Mobile Tower Scaffold: PASMA Rules, 3T Method, Stabilisers and Maximum Heights for Indoor and Outdoor Use

Summary

Mobile access towers (also called mobile scaffold towers or MAST — Mobile Access and Scaffold Towers) are the most common form of temporary access equipment on construction sites and in maintenance settings. They are quicker to erect than conventional scaffolding, can be moved between positions, and are widely available on hire.

PASMA (Prefabricated Access Suppliers' and Manufacturers' Association) is the UK industry body that sets standards for mobile towers. PASMA training is the recognised industry qualification for tower users, and PASMA-trained operatives are a contractual requirement on many sites. The PASMA guidelines are not legislation themselves, but they implement the Work at Height Regulations 2005 and are considered the accepted industry standard.

Despite their apparent simplicity, mobile towers are involved in a disproportionate number of work-at-height accidents. Common causes: incorrect erection, failure to level, working outside the safe height, not using the 3T method, and moving the tower with someone on it. Understanding the rules is critical for any tradesperson who uses towers.

Key Facts

• PASMA — Prefabricated Access Suppliers' and Manufacturers' Association; sets UK tower standards
• PASMA training — 1-day training course; PASMA card valid for 5 years; widely required by principal contractors
• 3T method — Through the Trapdoor; the only approved method of accessing the working platform
• Maximum height (indoors) — working platform height up to 2.5× the smallest base dimension (without stabilisers)
• Maximum height (outdoors) — working platform height up to 3.0× the smallest base dimension (without stabilisers) — note: see footnote below regarding outdoor limits
• Stabilisers/outriggers — increase the effective base dimension and thus the maximum height
• Levelling — tower must be level within ±1° or per manufacturer's specification before use
• Casters — must be locked when in use; tower must never be moved with anyone on the platform
• Wind — outdoor towers should be taken down or secured in wind speeds above Beaufort 5 (fresh breeze, ~29–38 km/h); manufacturer guidance may specify lower limit
• Manufacturer instructions — ALWAYS follow the specific manufacturer's instructions; they take precedence over generic PASMA guidance
• IPAF vs PASMA — IPAF cards are for powered MEWPs (cherry pickers, scissor lifts); PASMA cards are for mobile towers; they are separate qualifications
• HSE guidance — mobile towers are covered by WAH Regs; formal inspection required before first use and every 7 days

Quick Reference Table: Maximum Tower Heights (Approximate)

Exact limits vary by manufacturer. Always check the specific manufacturer's documentation.

Note: PASMA guidance has been updated; check current PASMA 7th Edition guidelines for the latest height ratios — the 3:1/2.5:1 rule is the traditional standard but some products have updated ratios.

Detailed Guidance

The 3T Method — Through the Trapdoor

The 3T (Through the Trapdoor) method is the only PASMA-approved method for accessing a mobile tower platform. The prohibition on climbing the outside frame is absolute.

Why 3T? Climbing the outside frame of a tower:

• Places the climber outside any edge protection
• Applies lateral loads to the tower that it is not designed for
• Creates a significant fall risk with no protection

How to use 3T:

1. Climb the internal ladder to the trapdoor hatch in the working platform
2. Push the trapdoor up and through it
3. Step onto the working platform
4. Close the trapdoor behind you (to prevent another person stepping into an open hatch)
5. Do not stand on the trapdoor itself

If the tower does not have an internal ladder with a trapdoor, it should not be used. Many older or cheap hire towers do not have a proper 3T hatch — check before hiring.

Levelling the Tower

A mobile tower that is not level is a significantly less stable structure. Requirements:

• Level the tower using the height-adjustable casters (threaded spindle within the caster)
• Maximum caster extension: typically 300mm from the locked position (check manufacturer)
• Tower must be level within ±1° in both directions (or per manufacturer spec)
• If the ground is too uneven for the casters to compensate, use timber pads or adjustable leg jacks — but this takes the tower outside standard PASMA use; check manufacturer guidance

On sloping ground outdoors, if the slope exceeds the caster adjustment capacity, the tower should not be used at that position without specific assessment. Using a tower on a steep slope is one of the most common causes of tower overturning.

Stabilisers and Outriggers

Stabilisers (also called outriggers) extend the effective base dimension of the tower, allowing greater working heights. They consist of horizontal tubes attached to the base of the tower, often with a pad or wheel at the end.

Key rules for stabilisers:

• Use only stabilisers specifically designed for the tower model — no improvised versions
• Fit per manufacturer instructions — typically level and fully extended before climbing
• Stabilisers must be fitted before the tower is raised to working height, not after
• All four stabilisers required — fitting three is not acceptable
• Casters must still be locked when using stabilisers; stabilisers do not replace the need for locked casters
• With stabilisers, maximum height increases to the ratio specified in the manufacturer's documentation

Moving the Tower

One of the most frequently violated mobile tower rules: never move a tower with anyone on the working platform or on the ladder.

Before moving:

1. Ensure no one is on the tower at any level
2. Remove all materials and tools from the platform
3. Unlock all four casters
4. Move by pushing at the frame level — not by pulling a rope from the platform
5. Move slowly on level ground only; do not move on slopes
6. Re-lock all four casters before anyone ascends
7. Re-check level before use

Horizontal forces from pushing the tower must not be applied above the mid-height of the tower. Pushing from the top creates a destabilising moment.

Outdoor Use Considerations

Outdoor use introduces wind loading that is absent indoors. Additional precautions:

• Wind speed limit — take down or secure when wind reaches Beaufort 5 (29–38 km/h, leaves and small branches in constant motion); some manufacturers specify lower limits (check)
• Anchor if leaving overnight — secure to an anchor point or dismantle if leaving erected overnight or over a weekend
• Rain and surface conditions — wet ground reduces caster traction; always check that casters are on a firm, level surface
• Sun/UV and material condition — aluminium towers are not affected by UV, but check locking pins and couplings for corrosion
• Position away from overhead lines — minimum safe clearance from overhead power lines per HSE guidance (see scaffolding near overhead lines)

PASMA Training

PASMA training is a 1-day course typically conducted at a PASMA-approved training centre or on-site. Content covers:

• Work at Height Regulations 2005
• Tower design and components
• Erection, dismantling, and inspection
• 3T access method
• Stabilisers and working heights
• Pre-use checks
• Practical assessment

On successful completion, the trainee receives a PASMA card (credit-card format) valid for 5 years. The card states the card holder's name, PASMA registration number, and expiry date.

Is PASMA training a legal requirement? Not as such — the Work at Height Regulations require that persons erecting and using work equipment at height are competent. PASMA training is the recognised industry method of demonstrating that competence. Principal contractors increasingly require a valid PASMA card as a condition of site access for anyone who will use a tower.

Pre-Use Checks

Before each use, the tower operator should carry out a pre-use inspection:

• All components present per manufacturer's documentation
• No visible deformation, cracks, or impact damage to frame tubes or locking pins
• All locking pins and clips engaged and secure
• Casters in working condition; not damaged or seized
• Working platform boards or deck in place, no damaged boards
• Guardrail system complete (top rail, intermediate rail, toeboard if required)
• Trapdoor hatch opens and closes freely

A formal Schedule 7 inspection under WAH Regs is required before first use and every 7 days.

Frequently Asked Questions

Can I lean a ladder against a mobile tower to reach higher?

No. Leaning a ladder against a mobile tower is not an approved access method. It changes the stability of the tower, applies undesigned lateral loads, and creates a complex fall risk. If you need to go higher, extend the tower to the required height (within safe limits) or use a different access method.

Can I use a mobile tower on a scaffold platform?

Using a mobile tower on top of a scaffold or elevated platform is hazardous and generally not permitted without a specific risk assessment and written method statement. The tower's base must be on a firm, level surface; the scaffold platform may not be adequate. If access requires stacking, a different solution (purpose-designed extension scaffold, MEWP) should be considered.

Is a PASMA card the same as an IPAF card?

No. PASMA cards are for mobile access towers (unpowered). IPAF (International Powered Access Federation) cards are for powered mobile elevating work platforms (MEWPs) such as scissor lifts, boom lifts, and cherry pickers. They are separate qualifications. A PASMA card does not authorise MEWP operation, and vice versa.

What is the difference between a mobile tower and a static scaffold tower?

A mobile tower is on casters and can be moved. A static scaffold tower is fixed in position, typically assembled from tube and fitting or system scaffold components, and may be tied to the building. The PASMA rules and height ratios apply specifically to mobile towers on casters. A static tower is governed by TG20:21 (tube and fitting) or manufacturer documentation (system scaffold) and WAH Regulations.

Regulations & Standards

• Work at Height Regulations 2005 (SI 2005/735) — applies to all mobile tower use; inspection requirements; hierarchy of control

• BS 1139-6:2014 — Specification for mobile access and working towers; requirements for manufacture and labelling

• PASMA Operational Code of Practice (7th Edition) — industry standard for mobile tower use; erection, use, and inspection

• BS EN 1004:2004 — Mobile access and working towers made of prefabricated elements — Materials, dimensions, design loads, safety and performance requirements

• HSE HSG150 — Health and Safety in Construction; includes guidance on mobile towers

• PASMA — Mobile Tower Guidance — official PASMA guidelines, training, and card verification

• HSE Work at Height — Mobile Towers — HSE guidance specific to mobile towers

• Work at Height Regulations 2005 — full text

• work at height regs 2005 — WAH Regulations duties including tower inspection requirements

• scaffolding near overhead lines — clearance distances for towers near overhead cables

• scaffold edge protection — guardrail requirements on tower platforms

• scaffold inspection records — Schedule 7 inspection requirements for towers