Can Your Factory Roof Take Solar? — Structural Load, the 20 kg/m² Rule, and Wind Uplift (มยผ. 1311) Before Installing PV

Before a factory commits to rooftop solar, the first question to answer is not "what's the payback" but "can the roof take the weight, and do I need a permit" — because if the structure isn't ready, the install can damage the roof, cause leaks, or in the worst case let the wind tear panels off.

This article summarises how to assess a Thai factory roof before installing PV — the added load, the 20 kg/m² legal line, each roof type's weak point, and the wind load you must calculate — so you can talk to installers with your eyes open.

1. How much weight solar adds to the roof

A PV system adds permanent (dead) load to the roof structure; the amount depends on how it's mounted:

A typical clamp system on a metal-sheet roof is designed so total weight sits just under 20 kg/m² — Thailand's legal threshold (next section). A ballasted system on a concrete deck usually exceeds it and needs a heavier structural check.

2. Thailand's legal line — 20 kg/m²

This is the number a factory owner must know first:

⚠️ "Permit-exempt" does NOT mean "engineer-exempt." Even at ≤ 20 kg/m², the law still requires a licensed civil engineer to inspect and certify the strength of the roof structure and the electrical system, and the local authority to be notified per the conditions. Skipping this is both a safety and an insurance risk.

So "do I need a permit" and "do I need an engineer's sign-off" are two different questions — the second is always yes.

3. The real weak point — not where you'd think

Most Thai factory roofs are one of two types, with different critical points:

Metal sheet on purlins:

• The sheet itself is usually not the problem — the critical point is the purlins and fixings
• Panel weight transfers to the purlins; thin purlins or wide spacing can deflect
• Wind uplift can pull out screws/fixings → check fixing pull-out capacity and purlin spacing
• Fix at the purlin line, not mid-sheet

Concrete deck:

• Spreads load better, but ballast creates high point loads → check slab and beam capacity
• Penetration points need good waterproofing or they leak later

flowchart TD
  A[Assess factory roof] --> B{Roof type?}
  B -->|Metal sheet on purlins| C[Check purlin capacity + fixings + spacing]
  B -->|Concrete deck| D[Check slab/beam capacity + waterproofing]
  C --> E{Total weight <= 20 kg/m2?}
  D --> E
  E -->|Yes| F[Permit-exempt — but engineer must still certify]
  E -->|No| G[Apply for building-modification permit]
  F --> H[Calculate wind uplift per มยผ. 1311-50]
  G --> H
  H --> I{Fixings resist edge/corner uplift?}
  I -->|Yes| J[OK to install]
  I -->|No| K[Reinforce purlins / add fixings first]

4. Wind uplift — the most overlooked factor

A solar panel is a wind-catching surface; wind creates uplift that can tear off panels or fixings — worst at the roof edges and corners.

It must be calculated per มยผ. 1311-50 (the DPT wind standard) or an equivalent such as AS/NZS 1170.2 / ASCE 7 by:

• Using the site's reference wind speed (southern/coastal areas are higher than the central region)
• Allowing for the edge and corner zones, where wind coefficients are far higher than mid-roof
• Designing the number and spacing of fixings to resist total uplift with a factor of safety

Coastal work (e.g. the south, Rayong, Chonburi) sees higher wind and salt — allow for both wind and corrosion of fixings/structure (see corrosion systems at ISO 12944).

5. Checklist before signing a solar contract

Require the installer to deliver these before work starts — missing any one is a warning sign:

• Structural calculation signed by a licensed civil engineer (stating added weight + wind load per มยผ. 1311-50)
• Stated total system weight (kg/m²) and whether it's under/over the 20 line — sets whether a permit is needed
• As-built drawings of the existing roof, or a site survey (purlin spacing, steel size, corrosion state)
• Fixing method + fixing at the purlin + waterproofing of penetrations
• A purlin/structure reinforcement plan if capacity is short
• Local-authority notification (where required) + permit (if > 20 kg/m²)
• Electrical connection per IEC 62548 + MEA/PEA grid-tie approval

The way to avoid trouble: never accept "the roof can definitely take it" — always demand numbers and an engineer's signature. A structural assessment costing a few tens of thousands of baht is far cheaper than repairing a leaking roof or a blown-off array.

Summary

Roof readiness is the first condition of factory rooftop solar. Check three things: (1) total system weight against the 20 kg/m² line (which sets the permit question) → (2) the roof's real weak point (purlins/fixings for metal sheet, slab/waterproofing for a deck) → (3) wind uplift per มยผ. 1311-50 at the edges and corners.

And remember: ≤ 20 kg/m² = permit-exempt but not engineer-exempt — a licensed civil engineer's certification is always required.

Planning rooftop solar for a factory in Thailand — structural assessment, weight, wind load, with correct fixings and corrosion protection — request a quote and our team can assess roof readiness and design it to pass the engineering criteria.