One of the most common (and most time-consuming) smoke ventilation queries we receive is some version of:
- “We need an AOV to achieve 1.0m² free area.”
- “Can you size a vent to provide 1.5m² free area?”
- “The engineer wants Aa / aerodynamic free area of 0.7m².”
- “Does the structural opening reduce the free area?”
- “Will wind deflectors or access arrangements affect Aa?”
The issue is rarely the number. The issue is what “free area” means on your project and what evidence is required to support compliance.
If you are still selecting an AOV type, start here: Smoke Vents (AOVs) – category.
A 60-second sizing decision tree (before you pick a vent size)
- Is the requirement geometric free area or aerodynamic free area (Aa)?
If it does not explicitly say “Aa” (or “aerodynamic”), ask the fire engineer/designer. Do not guess. - Is the figure “per vent” or “total” for the space/shaft?
“1.5m² free area” can mean one vent, or multiple vents adding up to 1.5m². - What is the restricting aperture?
In roof applications this is often the structural opening (builder’s work). If you measure to the wrong datum (frame/kerb/external), you will get disputes later.
What “free area” can mean
1) Geometric free area
This is the physical clear opening provided when the vent is open. Importantly, it is not always the same as the nominal “vent size” on a drawing because frames, liners, hinges and construction build-ups can reduce the clear aperture.
For a deeper explanation, see: Geometric vs aerodynamic free area in smoke vents.
2) Aerodynamic free area (Aa)
Aa is a declared performance value (commonly associated with EN 12101-2 declarations). In practice, Aa is usually lower than geometric free area because it reflects airflow efficiency through the ventilator in a stated configuration.
Where 1.0m² / 1.5m² “free area” requirements usually show up (and why they get misread)
On UK projects, “1.0m²” and “1.5m²” often appear as shorthand notes in smoke ventilation discussions (for example around stairs, corridors, lobbies and shafts). The same numbers can be used in different ways depending on the smoke strategy.
Important context: Approved Document B allows approaches that use aerodynamic performance (Aa), but when design notes cite “1.0m² / 1.5m² free area” without stating Aa, most teams treat those prescriptive-style numbers as geometric free area unless the smoke strategy explicitly states otherwise.
If you are coordinating AOV selection to a specific strategy note, it is usually faster to forward the relevant excerpt when you request a quotation or technical check.
Use declared data, not nominal dimensions (SRH-PAOV example)
To make this practical, the worked examples below use the declared geometric free area and declared aerodynamic free area (Aa) from the SRH-PAOV Thermally Broken AOV data sheet.
Related product page (for the “AOV + access” use case): Smoke Vent with Access.
Declared areas (table)
Why the table matters: the declared free areas are not identical to simply multiplying “size × size”. That difference is exactly why “nominal size = free area” is the number one cause of late-stage redesigns.
Product code |
Structural Opening (mm) |
Weight |
Declared geometric free area (m²) (Approved Document B context – without wind deflectors) |
Declared aerodynamic free area Aa (m²) (EN 12101-2 context – with wind deflectors) |
|---|---|---|---|---|
SRHPAOV/105.105 |
1050 × 1050 |
110kg |
1.03 |
0.73 |
SRHPAOV/126.126 |
1260 × 1260 |
132kg |
1.52 |
1.05 |
Note: SRH-PAOV standard sizes are stated up to 1260 × 1260, with bespoke sizes available. The data sheet also states the product is CE marked to EN 12101-2, thermally broken, and suitable for daily ventilation and access, with a curb liner as standard and installation up to 15° pitch.
Worked examples (how designers actually use the declared figures)
Example 1: “Provide minimum 1.0m² free area” (geometric)
Interpretation: You need geometric free area ≥ 1.0m².
- SRHPAOV/105.105 declares 1.03m² geometric free area → passes a 1.0m² geometric target.
- SRHPAOV/126.126 also passes (1.52m²), but may be unnecessary if 1.0m² geometric is the only requirement.
Do not skip the datum check: confirm the restricting aperture (typically the structural opening) and confirm your roofing build-up and kerb details match the supplier drawing.
Example 2: “Provide minimum 1.5m² free area” (geometric)
Interpretation: You need geometric free area ≥ 1.5m².
- SRHPAOV/126.126 declares 1.52m² geometric free area → passes a 1.5m² geometric target.
- SRHPAOV/105.105 (1.03m²) does not pass.
This is a common point where projects stall: teams request “1.5m² free area” while also issuing an opening size that cannot physically accommodate it unless the requirement is a total across multiple vents.
Example 3: “Provide 0.7m² Aa (aerodynamic free area)”
Interpretation: You need Aa ≥ 0.7m².
- SRHPAOV/105.105 declares Aa = 0.73m² → passes.
- SRHPAOV/126.126 declares Aa = 1.05m² → passes.
Make the configuration explicit: If your project requires Aa, you must specify the vent “with wind deflectors”. Without them, the vent achieves a lower Aa value and may fail compliance checks.
Example 4: “Provide 1.0m² Aa”
Interpretation: You need Aa ≥ 1.0m².
- SRHPAOV/126.126 declares Aa = 1.05m² → passes.
- SRHPAOV/105.105 (0.73m²) does not pass.
Example 5: “Provide 1.5m² Aa” (what happens next)
With the SRH-PAOV standard sizes shown above, a single vent does not reach 1.5m² Aa (largest declared Aa shown is 1.05m²). At this point the options are typically:
- Multiple vents providing a combined Aa total (confirm whether the requirement is total or per vent).
- A different ventilator configuration/product with higher declared Aa.
- Bespoke sizing (where appropriate), supported by declared/test data for the stated configuration.
Dimensions: avoid the “standard size” trap
In practice, the biggest disputes come from mixing up these three dimension references:
- Structural opening (builder’s work / slab opening)
- Overall frame size (external)
- Clear opening / throat (usable clear aperture)
On the SRH-PAOV 1050 × 1050 drawing, the relationships are shown as:
- Overall frame = structural opening + 224mm
- Overall lid = structural opening + 190mm
Tip: treat these as drawing-specific values and always confirm against the latest GA for the size/configuration you are ordering.
Opening angle: check actuator stroke and controls settings
The SRH-PAOV declared performance relies on an opening angle of 140°. Ensure your control system/actuator stroke is set to achieve this full extension.
If you are coordinating controls, wiring, or fire inputs, start with the relevant diagrams from our resource centre.
Common mistakes (and how to avoid them)
1) Assuming nominal size equals free area
Declared geometric free area can be lower than “size × size”. Always size from declared values where available.
2) Mixing up geometric free area and Aa
If the requirement is Aa, you need a declared aerodynamic free area figure for the stated configuration (and you must ensure your specification matches that configuration).
3) Measuring to the wrong datum
Confirm whether your project team is using structural opening, overall frame, or clear opening. Put the chosen datum explicitly in drawings, schedules, and enquiries.
4) Leaving accessories implied
Wind deflectors, liners, and access options can materially affect what is being claimed. If you need Aa, specify the vent with wind deflectors rather than assuming it will be included.
What to request from a supplier (so your submission is defensible)
When your requirement is “free area”, request these items up front:
- Declared geometric free area (and what datum it is based on).
- Declared aerodynamic free area (Aa) and the configuration it relates to (including whether wind deflectors are included).
- A dimensioned drawing confirming structural opening, overall sizes, and key build-up constraints.
- Opening angle/actuation notes, so you can confirm controls and stroke allow the declared opening.
- Any supporting documentation required by the project submission process.
Quote-ready checklist (copy/paste)
If you want a fast technical confirmation with minimal back-and-forth, include the following in your enquiry:
- Required value and definition: ___ m² geometric free area or ___ m² Aa
- Is that requirement per vent or total?
- Location/application: stair/corridor/lobby/shaft (attach the smoke strategy excerpt if possible)
- Structural opening (W × H) and any slab/beam constraints
- Roof build-up constraints and kerb/upstand requirements
- Any access requirements (roof access use, ladder arrangements, handrails/guarding)
- Controls and interface requirements (power, fire input, overrides)
- Evidence pack required (datasheet, drawings, declarations)
Next step
Send your smoke strategy excerpt plus the required free area (state geometric or Aa) and your structural opening, and we will confirm a compliant configuration.
