Aluminium Roof Edge Drip Trim – 90° Internal Corner

Corners are where drip trim installations go wrong. Try to mitre a corner on site with tin snips and a hacksaw, and you end up with a gap, a rough edge, or a joint that lets water behind the fascia the first time it rains properly. It looks poor, it performs worse, and it takes twice as long as using the right component in the first place.

Our pre-formed aluminium drip trim internal corner takes all of that hassle out of the job. It is factory-made to an exact 90-degree angle, profile-matched to our drip trim system, and ready to fit straight out of the box. Whether you are finishing a flat roof on a house extension in Chelmsford, a garden office in Brentwood, or a commercial unit in Basildon, this corner gives you a clean, watertight turn every time.

Product Overview

This internal corner is a pre-formed 90-degree aluminium corner piece designed to connect two lengths of our aluminium roof edge drip trim where the roof edge turns inward. It matches the exact profile of the drip trim, so the two components slot together seamlessly with no trimming, filing or adjustment needed on site.

We stock the corner in a full range of sizes to match every drip trim in our system. Each corner can be ordered in mill finish or powder coated to any RAL colour, so it blends perfectly with the rest of the trim run. The pre-formed design eliminates the need for on-site mitre cutting, which saves time, produces a neater result and removes the risk of a poorly cut corner allowing water ingress.

Online Metal Store Ltd manufactures these corners at our factory in Chelmsford, Essex, on the same production line as the drip trim itself. That guarantees a precise profile match between the corner and the straight trim lengths, which is not always the case when you mix components from different suppliers.

Key Benefits

Factory-formed for a precise 90-degree angle

Cutting a clean mitre on site is difficult, even for experienced roofers. Aluminium is thin and can deform easily if the cut is not exact. A factory-formed corner eliminates that risk entirely. The angle is precise, the edges are clean, and the profile matches the trim perfectly. The result is a corner that looks like it was made in one piece.

Watertight corner detail

Internal corners on a flat roof are a natural collection point for rainwater. If the corner detail is not watertight, that is exactly where water will get behind the trim and into the fascia timber. A pre-formed corner with sealant at the joints gives you a reliable, lasting seal at the most vulnerable point on the roof perimeter.

Saves time on site

Cutting and fitting a mitred corner on site takes time: measuring, marking, cutting, filing, test-fitting, adjusting. A pre-formed corner goes in within minutes. On a job with four or more internal corners, that time saving is significant. For contractors working to tight schedules, it is one less thing to worry about.

Full size range

We stock the internal corner in every size that matches our drip trim range, from 75mm right up to 400mm. Whatever drip trim size is specified on your project, the matching internal corner is available. There is no need to improvise or cut down a larger corner to fit.

Powder coating in any RAL colour

If the drip trim is powder coated, the corner should be too. A colour-matched corner maintains the consistent appearance of the trim run around the full perimeter. On visible roof edges, especially at eye level on single-storey extensions, that consistency makes a noticeable difference to the finished look.

Made in Chelmsford alongside the full system

We manufacture these corners at our facility on Southend Road in Chelmsford, Essex. Because they come off the same line as the drip trim and internal joiners, the profiles are guaranteed to match. Orders go out quickly across the UK.

Technical Specifications

Material: High-grade aluminium

Type: Pre-formed 90-degree internal corner for aluminium drip trim

Available Sizes: Full range to match drip trim system (75mm to 400mm)

Finish Options: Mill finish as standard, or polyester powder coated (PPC) to any RAL colour

Compatible With: Online Metal Store Ltd aluminium roof edge drip trim system

Manufactured In: Chelmsford, Essex, United Kingdom

Custom Fabrication: Yes. Powder coated to order. Non-returnable unless faulty.

How to Fit a Drip Trim Internal Corner

Fitting is straightforward. Here is the process:

Position the pre-formed corner piece at the internal angle of the roof edge. Fix it to the timber substrate using screws or appropriate fasteners, ensuring it sits flush and level. Apply a bead of sealant along the inside edge of both return legs where they will meet the incoming straight trim lengths. Our colour coded sealant works well here, especially on powder coated corners where you want a colour-matched joint.

Slide an internal joiner into each leg of the corner to bridge the connection to the incoming trim lengths. Push the straight trim lengths onto the joiners, butt them up to the corner and fix them down. The sealant sandwiched between the joiner and the trim provides the waterproof seal, and the joiner keeps everything aligned. The result is a clean, continuous trim line that turns the corner without any visible break.

On roofs with multiple internal corners, repeat the process at each turn. Keep the sealant application consistent and make sure every joiner is centred across the joint so both the corner leg and the trim length get equal internal support.

Where Drip Trim Internal Corners Are Used

L-shaped and U-shaped flat roof extensions

The most common application. When a flat roof wraps around a building in an L or U shape, the roof perimeter has internal corners where the trim needs to change direction. A typical rear extension with a side return creates two internal corners. A wrap-around extension might have three or four. Each one needs a pre-formed corner piece.

Flat roofs around existing structures

Where a flat roof butts up against an existing wall, bay window, chimney breast or other projection, the drip trim needs to turn inward to follow the roof edge around the obstruction. Pre-formed internal corners handle these turns neatly. On these details, you may also need an external corner on the opposite side of the same projection.

Commercial flat roofs with complex perimeters

On larger commercial buildings, the flat roof perimeter often has multiple internal and external angles where the building footprint changes direction. Each internal angle needs a corner piece. On these jobs, specifying the complete system from a single supplier, including drip trim, internal corners, external corners and internal joiners, keeps the whole installation consistent.

Garden rooms, offices and outbuildings

Most garden rooms and home offices have simple rectangular footprints, which means the drip trim run is all straight lengths and external corners with no internal turns. However, more complex designs with recessed entrances, covered porches or L-shaped layouts do need internal corners. The pre-formed piece keeps the finished look clean and professional, which matters on a building that is often highly visible from the house.

Frequently Asked Questions

What is a drip trim internal corner used for?

A drip trim internal corner is a pre-formed 90-degree aluminium corner piece that connects two lengths of drip trim where the roof edge turns inward. It provides a precise, watertight corner detail without the need for on-site mitre cutting. Internal corners are needed wherever the flat roof perimeter changes direction at an inward angle, such as at L-shaped extensions, U-shaped buildings or where the roof wraps around an existing structure.

Do I need a pre-formed corner or can I mitre on site?

You can technically mitre a corner on site, but the result is almost always inferior to a factory-formed piece. On-site mitres are difficult to cut cleanly in thin aluminium, they leave exposed cut edges, and they rarely achieve a tight, gap-free joint. A pre-formed corner gives you a precise angle, clean edges and a guaranteed profile match to the trim, all without any cutting on site. It saves time and produces a better result.

What size drip trim internal corner do I need?

The corner size must match the drip trim size. If you are using a 150mm drip trim, order the 150mm internal corner. We stock corners in a full range of sizes to match every trim in our system. If you are unsure which drip trim size is installed or specified, measure the visible front face of the trim and order the corresponding corner.

How do you fit a drip trim internal corner?

Fix the pre-formed corner to the timber substrate at the internal angle. Apply sealant along the inside of each leg. Slide an internal joiner into each leg of the corner to bridge the connection to the incoming straight trim lengths. Push the trim lengths onto the joiners, butt them against the corner and fix them down. The joiner keeps everything aligned, and the sealant provides the waterproof seal.

Can drip trim internal corners be powder coated?

Yes. We offer powder coating in any RAL colour. The corner should always be colour-matched to the rest of the drip trim run for a consistent appearance. Select your RAL colour when you order and we will fabricate the corner to match.

Part of a Complete Drip Trim System

This internal corner is one component in a complete aluminium drip trim system manufactured by Online Metal Store Ltd. The full system includes the drip trim in 3 metre lengths, internal joiners for splicing straight runs, internal corners (this product) and 90-degree external corners for outward turns. Ordering all the components together from the same manufacturer guarantees that every profile matches precisely, which makes installation faster and the finished result cleaner.

The entire system is available in a full size range and can be powder coated to any RAL colour across all components. Whether you are specifying for a small domestic extension or a large commercial flat roof, the complete system is available from stock with fast UK-wide delivery from our Chelmsford factory.

Standards and Compliance

Our aluminium drip trim system, including this internal corner, is manufactured from high-grade aluminium and powder coated with external-grade polyester paint for long-lasting durability. The system is suitable for use on flat roofs in accordance with BS 6229:2018 (flat roofs with continuously supported flexible waterproof coverings) and is compatible with EPDM, single ply, felt and liquid-applied membrane systems. For new-build projects, check the relevant NHBC Standards chapter for guidance on acceptable perimeter trim details.