How to Connect New Guttering to Old (Adaptor Guide)
To join new guttering to existing, you need an adaptor fitting that bridges the difference in profile, size, or material between the two systems. Most PVC-U manufacturers produce universal adaptors, cast-iron-to-PVC conversion kits, and transition fittings that let you connect a new section of guttering to an older system without replacing the entire run. The key is identifying your existing gutter profile and size accurately before ordering — the wrong adaptor will not seal properly and will leak.
This is one of the most common practical challenges on roofline and rainwater jobs. You are replacing one elevation’s guttering but the return along the side runs into an older system in good condition. Or you are adding guttering to an extension and need to connect it to the existing house gutter. Or the original cast iron has been partly replaced with PVC over the years and now nothing matches. Whatever the scenario, adaptors are the solution.
Why Direct Connection Rarely Works
PVC gutter systems are proprietary — each manufacturer’s fittings are designed to work with their own range. Mixing brands causes problems because:
- Profile shapes differ — a half-round from one manufacturer may be 112 mm, another may be 110 mm
- Union clips are sized to specific profiles — they grip the gutter’s front and rear edges, which vary
- Seal positions differ — gaskets designed for one system do not align with another
- Wall thicknesses vary — even same-size gutters from different makers may have different thicknesses
This means you cannot simply push new gutter into old fittings and expect a watertight joint. You need either a purpose-made adaptor or a universal connector.
Types of Adaptor
1. Profile-to-Profile Adaptors
These connect two different PVC gutter profiles — for example, joining a half-round run to a square-line run. They are shaped to match one profile on each end.
Common combinations:
- Half-round to square-line
- Half-round to ogee
- Square-line to deep-flow
These are available from most major PVC-U manufacturers and sit at the junction between the two profiles, typically at an external or internal angle where the transition is least noticeable.
2. Size Adaptors
These connect same-profile gutters of different sizes — for example, transitioning from 112 mm half-round to 150 mm half-round where additional capacity is needed.
3. Cast Iron to PVC Adaptors
Purpose-made fittings that connect PVC gutter to existing cast-iron gutter. They typically clamp onto the cast-iron gutter end and provide a PVC socket on the other side.
Key features:
- Adjustable clamp mechanism to accommodate cast-iron size variations
- Rubber gasket to create a watertight seal against the cast-iron surface
- Available for both gutter-to-gutter and downpipe-to-downpipe connections
4. Universal Connectors
Flexible adaptor strips or universal unions that can accommodate a range of gutter widths and profiles. These are the fallback option when no specific adaptor exists. They use adjustable clamps and sealing strips to bridge the gap between dissimilar systems.
5. Downpipe Adaptors
For connecting new PVC downpipes to existing pipes of different sizes or materials:
- 68 mm to 65 mm reducer/expander
- Round to square transition
- Cast-iron to PVC sleeve adaptor
- Push-fit to solvent-weld transition
How to Identify Your Existing Gutter
Before ordering any adaptors, you need to know exactly what you have. Here is how to identify the existing system:
Step 1: Material
| Material | Visual Clues |
|---|---|
| Cast iron | Very heavy; dark grey/black; often flaking paint; rust visible; bolted joints |
| PVC-U | Lightweight; uniform colour; clip or union joints; smooth surface |
| Aluminium | Lightweight; metallic appearance; often painted; riveted or sealed joints |
| Asbestos cement | Heavy; grey; fibrous texture; older properties (pre-1980s) |
Warning: If you suspect asbestos-cement guttering (common on 1950s–1970s council housing and commercial buildings), do not cut, drill, or break it. Asbestos fibres are a serious health hazard. Engage a licensed asbestos removal contractor.
Step 2: Profile Shape
- Half-round — semi-circular cross-section
- Square-line — rectangular with flat front and back
- Ogee — S-shaped decorative front, flat rear
- Deep flow — deeper than standard, various shapes
- Box — rectangular channel, usually industrial
Step 3: Measure the Size
| Measurement | How |
|---|---|
| Width (across the top) | Measure inside edge to inside edge |
| Depth (top to bottom) | Measure at the deepest point |
| Wall thickness | Measure with a caliper if possible |
| Downpipe diameter | Measure the outside diameter |
Common sizes:
- Mini: 76 mm wide
- Standard half-round: 112 mm wide (some older systems: 110 mm or 115 mm)
- Square-line: 114 mm wide
- Ogee: 116 mm wide
- Deep flow: 115–170 mm wide
Step 4: Identify the Manufacturer (If Possible)
Some PVC gutter has the manufacturer’s name moulded into the inside surface. Check the inside of a union clip or the underside of a stop end. If you can identify the brand, ordering matching fittings or the correct adaptor is straightforward.
Connecting to Cast Iron — Step by Step
This is the most common adaptor scenario — replacing part of an old cast-iron system with new PVC while retaining some original sections.
Method 1: Cut and Connect at the Cast Iron End
- Decide where the transition will be — ideally at a point where you can make a clean cut on the cast iron
- Cut the cast iron with a hacksaw or angle grinder with a metal-cutting disc. Support the off-cut to prevent it dropping and breaking
- Clean the cut end — remove rust, loose paint, and burrs
- Slide the cast-iron adaptor over the cut end. Tighten the clamp to grip the cast iron and seal with the rubber gasket
- Connect the PVC gutter to the other end of the adaptor using a standard union clip
Method 2: Use the Existing Cast-Iron Joint
- Remove the bolts at an existing cast-iron joint
- Separate the sections
- Clean the remaining cast-iron gutter end
- Fit the adaptor over the cast-iron end
- Connect new PVC
Downpipe Transition
Cast-iron downpipes are typically 63 mm, 75 mm, or 100 mm diameter — different from PVC standards. Use a purpose-made cast-iron-to-PVC downpipe adaptor:
- Cut the cast-iron downpipe at a convenient height
- Fit the adaptor sleeve over the cut end (or into the existing socket)
- Push-fit the PVC downpipe into the adaptor
Connecting PVC to PVC (Different Brands or Ages)
If the existing PVC gutter is from a different manufacturer or era:
Option A: Find the Matching System
If you can identify the brand and it is still in production, order matching fittings. Many PVC-U gutter profiles have been standard for decades, so even 20-year-old systems may be compatible with current production.
Option B: Universal Union
If the profiles are similar but not identical, a universal union connector with an adjustable seal may bridge the gap. These work by gripping the outside of each gutter length with an adjustable band, sealed by a continuous rubber gasket.
Option C: Transition at an Angle
Replace gutter from the nearest corner — fit an angle piece that transitions between the two profiles. This gives the cleanest result because the change happens at a natural break point rather than mid-run.
Option D: Replace the Full Run
If the existing gutter is in poor condition and no adaptor works cleanly, replacing the full elevation with new matching guttering is often the most practical option. The cost difference between one adaptor and 8 metres of new gutter may be only £15–25 in materials.
Tips for a Clean Connection
- Do not force it. If an adaptor does not fit comfortably, you have the wrong size or profile. Forcing a poor fit will leak.
- Always use a seal or sealant. Even well-fitting adaptors benefit from a bead of silicone sealant inside the joint as extra insurance.
- Allow for expansion. The PVC side of the connection will expand and contract with temperature. Do not glue PVC gutter joints (unless using solvent-weld fittings specifically designed for it).
- Support both sides. Fit a bracket within 150 mm of each side of the adaptor. Unsupported joints sag and leak.
- Test with water. Before finishing, run a hose through the system and watch the adaptor joint closely. Fix any drips now, not when it is raining.
Product Compatibility
Kalsi Plastics’ rainwater systems are manufactured to standard UK dimensions, making them compatible with a wide range of existing PVC-U gutter systems. The half-round, ogee, and square-line profiles use industry-standard sizing, so adaptors and transition pieces work reliably.
For cast-iron conversion projects, the cast-iron effect range provides both the aesthetic match and practical transition fittings to modernise a period property’s rainwater system incrementally.
Frequently Asked Questions
How to join new gutters to existing?
Use an adaptor fitting designed for the specific combination — profile-to-profile, size-to-size, or material-to-material (e.g., cast iron to PVC). Identify your existing gutter’s profile, material, and dimensions first, then source the correct adaptor. Always seal the joint and support it with brackets on both sides.
Can I connect PVC guttering to cast iron?
Yes. Purpose-made cast-iron-to-PVC adaptor fittings clamp onto the end of the cast-iron gutter and provide a PVC socket on the other side. These are available for both gutter and downpipe connections. Clean the cast-iron surface before fitting to ensure a good seal.
Do I need to replace all the guttering at once?
Not necessarily. If sections are in good condition, you can replace only the damaged or undersized parts and connect the new to the old using adaptors. However, if the entire system is ageing and showing multiple points of failure, a full replacement is usually more cost-effective and gives a consistent appearance.
What if I can’t identify my existing gutter brand?
Measure the profile width, depth, and shape. Take a photo and bring it (or a sample fitting) to your builders’ merchant. Most experienced merchants can identify common systems by sight. If the profile is non-standard or discontinued, a universal adaptor is the fallback option.
Can I mix different gutter profiles on the same house?
You can use different profiles on different elevations (e.g., ogee on the front, half-round on the rear) as long as each elevation is a self-contained run. Where two runs meet at an angle, both must be the same profile. Use profile-to-profile adaptors only if no other layout works.
