2mm Aluminium Retention Trim - 2.4m Length

The heavier 2mm version of the aluminium retention trim, supplied in 2.4m lengths. It secures flat roof waterproofing membranes at the roof perimeter and retains growing substrate, pebble ballast or drainage layers on green and living roofs. Available in eight sizes from 50x100mm to 300x150mm, where the first dimension is the face height and the second is the base depth. Choose the face height to match the installed depth of your substrate or membrane layer. Powder coated in any RAL colour and manufactured in the UK at the Online Metal Store Ltd facility in Chelmsford. For the lighter gauge version, see the 1.5mm aluminium retention trim.

The 2mm gauge is the correct specification for heavier-loaded applications: green roofs with a deeper substrate layer or pebble ballast of 150mm or more, exposed sites where wind pressure on the trim face is higher than standard, and commercial flat roofs where the specification calls for the heavier section. Like the 1.5mm version, the face has drainage slots for free water flow and the base has fixing holes for mechanical attachment or bonding straps.

Understanding the Sizes: Face Height x Base Depth

The eight size options listed as two dimensions (e.g. 50x100mm, 75x100mm) can be confusing without an explanation. Here is what each figure means:

First Dimension: Face Height

The vertical height of the exposed face of the trim. This is the dimension that needs to match the installed depth of the substrate, growing medium or pebble layer on a green roof, or the height of the membrane upstand on a standard flat roof. Select this dimension to match the layer depth at the perimeter. If the growing medium is 100mm deep, use a 100mm face height. If it is 150mm deep, use a 150mm face height.

Second Dimension: Base Depth

The horizontal depth of the flat base that sits on the roof deck. A wider base gives a larger fixing footprint and greater stability against lateral pressure from the substrate layer. The 100mm base suits most standard applications; the 150mm base is better suited to deeper substrate loads and exposed sites where additional base area improves resistance to overturning.

Size Selection Guide

• 50 x 100mm: shallow membrane retention on standard flat roofs. Smallest available face height.

• 75 x 100mm: standard green roof application with a shallow growing medium or lightweight drainage layer.

• 100 x 100mm: medium-depth green roof substrate retention. Most common commercial specification.

• 120 x 100mm: green roofs with substrate depths up to 120mm or pebble ballast layers of similar depth.

• 150 x 150mm: deeper substrate applications. Wider base for additional stability.

• 200 x 150mm: intensive green roofs with deeper growing media and heavier substrate loads.

• 250 x 150mm: deep substrate retention. Used on intensive green roofs with planted areas.

• 300 x 150mm: maximum standard face height. For the deepest substrate layers and planted roof applications.

1.5mm or 2mm: Which Gauge Do You Need?

Use 1.5mm When

The application is a standard domestic or light commercial flat roof with a shallow membrane retention requirement, or a green roof with a growing medium depth up to approximately 100mm in a sheltered position. The 1.5mm retention trim covers the majority of domestic flat roof extension and light commercial green roof applications at a lower material cost.

Use 2mm When

The application involves a deeper or heavier substrate load (150mm or more), an exposed site where wind pressure on the trim face is above the standard sheltered baseline, a commercial project where the specification calls for 2mm gauge, or an intensive planted green roof where the substrate and loaded planting exerts meaningful lateral pressure on the trim face. The 2mm section provides greater rigidity and resistance to deflection under these conditions.

Key Benefits

• Heavier 2mm gauge for demanding applications: greater rigidity than the 1.5mm version under lateral substrate pressure, deeper growing media loads, and higher wind exposure.

• Drainage slots in the face: allows free water drainage through the trim face while retaining substrate, pebbles or ballast. Prevents water damming at the trim that could back up under the waterproofing layer.

• 8 sizes covering 50mm to 300mm face heights: face height matches the installed substrate or membrane depth at the perimeter. Base depth of 100mm or 150mm gives the correct fixing footprint for the load.

• Pre-drilled fixing holes in the base: for mechanical fixing with corrosion-resistant screws or bolts, or for bonding straps where adhesive bonding is specified.

• Any RAL colour powder coated: matches fascia, coping, guttering and other external aluminium components on the same building.

• 2.4m lengths with joiner connector: reduces joint count on longer runs. Joiner connectors allow straight joints and can be bent to form internal and external angles at corners.

• UK manufactured in Chelmsford: consistent quality and fast lead times for projects across Essex, the South East and the wider UK.

Technical Specifications

Material: 2mm grade aluminium

Length: 2.4m (2400mm) per section

Sizes Available: 50x100mm, 75x100mm, 100x100mm, 120x100mm, 150x150mm, 200x150mm, 250x150mm, 300x150mm

Size Notation: Face height x Base depth

Drainage Slots: Laser-cut slots in face for free water drainage while retaining substrate

Fixing Holes: Pre-drilled holes in base for mechanical fixing or bonding straps

Joiner Connector: Included with each 2.4m length for straight joints and angle formation

Finish: Polyester powder coating, external grade, any RAL colour

Fire Rating: A2-s1,d0 (non-combustible)

Applications: Green roof substrate retention, flat roof membrane retention, pebble ballast containment, intensive planted roofs

Compatible Systems: Single ply (TPO, PVC, EPDM), built-up felt, liquid-applied waterproofing

Price: £18.56 ex VAT per 2.4m length (£22.27 inc VAT)

Fabrication: Made in the UK, Chelmsford Essex

How Many Lengths Do You Need?

Measure the total linear metres where retention trim is needed. Divide by 2.4m and round up to the nearest whole number. Allow for a small expansion gap (approximately 2mm) between lengths at each joint.

• 12m perimeter: 12 / 2.4 = 5 sections exactly.

• 15m perimeter: 15 / 2.4 = 6.25 -- order 7 sections.

• 24m perimeter: 24 / 2.4 = 10 sections exactly.

• On complex roof plans with internal and external angles, the joiner connectors handle direction changes without cutting, so no additional allowance is needed for corners.

Installation Overview

Fix the retention trim to the flat roof perimeter before the membrane or substrate layer is installed. The base of the trim sits flat on the roof deck surface. Fix through the pre-drilled holes in the base using corrosion-resistant fixings at the centres specified by the waterproofing system manufacturer, or bond the base to the deck surface using a compatible adhesive where mechanical fixing is not appropriate for the deck construction.

Join adjacent lengths using the included connector. Slide the connector into the back of the trim profile at the joint position and leave a 2mm expansion gap between the section ends. For direction changes at corners, bend the connector by hand to the required angle: connectors can form both internal and external angles without separate corner pieces, which simplifies installation on complex roof plans.

Once all trim sections are fixed and joined, install the membrane or substrate layer. The top of the trim face should align with the finished surface level of the substrate or membrane upstand, containing the layer completely at the perimeter. For a complete flat roof edge detail at the exposed outer perimeter, use the aluminium drip trim at the drip edge alongside this retention trim at the upstand.

Where the 2mm Retention Trim Is Used

Extensive and Intensive Green Roofs

On green and living roofs, the retention trim contains the growing medium, substrate, or drainage layer at the roof edge while allowing water to drain freely through the slotted face. The 2mm gauge is the correct specification for intensive green roofs with a deeper planted substrate of 150mm or more, where the lateral pressure on the trim face from the loaded growing medium is higher than on a shallow sedum roof. For very deep planted roof applications with substrate over 250mm, specify the 300x150mm size and consult the waterproofing system designer for structural advice. For green roof maintenance access, the aluminium inspection chambers complement the retention trim system.

Exposed Site Flat Roofs

On flat roofs on buildings in exposed coastal, elevated or high wind-speed locations, the 2mm gauge provides greater resistance to wind-induced movement at the trim face. On a sheltered domestic flat roof in a suburban setting, the 1.5mm trim is adequate. On a commercial building near the Essex coast or on an open aspect site where wind loading has been calculated as higher than the standard sheltered baseline, the 2mm trim is the correct specification.

Commercial Flat Roofs

The 2mm section is the standard commercial specification for flat roof retention trims on office, retail, healthcare and light industrial buildings where a heavier-gauge edge detail is called for by the waterproofing contractor or the project specification. On commercial projects where the retention trim is ordered alongside parapet aluminium coping, fascia boards and guttering, specifying all external aluminium components from the same Chelmsford manufacturer gives colour-matched components in the same RAL across the full building envelope.

Ballasted Single Ply Roof Systems

On warm roof constructions with a loose-laid single ply membrane protected by a pebble ballast layer, the retention trim holds the ballast at the roof perimeter. The drainage slots prevent water from damming against the trim inside the ballast layer, and the wider base size options (150mm base) give the trim greater resistance to overturning under the weight and lateral pressure of a pebble ballast layer.

Why Aluminium for Flat Roof Edge Retention

Plastic and uPVC retention trims are widely available at lower initial cost, but they have a shorter practical service life on exposed flat roofs. UV degradation causes plastic to yellow and become brittle, particularly at low temperatures when the material is most likely to crack if a load is applied. On a green roof where the trim is permanently loaded by substrate and periodically impacted by maintenance work and foot traffic, a trim that becomes brittle within 10 to 15 years is not fit for a flat roof expected to last 25 years or more.

The 2mm aluminium retention trim carries a fire classification of A2-s1,d0. It is non-combustible and meets the current Building Regulations requirements for external products on residential buildings over 11 metres and on commercial buildings where non-combustible external materials are specified. Plastic trims do not meet this classification and cannot be used in these positions.

Why Choose Online Metal Store Ltd

All our aluminium retention trims are manufactured at the Chelmsford facility and powder coated in any RAL colour as a standard offering. For projects combining green roof retention trims with parapet coping, fascia boards, rainwater goods and flat roof accessories, ordering everything from the same source gives colour consistency and a single supplier contact for the full external aluminium specification. Contact our team at sales@onlinemetalstore.co.uk or call +44 7907 239290, Monday to Saturday 6:30am to 4:30pm, to discuss face height selection, confirm quantities, or get a quote for a complete flat roof edge package.

Related Products

• 1.5mm Aluminium Retention Trim – 2.4m Length -- the lighter gauge version for standard domestic flat roofs and shallow green roofs

• 1.5mm Aluminium Roof Edge Drip Trim – 3m Length -- the companion drip trim for the exposed outer edge of the flat roof

• Aluminium Inspection Chamber 300mm x 300mm (Deck Version) -- maintenance access point for green roof systems

• Aluminium Inspection Chamber 300mm x 300mm (Parapet Version) -- parapet-mounted access for living roof systems

• Aluminium Inspection Chamber 450mm x 450mm (Deck Version) -- larger access chamber for extensive green roofs

• Aluminium Copings range -- for the parapet wall coping that sits above the flat roof edge detail

• Colour Coded Sealant 300ml -- for additional weathersealing at trim joints where required

Frequently Asked Questions

What do the two dimensions in each size mean, e.g. 50x100mm?

The first figure is the face height: the vertical dimension of the exposed trim face that must match the depth of your substrate, growing medium or membrane upstand. The second figure is the base depth: the horizontal dimension of the flat base that sits on the roof deck. A deeper base gives a larger fixing footprint and better resistance to lateral pressure from a loaded substrate layer. For a 100mm deep growing medium, select a 100x100mm or 120x100mm size. For a 200mm deep planted substrate, select the 200x150mm size.

When should I use 2mm rather than 1.5mm?

Use 2mm when the substrate layer is deep (150mm or more), when the site is exposed or the specification calls for a heavier gauge, when the application is an intensive planted green roof with a loaded growing medium, or when the project is commercial and a 2mm section is specified. For standard domestic flat roofs and shallow sedum green roofs with a growing medium up to 100mm, the 1.5mm retention trim is normally adequate.

How do I handle corners without cutting the trim?

The joiner connector included with each length can be bent by hand to form both internal and external angles. Slide the connector into the back of the trim profile at the corner position and bend it to the required angle. This creates a neat, continuous trim around the corner without requiring the trim face to be mitred on site. On complex roof plans with multiple direction changes, the flexible connectors significantly simplify the installation.

Is the 2mm retention trim suitable for ballasted single ply roofs?

Yes. The trim retains the pebble ballast layer at the roof perimeter and the slotted face allows water to drain freely from the ballast layer. For ballasted roofs, the 150x150mm or larger sizes are generally recommended as the wider base provides better resistance to overturning under the weight and lateral movement of a pebble ballast layer. Check the base fixing specification with the waterproofing system manufacturer.

Can I powder coat the trim to match my coping?

Yes. Specify your RAL colour at order and the trim is powder coated in the same external grade polyester finish as all other OMS aluminium products. If you are ordering this trim alongside coping, fascia boards, drip trims and rainwater goods from Online Metal Store Ltd in the same RAL, ordering everything together means the components go through the same coating batch and arrive colour-matched.

Ready to Order?

Select your face height and base depth from the size dropdown, choose your RAL colour, and add to basket. For face height advice, substrate depth confirmation, or a quote for the full flat roof edge package including drip trims, retention trims and accessories, contact our team at sales@onlinemetalstore.co.uk or call +44 7907 239290, Monday to Saturday 6:30am to 4:30pm.

Custom Fabricated Item: Made to order at our Chelmsford facility. Confirm face height before ordering. Non-returnable unless faulty. See our Refund and Returns Policy for full details.