Factory Roof Assessment for Solar Complete Guide 2026
Regulation 72 exempts building permits for solar ≤20 kg/sqm — but factories 15+ years old still need structural assessment. 9-point checklist covering 6 roof types, mounting systems, load calculation, and waterproofing.
Standard 580W panels + mounting weigh just 12-16 kg/sqm — well under the 20 kg/sqm threshold set by Ministerial Regulation 72 (B.E. 2568), which exempts solar installations from building modification permits. However, factories 15+ years old should always get a structural assessment first: rust on purlins and trusses can silently reduce load capacity with no visible exterior signs.
Ministerial Regulation 72 — The 20 kg/sqm Rule
Ministerial Regulation No. 72 (B.E. 2568), published in the Royal Gazette on November 19, 2025, marks a turning point for Thailand's solar industry — dramatically simplifying rooftop solar installation for all building types. Read the full permit guide at Solar Permit Approvals Thailand.
What Changed
What changed: Solar panel installations on ALL building types (residential, commercial, factory) with total weight no more than 20 kg/sqm are now exempt from building modification permits (อ.1). No application needed, no waiting period.
What Was Removed
What was removed: No more area limitation (previously residential was capped at 160 sqm), no mandatory structural engineer certification, no local authority notification — all waived when weight is ≤20 kg/sqm.
What This Means for Factories
For factories: Previously you needed an อ.1 building modification permit, structural engineer sign-off, and local authority notification. Now if weight stays under 20 kg/sqm — none of this is needed. But regulatory exemption doesn't mean engineering prudence is optional.
Weight Reality
Weight reality: A standard 580W mono-PERC panel weighs about 28-32 kg for a 2.3 sqm panel = approximately 13 kg/sqm. With flush mounting system = 14-16 kg/sqm. Ballasted flat-roof systems can reach 18-20 kg/sqm.
Separate from factory license: The Factory Operations Act amendment (December 2024, per Tilleke & Gibbins analysis) also removed the factory license (Ror.4) requirement for factory rooftop solar.
Despite the legal exemption, CapSolar still recommends structural assessment for any factory older than 10 years — because rust and deterioration in purlins and trusses are invisible from outside and may prevent the roof from handling additional weight safely.
Thai Factory Roof Types & Solar Compatibility
Thai factories use various roof types, each with different advantages and limitations for solar. The table below compares the 6 most common factory roof types — to help you make an initial assessment of your roof's readiness. Try our roof area estimator tool to calculate installation area.
| Roof Type | Prevalence | Compatibility | Mounting | Key Notes |
|---|---|---|---|---|
| Corrugated Metal Sheet | Very common (60%+ pre-2010 factories) | Good | Rail + bracket with roof penetration (L-foot/stanchion) | Must check for rust, especially at fastener points. Requires waterproof sealant at penetrations. Check panel drainage path. |
| Metal Deck / Klip-Lok | Common in modern factories (2010+) | Excellent | Clamp-on system, no penetration needed | Best option for solar — no roof penetration, fast installation, preserves roof warranty. |
| Standing Seam | Growing in premium factories | Excellent | S-5! style clamps directly on seam ribs — zero drilling | Zero penetration, fastest install, highest compatibility. |
| Concrete Flat Roof | Common in older factories, multi-story | Good | Ballasted tilt frame or chemical anchor | Higher weight per sqm due to ballast. Check waterproofing membrane first. Tilt frames allow optimal angle. |
| Asbestos Cement | Legacy factories (pre-2000) | Poor — requires roof replacement | Must replace roof first — cannot drill | Asbestos sheets cannot be drilled. Requires licensed removal contractor. Factor roof replacement cost into solar project budget. |
| Polycarbonate / Translucent | Skylights in factories | Not suitable | Avoid — cannot bear weight | Cannot support solar panel weight. Route solar panels around skylight areas. |
Approximately 70% of Thai factories have corrugated metal or metal deck roofs — both are solar-compatible with the right mounting system.
9-Point Factory Roof Solar Readiness Checklist
01Roof Age Assessment
When was the roof last replaced? Roofs older than 15 years require detailed inspection — metal degrades faster than expected in Thailand's hot, humid climate, especially in coastal industrial zones (Rayong, Samut Prakan, Chonburi).
02Steel Structure Condition
Check purlins, trusses, and columns — look for rust, deformation, welding cracks. Purlin spacing must be less than 1.5m for standard panel mounting rails.
03Load Capacity Verification
Dead load + live load + solar load must not exceed design capacity. Rule of thumb: standard Thai industrial buildings are designed for 30 kg/sqm = safe margin for solar at 14-16 kg/sqm.
04Column Footing Condition
Check ground level for column settling, look for cracks at footings, and inspect for water damage — settling footings can cause uneven load distribution across the entire roof structure.
05Roof Surface Integrity
Check for missing fasteners, loose sheets, holes, dents, and previous penetration points — all must be repaired before panel installation.
06Waterproofing Status
Existing roof leaks MUST be fixed before solar installation — panels will mask leaks and make future repair much harder, plus standing water under panels accelerates mounting system corrosion.
07Drainage Assessment
Panel layout must not block existing water drainage paths. Standing water under panels = accelerated corrosion + algae growth. Maintain minimum 200mm gap between panel rows for water flow.
08Tilt and Orientation
Optimal for Thailand = 10-15 degrees south-facing. East-west roofs split electricity production (morning/afternoon). Flat roofs need tilt frames, which add weight per sqm.
09Shading Analysis
Check adjacent buildings, cooling towers, signage, and trees for shading. Analyze shadow paths 06:00-18:00 across all seasons. More than 30% shading during 09:00-15:00 = significantly reduced viability.
Structural Load Calculation Simplified
Total roof load = Dead Load + Live Load + Solar Load + Wind Uplift — must be ≤ structural design capacity.
| Load Component | Typical Value |
|---|---|
| Dead load (roof sheeting + insulation) | 8-15 kg/sqm |
| Live load (maintenance access) | 30-50 kg/sqm |
| Solar panel + mounting system | 12-16 kg/sqm (flush) or 16-20 kg/sqm (tilted ballast) |
| Wind uplift | Location/height dependent |
Key insight: The 20 kg/sqm regulatory threshold refers to solar equipment weight only, not total roof load. You must calculate total combined load against the original design capacity.
When reinforcement is needed: if existing dead load + solar load exceeds the original design capacity minus safety factor. For factories 15+ years old, hire an engineer for actual calculation. Try our solar system sizing calculator to estimate system size after confirming your roof.
Reinforcement costs: purlin reinforcement 200-500 THB/sqm, truss upgrade 500-1,500 THB/sqm — pricing depends on factory size, existing condition, and materials. See more cost details at Solar Panel Price Thailand 2026.
Structural engineer survey cost: 20,000-80,000 THB for a standard factory — includes visual inspection, purlin spacing measurement, load calculation, and engineer certification.
Solar Mounting Systems for Thai Factory Roofs
Non-Penetrating Systems (Preferred)
Standing seam clamps (S-5!, K2, Schletter): grip the seam rib without drilling. Best for metal deck / klip-lok / standing seam. Most common in modern Thai factory installations. Compare panel weights by brand at 5 Tier-1 Solar Panel Brands Compared.
Ballasted systems for concrete flat roofs: weighted frames without penetration. Must verify total weight (panels + frame + ballast) stays within design capacity.
Penetrating Systems (When Non-Penetrating Is Not Possible)
L-foot / stanchion mounts: bolt through corrugated metal into purlin. Must waterproof with EPDM gaskets + polyurethane sealant. Fasteners must hit the purlin, not just the roof sheet.
Chemical anchor for concrete roofs: epoxy or polyester resin anchor. Requires clean, dry holes and 24-hour cure time.
Rail Systems
Rail systems: aluminum rail (lightweight, corrosion-resistant for Thai humidity) or steel rail (heavier but cheaper — must be hot-dip galvanized for coastal/high-humidity areas like Samut Prakan, Rayong).
Wind zone note: Thailand building code specifies wind zones — Eastern Seaboard Zone II requires additional wind clip spacing for stronger wind loads. Have your system designer calculate site-specific wind loads.
Waterproofing & Roof Warranty Considerations
Every roof penetration is a potential leak point: each hole requires EPDM gaskets + polyurethane sealant + flashing tape — all 3 layers. Never skip any step. Read more about long-term care at Solar Maintenance Guide.
Roof warranty impact: some metal roof manufacturers void warranty for non-approved penetrations. Check your roof warranty terms before choosing a mounting system. See more on insurance at Solar Insurance & Warranty Guide.
Drainage: panel rows must maintain minimum 200mm gap to allow water flow and debris clearing. Standing water between rows = accelerated corrosion and algae growth.
Condensation: in Thailand's tropical climate, under-panel condensation is common (morning dew cycle). Maintain minimum 100mm ventilation gap between panel and roof to prevent mold and corrosion.
Leak testing: post-installation water spray test on all penetration points is mandatory before project handover — every professional EPC provider must perform this step.
Shading Analysis & Layout Optimization
Check morning and afternoon shadows from adjacent buildings: observe/photograph shadow patterns at 08:00, 10:00, 14:00, 16:00 across all seasons — winter shadows (Dec-Feb) are longest.
Obstacle mapping: list EVERY item on the roof — HVAC units, cooling towers, exhaust stacks, lightning rods, antenna masts, water tanks — then mark a 2m buffer zone around each.
Inter-row spacing: for tilted systems on flat roofs, maintain minimum 1.5x panel height to avoid self-shading. In Thailand (13-15 degree latitude), this is less severe than higher-latitude countries.
String design: shading on one panel in a string affects entire string output. Design strings to avoid grouping panels with mixed shade exposure.
Future changes: ask about planned factory expansion, adjacent construction, and tree growth — what doesn't shade today may shade in 5 years.
Tools: professional EPC providers use PVsyst, Helioscope, or Aurora Solar for shade modeling — request a shading report from every EPC bidder. For free self-assessment, use Google Earth + SunCalc. Try our roof area estimator for initial area assessment.
When to Walk Away — 6 Red Flags
- Asbestos roofing: must be removed first. Removal cost 500-1,000 THB/sqm. Licensed contractor required.
- Severe structural deterioration: multiple rusted-through purlins, sagging trusses, settling columns — structural repair needed first, costs may exceed viability.
- Roof scheduled for replacement within 5 years: install solar AFTER the new roof, not before — otherwise you'll need to remove panels, replace roof, and reinstall, doubling costs.
- Unresolvable shading: more than 30% of usable roof area shaded during 09:00-15:00 (peak production hours) = reduced viability that may not justify investment.
- No crane or staging access: if there's no crane access path or staging area for installation, transport and mounting become extremely difficult and expensive.
- Active roof leaks: fix all leaks first, then revisit solar — installing panels over leaks just buries the problem.
A good EPC provider will tell you straight when NOT to proceed. If someone says 'any roof works,' find a different provider. Read our EPC Selection Guide to choose a trustworthy EPC.
Written by Frank Lee — Founder, CapSolar
Frank Lee founded CapSolar in 2023, specializing in EPC and PPA services for Thai factories. With 80+ projects totaling 16.5 MWp installed, this article draws from real-world roof assessments across more than 200 factories nationwide.
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Ready for a Professional Roof Assessment?
CapSolar offers free roof assessment for projects 100 kWp and above — including purlin measurement, load calculation, shading analysis, and readiness report.