1.5mm Aluminium Retention Trim - 2.4m Length

A retention trim secures the edge of a flat roof waterproofing membrane and, on green and living roofs, retains the growing substrate or pebble ballast at the roof perimeter. This 1.5mm aluminium version is supplied in 2.4m lengths with 30 laser-cut drainage slots per metre so water flows freely through the trim face while substrate stays contained. A joiner connector is included with each length to bridge the gap between adjacent sections.

The trim is fixed to the perimeter structure of the flat roof before the membrane or substrate is installed. The flat base with six pre-drilled bonding holes allows the trim to be mechanically fixed or adhesive-bonded depending on the substrate and the waterproofing system specification. Available in any RAL colour as a polyester powder coat finish, and manufactured at the Online Metal Store Ltd facility in Chelmsford. Compatible with built-up felt, single ply (TPO, PVC, EPDM) and liquid-applied flat roof systems. For the longer 3m drip trim used at the exposed roof perimeter, see the separate product.

Retention Trim or Drip Trim: Which Do You Need?

These are two different products used at different positions on a flat roof. Confusing them is a common ordering error.

Retention Trim (This Product)

Used at the upstand edge of a flat roof or at internal perimeter positions to hold the waterproofing membrane down and prevent wind uplift. On a green roof, it retains the growing substrate or pebble ballast layer at the roof edge while letting water drain freely through the slots. The trim sits vertically at the perimeter, not at the exposed drip edge.

Drip Trim

Used at the exposed outer edge of a flat roof where water needs to drip clear of the fascia. It creates the visible drip detail at the roof edge and holds the membrane edge at the point where it meets the outside air. For the drip trim, see the 1.5mm Aluminium Roof Edge Drip Trim 3m Length.

Key Benefits

• Secures flat roof membrane: holds the waterproofing layer at the roof perimeter, preventing wind uplift and edge peel on flat and low-pitch roofs.

• 30 drainage slots per metre: laser-cut slots across the trim face allow rainwater to drain freely while retaining substrate, pebbles or ballast on green and living roofs.

• 6 bonding holes in the base: pre-drilled holes for mechanical fixing or bonding straps, giving a firm connection to the roof perimeter structure for additional wind resistance.

• Joiner connector included: each 2.4m length comes with a connector for bridging the joint between adjacent sections. Connectors also bend to form internal and external angles.

• 1.5mm aluminium, lightweight and rigid: strong enough to resist substrate pressure and wind loading at the roof edge without adding significant structural weight.

• Any RAL colour: powder coated in any RAL to match fascia boards, copings, gutters or other external aluminium components on the same building.

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

Technical Specifications

Material: 1.5mm grade aluminium

Length: 2.4m (2400mm) per section

Profile: Vertical face with flat perforated base

Drainage Slots: 30 laser-cut slots per metre (72 slots per 2.4m length)

Bonding Holes: 6 holes in base per 2.4m length for mechanical fixing or bonding straps

Joiner Connector: One connector included per 2.4m length for joining sections and forming angles

Face Heights: Multiple face heights available -- specify at order

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

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

Applications: Standard flat roof membrane retention, green roof substrate retention, living roof edge detail, pebble ballast retention

Compatible Systems: Built-up felt, single ply (TPO, PVC, EPDM), liquid-applied coatings

Fabrication: Made in the UK, Chelmsford Essex

Price: £14.72 ex VAT per 2.4m length (£17.66 inc VAT)

How Many Lengths Do You Need?

Measure the total linear metres of perimeter where retention trim is required. Divide by 2.4m and round up to the next whole number to get the minimum section count. Leave a small expansion gap (approximately 2mm) between sections and use the included joiner connector to bridge the joint.

• 10m perimeter: 10 / 2.4 = 4.17 -- order 5 sections.

• 24m perimeter: 24 / 2.4 = 10 -- order 10 sections.

• On a green roof where you need retention on all four sides, measure each side separately and add them together before dividing.

• Order a small number of additional sections if the layout requires angled cuts at corners, as cut sections cannot be returned.

Installation Overview

Fix the retention trim to the perimeter structure of the flat roof before the membrane or substrate is laid. The base of the trim sits flat against the roof deck or upstand. Fix through the pre-drilled bonding holes using corrosion-resistant screws or bolts at the recommended centres for the waterproofing system specification, or bond the trim to the substrate using a compatible adhesive where mechanical fixing is not appropriate.

Join adjacent lengths using the included joiner connector. Slide the connector into the back of the trim profile to bridge the joint between two sections. The connectors can also be bent by hand to form internal and external angles at corners, which avoids the need to site-mitre the trim at every directional change. Leave a 2mm expansion gap at each joint to allow for thermal movement along a long run.

Once the trim is fixed in position, lay the membrane or substrate up to the base of the trim face. On green roofs with a substrate layer, the trim face height should correspond to the installed depth of the growing medium or pebble layer so the trim retains the substrate flush with the top surface. Check the face height specification before ordering.

Where This Trim Is Used

Green and Living Roofs

The primary application. Green roofs require a perimeter edge trim that retains the growing substrate or pebble drainage layer at the roof boundary while allowing rainwater to drain freely through the face. The 30 slots per metre maintain positive drainage while the trim face holds the substrate in place under wind and loading conditions. The joiner connector allows the trim to navigate corners without cutting, simplifying installation on complex roof plans.

Flat Roof Membrane Retention

On standard flat roofs with a waterproofing membrane, the retention trim secures the membrane edge at upstand positions and internal perimeter details. It prevents wind uplift at the edge and gives the membrane a defined, fixed termination point at the roof boundary. Used alongside the drip trim at the exposed outer edge for a complete flat roof edge detail.

Pebble Ballast Retention

On flat roofs with a pebble ballast layer over a waterproofing membrane (typically a warm roof build-up with loose-laid single-ply), the retention trim holds the pebbles at the roof perimeter. The slotted face keeps pebbles contained while water from the ballast layer drains freely through the trim. The 1.5mm aluminium face is rigid enough to resist the lateral pressure of a pebble layer without deflecting.

New-Build and Refurbishment Flat Roofs

Used on domestic extensions, commercial flat roofs and refurbishment projects across Essex, the South East and the wider UK. On an NHBC new-build project with a green roof element, the retention trim is part of the edge detail specification. For a complete flat roof edge solution on a standard roof, the retention trim at the upstand works with the 1.5mm aluminium drip trim at the outer edge to give a properly detailed perimeter.

Why Aluminium Retention Trim

Plastic and uPVC edge trims are common but carry well-known limitations: they become brittle at low temperatures, fade and discolour under UV, and have a shorter service life on exposed flat roofs. Aluminium does not become brittle in cold weather, does not fade under UV, and the polyester powder coat finish does not degrade over the service life of the flat roof. On a project where the flat roof is expected to perform for 20 to 25 years, the edge detailing should be specified to the same standard as the membrane and insulation.

The fire rating of A2-s1,d0 makes this trim suitable for use on buildings of any height, including residential buildings over 11 metres where the Building Regulations now require non-combustible external wall and roof edge materials. Plastic trims typically have a lower fire classification and cannot be used in these positions.

Why Choose Online Metal Store Ltd

Manufactured at our Chelmsford facility and powder coated to any RAL colour to match the rest of the external aluminium specification on the building. For projects that combine flat roof edge trims, parapet coping, fascia and rainwater goods, ordering all components from the same Chelmsford source gives colour-matched components from the same coating process. Contact our team at sales@onlinemetalstore.co.uk or call +44 7907 239290, Monday to Saturday 6:30am to 4:30pm, to confirm face height requirements, discuss quantity, or get a quote for the full flat roof edge detail package.

Related Products

• 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) -- for green roof access points

• Aluminium Inspection Chamber 300mm x 300mm (Parapet Version) -- for parapet-mounted green roof inspection points

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

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

• Aluminium Copings range -- for the parapet wall coping that works alongside the flat roof edge detail

Frequently Asked Questions

What is the difference between a retention trim and a drip trim?

A retention trim holds the waterproofing membrane or substrate layer at the roof perimeter upstand, preventing wind uplift and containing green roof substrate. A drip trim sits at the exposed outer edge of the roof and creates a defined drip point that throws rainwater clear of the fascia. They serve different positions on the flat roof edge detail and are often used together. For the drip trim, see the 1.5mm aluminium roof edge drip trim.

What face height do I order?

The face height should match the installed depth of the substrate, growing medium or pebble ballast layer on a green roof, or the height required to secure the membrane at the upstand on a standard flat roof. Measure the installed depth of the layer that the trim needs to contain and specify that height at order. Multiple face heights are available. If you are not certain, contact our team before ordering as this is a made-to-order product.

How do I join two lengths together?

Each 2.4m length includes a joiner connector. Slide the connector into the back of the trim profile where two lengths meet to bridge the joint. Leave a 2mm expansion gap between the ends of the two sections before fitting the connector. The connectors can also be bent by hand to form internal and external angles at corners, avoiding the need to cut and mitre the trim at each direction change.

Is this product anodised or powder coated?

Powder coated only. The finish is an external grade polyester powder coat available in any RAL colour. These products are not anodised. Any reference to anodising on the current product page is a factual error. Powder coating is the only available finish.

Is this trim suitable for use on buildings over 11 metres?

Yes. The 1.5mm aluminium retention trim carries a fire classification of A2-s1,d0, which means it is non-combustible and compliant with the current Building Regulations requirements for external products on residential buildings over 11 metres and commercial buildings above certain heights. Plastic and uPVC trims typically cannot meet this classification.

Can I powder coat the trim to match my coping and fascia?

Yes. Specify your RAL colour at order and the trim will be powder coated in the same external grade polyester finish as the rest of the OMS product range. If you are ordering retention trim alongside parapet coping, fascia boards, drip trims and guttering from Online Metal Store Ltd in the same RAL, ordering all components together means they go through the same coating batch and arrive colour-matched.

Ready to Order?

Select your face height and RAL colour and add to basket. For quantity advice, face height confirmation, or to order as part of a full flat roof edge package alongside drip trims, copings and accessories, contact our team at sales@onlinemetalstore.co.uk or call +44 7907 239290, Monday to Saturday 6:30am to 4:30pm. Each length is manufactured to order at our Chelmsford facility.

Custom Fabricated Item: Made to order. Please confirm face height and check quantities before ordering. Non-returnable unless faulty. See our Refund and Returns Policy for full details.