Half-Round Gutter Installation
Complete guide to installing 112mm half-round guttering with fascia-mounted brackets, running outlets, and downpipe connections.
Safety Warning — Working at Height
Plan and risk-assess all work at height. Use a suitable working platform with collective fall protection where reasonably practicable. A ladder should only be used for low-risk, short-duration work where the assessment shows that equipment offering greater fall protection is not justified. Inspect and secure access equipment, keep three points of contact, do not overreach, and keep the area below clear.
What You'll Need
Tools
- Tape measure (minimum 5m)
- Spirit level or laser level
- String line and chalk
- Hacksaw or fine-tooth panel saw
- Drill/driver with pilot bits suitable for the fixings
- File or deburring tool
- Scaffold tower or secured ladder
- Pencil and marker
Materials
- 112mm half-round gutter lengths
- Fascia brackets
- Running outlets
- Half-round union brackets
- Stop ends (internal / external)
- 68mm round downpipe and fittings
- Downpipe clips (pipe brackets)
- Corrosion-resistant bracket fixings suitable for the fascia substrate
Technical Specifications
Technical basis: Kalsi document TDS-RW-HR-001, issue February 2026. Check the current Half Round Rainwater System data sheet before installation.
Step-by-Step Installation
1 Plan the Layout & Outlet Position
Confirm where each downpipe can discharge to an appropriate rainwater system. Determine the number and position of outlets from the effective roof area, local design rainfall, gutter fall and whether the outlet is at the end or centre of the run. Do not size the system from gutter length alone. Use the hydraulic capacities in the current Kalsi data sheet or ask Kalsi Technical to confirm the layout, then mark the outlet positions on the fascia.
2 Establish the Fall
Set the gutter to fall towards the outlet at no flatter than 1:600; Kalsi recommends 1:350 (approximately 3mm per metre). Fix a string line from the high point to the outlet. At the recommended fall, a 6m run drops approximately 18mm. Position the gutter so the projected roof discharge enters it cleanly, following the roof and eaves detail rather than relying on a universal offset from the tile line.
3 Fix the Running Outlet
The running outlet acts as a bracket and connects gutter to downpipe. Position it at the marked outlet point and align it with the string line. Fix only into a sound fascia substrate using compatible corrosion-resistant fixings, pre-drilling timber where needed. Check it across its width and ensure the outlet spigot points straight down.
4 Install Fascia Brackets
Starting from the running outlet, fix fascia brackets at no more than 600mm centres. Reduce the spacing to 300mm centres in areas of heavy snow or extreme weather conditions. Position support within 150mm of joints and fittings. Fix only into a sound substrate with compatible corrosion-resistant fixings, pre-drilling timber where needed. Keep every bracket aligned with the string line so the gutter is supported without being pinched.
5 Cut & Fit Gutter Lengths
Measure and cut gutter lengths square using a fine-tooth saw, then remove all swarf and burrs. Clip each length into the brackets in the sequence shown for the current Kalsi fittings. At every union, set the cut ends to the moulded insertion marks so the joint retains the specified 10mm expansion allowance. Do not over-tighten or otherwise restrain the gutter within its supports.
6 Join Gutter Lengths
Use the matching half-round union bracket to join gutter lengths. Check that its seals are present, clean and correctly seated. Set the gutter ends to the fitting marks to maintain the 10mm joint allowance, then engage the gutter without twisting the seal. Fix the union bracket securely to the fascia and check that the joint follows the fall line.
7 Fit Stop Ends
Fit the matching internal or external stop end required by the layout. Check that the seal is clean, seated and fully engaged. Do not glue a sealed rainwater joint or add sealant unless the current Kalsi component instructions specifically require it, because the joint must accommodate movement.
8 Install Downpipe
Connect the matching 68mm round downpipe to the outlet, using Kalsi offset bends where needed. Maintain at least 25mm wall clearance. Fit clips at 1500mm centres where practicable and never more than 1800mm centres, with support within 150mm of bends and joints. Connect the base to the designed rainwater drainage point; do not discharge where water could affect the building or neighbouring property.
9 Test the System
Run water through the completed system using a hosepipe at the high end. Check every joint for leaks. Verify the fall is correct — water should flow steadily towards the outlet with no pooling anywhere in the run. Check that the downpipe discharges cleanly into the gully or drain below. Rectify any issues before leaving site.
Common Mistakes to Avoid
- Insufficient fall — Do not install flatter than 1:600. A 1:350 fall is recommended and provides greater flow capacity.
- Ignoring expansion gaps — PVC-U expands and contracts with temperature changes. Failing to use the moulded insertion marks and leave movement allowance at unions will cause buckling in warm weather. See our hot weather expansion guidance.
- Bracket spacing too wide — Never exceed 600mm centres, reduce to 300mm in areas of heavy snow or extreme weather conditions, and add support within 150mm of joints and fittings.
- Sizing by run length alone — Outlet quantity and location must account for effective roof area, rainfall, fall and outlet position.
- Not deburring cut ends — Rough cut edges prevent union seals from seating properly, causing leaks at joints.
- Gluing joints — Half-round gutter systems use push-fit rubber seals, not adhesive. Gluing joints prevents expansion movement and causes damage.
Related Products
Need Technical Help?
Our technical support team can talk you through any installation query. Available Monday to Friday, 8am–5pm.
0121 693 0373 Contact Technical Support