frame has an internal weep hole or drain, call it out. If the pergola is frameless (as on our Tendere glass overhead system), specify how water escapes the joint—typically over the outer edge of the glass.
4. Sealant specification (if used): If you are using a sealant in the joint, specify a flexible silicone or polyurethane rated for 25% joint movement. Rigid sealants (acrylic, polyester) will crack and fail within one monsoon season.

Do not rely on the contractor to "fit it tight" or "close the gap for a better look." Tight gaps fail. Specify the gap in writing, and inspect it at fit time with a feeler gauge.

Material choice and joint line visibility

The joint line itself becomes a design element. On clear glass pergolas with bronzed-steel frames, a 2.5mm gap reads as a fine line from below—visually clean and proportionate to the frame. On curved tinted-glass systems, the gap is less visible because the tint and curvature draw the eye to the glass surface rather than the joint.

If you are specifying a frameless detail, the gap becomes the only visual break in the glass plane. Here, the gap must be even more precise, because there is no frame edge to hide minor variations. Frameless pergolas typically demand a tighter tolerance spec: 2.0mm ±0.3mm rather than 2.5mm ±0.5mm.

Site commissioning and handover

When the pergola is fitted, the contractor should document the joint gap with a photograph and a written record of ambient temperature at the time of fit. This record becomes part of the as-built set and serves as a baseline for any future maintenance or warranty claim.

Inspect the joint line after the first monsoon. If water is pooling or mineral deposits are visible, the gap was undersized. If the gap has widened to 4–5mm by the following winter, the panel frame may have moved due to structural settlement or wind load—a separate issue, but one that the thermal-expansion baseline helps you diagnose.

On projects with hard-water risk, consider a light annual rinse of the joint line with distilled water in late August, before the monsoon peaks. This prevents mineral concentration and keeps the joint visually clean.

Questions we get asked

Should I use a sealant in the pergola joint to prevent water ingress?

Only if the gap is large enough (3mm or more) and the sealant is specified as flexible. A 2–2.5mm gap sealed with rigid sealant will crack by summer. If you are sealing, use a 100% silicone or polyurethane rated for ±25% movement, and apply it as a bead on the outer face only, not in the depth of the joint. This allows water to drain while preventing wind-driven spray from entering the joint.

Does the frame material (aluminium vs. steel vs. bronze) change the thermal expansion calculation?

The glass expansion is constant. The frame expands faster than glass, so the net gap closure is less than the glass expansion alone. For a pergola with a steel or bronze frame, the gap closes slightly less than with aluminium. However, the difference is typically under 0.5mm, so it does not change your specification. Specify the same gap regardless of frame material.

What if the pergola is in a location with extreme sun exposure—say, a west-facing courtyard in Sarjapur Road?

West-facing pergolas in Bangalore can see glass surface temperatures 10–15°C above ambient on a clear afternoon. This accelerates expansion and increases the risk of a fully-closed gap by mid-afternoon. Specify the gap using the peak glass temperature (approximately ambient + 12°C), not ambient air temperature. For a 37°C ambient day, assume the glass reaches 49°C, and calculate expansion accordingly. This typically means adding 0.5–1mm to your winter gap specification.

Can I use a thinner glass (8mm instead of 10mm) to reduce expansion risk?

Thinner glass expands proportionally less, but the absolute movement on a 1500–2000mm panel remains 0.2–0.3mm—not enough to justify the structural compromise. 10mm or 12mm tempered glass is the minimum for a pergola span. Stick with standard thickness and manage the gap instead.

Our Bangalore project has a pergola over a pool courtyard. Does chlorine or salt water change the joint protocol?

Chlorine and salt accelerate mineral deposition and corrosion in the joint line. Specify a slightly larger gap (2.5–3mm minimum) and a fully open drainage path with no sealant. Inspect the joint monthly during the swimming season, and rinse with fresh water after each use if possible. On pool pergolas, consider a frame finish that resists corrosion—anodised aluminium or powder-coated steel rather than bare bronze or brushed finishes.

Commissioning your pergola: next steps

If you are specifying a pergola for a Bangalore project and need to calculate the joint gap for your site conditions, climate exposure, and frame material, talk to the atelier. Bring your shop drawing, your site dimensions, and your reference temperature. We will specify the gap to the millimetre and provide a detailed joint protocol for your contractor.