How to Install an Aluminium Box Gutter: A Step-by-Step Guide
Getting the Installation Right
An aluminium box gutter that is correctly sized, correctly installed, and correctly jointed will give 40 to 50 years of service without significant maintenance. An aluminium box gutter that is correctly sized but poorly installed can fail at the joints within a season or two, allowing water to track behind the fascia or down the wall face in exactly the places it was designed to prevent.
Box gutter installation is a straightforward process for anyone who is comfortable working at height with hand tools. The critical details are in the planning before a single screw goes in: the bracket positions, the fall toward the outlet, the expansion gaps at joints, and the sealant application. Get these right and the rest follows naturally. Rush them, and you will be back on a ladder before long.
This step-by-step guide covers the complete installation process for an aluminium box gutter system on a standard domestic or light commercial application.
What You Will Need
Aluminium box gutter sections in the required size and RAL colour
Outlet fitting to connect to the downpipe
Stop ends for the gutter run terminations
External or internal corner fittings for any directional changes
Fascia brackets at the correct centres, or rafter brackets if fixing directly to timber
Stainless steel screws, typically M5x25mm for fascia-fixed installations
Low-modulus neutral cure silicone sealant suitable for aluminium
A hacksaw or fine-tooth chop saw for cutting gutter lengths to size
A spirit level, tape measure, and chalk line
Touch-up spray paint in the matching RAL colour for cut ends
Online Metal Store Ltd supply the full aluminium box gutter system including all fittings and accessories from their Chelmsford base in any RAL colour.
Step 1: Plan the Layout and Calculate the Fall
Before any materials go up, mark out the full gutter run on paper. Note the position of each outlet, each corner, each stop end, and each joint between gutter lengths. Calculate the total run from the high point to each outlet and determine the fall needed.
A fall of 1:600 toward each outlet is the standard recommendation: 1mm of drop for every 600mm of horizontal run. On a 6 metre run, that is 10mm total drop from the high end to the outlet. Mark this on the fascia with a chalk line before fitting any brackets, so you have a fixed reference line to work to throughout the installation.
Place outlets as close as possible to the low end of each run. On longer runs with a central outlet, the fall runs in both directions from the two ends toward the centre outlet. Always position the outlet before setting out the bracket fall, because the outlet position is determined by the location of the downpipe below it, not by the bracket layout.
Step 2: Fix the Brackets
Fascia brackets for aluminium box guttering are fixed at maximum 750mm centres along the fascia face, with an additional bracket positioned no further than 375mm from any joint, outlet, or corner fitting. This bracket spacing ensures the gutter is adequately supported under the weight of water and debris and that the loading at fixings is within the capacity of a standard timber fascia substrate.
Fix the first bracket at the high end of the run and the last bracket at the outlet end, setting both on the chalk fall line. Stretch a string line between these two fixed points and set the intermediate brackets to the string, maintaining the consistent fall across the full run. Use stainless steel screws throughout: electrolytic corrosion between aluminium brackets and steel fixings in a wet environment will cause the fixing to fail well before the aluminium component shows any sign of deterioration.
Step 3: Install the Outlet and Stop Ends
With the brackets in place, fit the outlet first. The outlet drops through or clips to the base of the gutter section at the outlet position and connects to the downpipe below. Seal around the outlet connection using low-modulus silicone sealant and allow it to achieve initial cure before running water over the joint.
Fit stop ends at each open termination of the gutter run. Stop ends for aluminium box guttering are typically fixed with sealant and small screws into the end face of the gutter section, and should be sealed on the internal face before the gutter is put into service. An unsealed stop end will leak immediately in any rainfall event, so it is worth taking the two minutes to seal it properly during installation rather than having to access it again after the system is live.
For corner sections where the gutter run changes direction, 90-degree and 135-degree corner fittings are available to suit the roof geometry. These are sealed into the gutter run in the same way as joint connectors, with the expansion gap principle observed on both sides of the fitting.
Step 4: Join the Gutter Sections
Gutter sections are connected using union connectors or joggle joints, depending on the system. In both cases, the critical requirement is the expansion gap: leave 3 to 4mm between the ends of adjacent gutter sections at each joint. This gap accommodates the thermal expansion of the aluminium across the temperature range it will experience in service, which on a UK roofline is typically from around minus 10 degrees Celsius in winter to plus 50 degrees Celsius on a hot summer roof surface.
Apply low-modulus silicone sealant to the joint connector surfaces before assembly, following the sealant manufacturer's instructions on application temperature and curing time. Low-modulus sealant remains flexible as the joint moves with thermal expansion, maintaining the watertight seal without cracking or debonding. Never use a rigid adhesive or high-modulus sealant at box gutter joints: the movement will cause it to fail, and it will take the watertight seal with it.
Secure the joint connector with the fixing bolts or clips provided for the specific system, applying consistent pressure to ensure even sealant compression around the full perimeter of the connector.
Step 5: Treat Cut Ends
Any gutter section or fitting that has been cut to length on site will have a raw aluminium cut face at the end. Apply touch-up spray paint in the matching RAL colour to all cut ends before installation. This step takes seconds per cut but ensures the end face of every length is finished and protected, maintaining the visual quality of the installation and preventing any galvanic issues at exposed cut faces in contact with fixings or adjacent materials.
Step 6: Connect the Downpipe
With the gutter run complete, connect the downpipe to the outlet fitting. Box gutter outlets are available for both round and square downpipes, so the downpipe profile needs to match the outlet type specified. Downpipe brackets should be fixed at maximum 1.5 metre centres and the pipe run kept as vertical as possible, with any necessary offsets made using standard offset fittings rather than forcing the downpipe out of plumb.
Once the downpipe is connected, test the whole system with a garden hose from the high end of the gutter run, checking all joints, the outlet connection, and the downpipe connections for any leaks before completing the job.
Conclusion
Aluminium box gutter installation rewards careful planning and attention to the critical details: the fall toward outlets, the bracket spacing, the expansion gaps at joints, and the sealant application. A system that is correctly installed to these principles will perform without intervention for decades. One where the joints have been rushed or the expansion gaps omitted will make its displeasure known at the first sustained rainfall.
For aluminium box gutter systems, fittings, and all the accessories needed for a complete installation, Online Metal Store Ltd supply UK-manufactured components from their Chelmsford base with the full system available in any RAL colour.
Frequently Asked Questions
What sealant should I use for aluminium box gutter joints?
Low-modulus neutral cure silicone sealant is the correct product for aluminium box gutter joints. Low-modulus means the cured sealant remains soft and flexible, which allows it to move with the thermal expansion and contraction of the aluminium without cracking or losing adhesion. High-modulus sealant sets much harder and will fracture as the joint moves over temperature cycles, leading to leaks at the joint within a relatively short time. The sealant must also be neutral cure rather than acetoxy cure: acetoxy sealants release acetic acid during curing, which can accelerate corrosion at aluminium surfaces. Most gutter manufacturers specify or supply a compatible low-modulus silicone for use with their system.
How far apart should box gutter brackets be spaced?
For aluminium box guttering fixed directly to a fascia board, brackets at 750mm maximum centres is the standard specification, with an additional bracket required within 375mm of any joint, outlet, or corner fitting. This spacing ensures the gutter is adequately supported under the full weight of a water-filled gutter in a blocked or overflowing scenario and keeps deflection between brackets within acceptable limits. In exposed locations with high wind loads or in areas with heavy snow loading, reducing the bracket spacing to 600mm provides additional support against wind uplift and overloading.
Can I cut aluminium box gutter sections on site?
Yes. Aluminium box gutter sections cut cleanly with a fine-tooth hacksaw or a chop saw fitted with a blade suitable for non-ferrous metals. Mark the cut line clearly with a pencil or scribe before cutting, secure the gutter section so it cannot move during the cut, and use smooth steady strokes rather than forcing the cut. After cutting, deburr the cut edge with a fine file and apply touch-up spray in the matching RAL colour to the raw aluminium face before installation. A clean, careful cut produces an end face that mates properly with the joint connector and maintains the watertight seal at the joint.
