MultiFlex Waste System: Flexible Push-Fit Advantages
MultiFlex waste pipe is a flexible push-fit waste system that uses corrugated or semi-flexible pipe sections to route waste through tight spaces where rigid pipe would require multiple bends and fittings. It is particularly useful for bathroom retrofits, kitchen upgrades, and any installation where the pipe route is awkward — behind vanity units, under baths with limited clearance, around structural obstacles, and through complex floor voids. The flexible sections connect to standard rigid push-fit fittings, giving you the routing flexibility of a hose with the connection reliability of a standard waste system.
Rigid waste pipe works perfectly when the route is straight or involves simple changes of direction. But in real-world bathroom and kitchen installations — especially in older properties being refurbished — the route is rarely ideal. Joists are in the wrong position, walls are not where you need them, and the trap-to-stack distance involves navigating around ductwork, cables, and structural members. MultiFlex pipe flexes around these obstacles without the accumulation of fittings that rigid pipe would require.
What MultiFlex Pipe Is
MultiFlex (or flexible push-fit) waste pipe is a corrugated PVC-U pipe with smooth-bore internal walls and a flexible outer wall. It can be bent by hand to navigate around obstacles, fitted through holes and chases that rigid pipe could not follow, and connected to standard push-fit or solvent weld waste fittings via adaptors.
Key Features
| Feature | Detail |
|---|---|
| Material | PVC-U (corrugated outer, smooth inner bore) |
| Sizes | 32 mm, 40 mm |
| Bend radius | Typically 75–100 mm minimum (varies by product) |
| Connection | Push-fit to standard rigid fittings |
| Flow performance | Comparable to rigid pipe on short runs |
| Temperature | Up to 75°C (standard domestic waste temperatures) |
| Colour | White or grey (depending on manufacturer) |
| Length | Sold in coils or cut lengths, typically 0.5–3 m |
Kalsi’s MultiFlex waste system includes flexible pipe and all the transition fittings needed to connect to standard rigid pipe systems.
When to Use MultiFlex
1. Under-Bath Connections
The space under a standard bath is typically 150–200 mm — too shallow for multiple rigid bends. MultiFlex pipe routes from the bath trap to the waste outlet in a single continuous run, bending as needed. This eliminates the common problem of having to assemble three or four rigid fittings in a space where you cannot see or reach what you are doing.
2. Behind Vanity Units
Back-to-wall vanity units and wall-hung basins have very limited space between the trap and the wall. Flexible pipe navigates this space without the need for short rigid sections and multiple fittings. It is also forgiving of slightly misaligned waste outlets — the flex accommodates minor offsets that would require an adaptor with rigid pipe.
3. Retrofit / Refurbishment
Connecting new fixtures to existing waste runs in older buildings often involves navigating around unexpected obstacles — original pipework, structural timbers, electrical cables. MultiFlex adapts to whatever route is available without the need for precise pre-measurement.
4. Through Floor Voids
Where the waste pipe must weave between joists, wiring, and other services in a floor void, flexible pipe routes through without notching joists or rerouting other services. This is a significant advantage in timber-frame properties where joist modification is restricted.
5. Shower Tray Connections
Low-profile shower trays have minimal clearance. MultiFlex pipe connects the shallow trap to the waste outlet with a smooth, gradual route rather than a series of tight rigid bends that may not physically fit in the available height.
6. Kitchen Appliance Connections
Dishwashers and washing machines discharge via a pump. The outlet hose typically connects to a flexible waste section that runs to the main waste pipe or gully. MultiFlex provides the flexibility needed while maintaining a proper push-fit sealed connection (unlike rubber hose clamps that can loosen over time).
7. Awkward Angles and Offsets
Where a fixture waste outlet does not align with the intended pipe run — common in older buildings where walls are not plumb and floors are not level — MultiFlex bridges the gap without requiring custom-cut short pipe sections and multiple couplings.
MultiFlex vs Rigid Pipe
| Feature | MultiFlex (Flexible) | Rigid Push-Fit |
|---|---|---|
| Routing flexibility | Bends by hand, navigates obstacles | Requires fittings for every direction change |
| Number of fittings | Fewer (the pipe IS the bend) | More fittings = more cost and more potential leak points |
| Flow performance (short runs) | Comparable | Slightly better (smoother bore) |
| Flow performance (long runs) | Reduced (corrugation creates turbulence) | Better for runs over 2 m |
| Blockage risk | Slightly higher (debris can catch on corrugations) | Lower (smooth bore throughout) |
| Cleaning access | Harder to rod (corrugations resist rods) | Easy to rod with standard tools |
| Appearance | Less neat (corrugated surface) | Clean, straight lines |
| Best use | Short runs, tight spaces, retrofits | Long runs, visible pipe, new installations |
The Honest Trade-Off
MultiFlex pipe sacrifices some flow efficiency and cleanability for routing convenience. The corrugated outer wall (even with a smooth-bore interior) creates slightly more friction than a perfectly smooth rigid pipe. On runs over 2 m, this can slow drainage marginally.
Best practice: Use MultiFlex for the awkward sections (under baths, behind vanities, through tight voids) and rigid pipe for the main runs. Transition between the two using standard push-fit connectors. This gives you the best of both systems — flexibility where you need it, performance where it matters.
Installation Tips
1. Support the Pipe
Flexible pipe needs more support than rigid — it can sag between clips, creating low points that trap water and debris. Clip at 300–400 mm intervals (closer than the standard rigid pipe spacing of 500–800 mm).
2. Maintain the Gradient
Even though MultiFlex bends easily, it still needs a consistent fall of 18–90 mm per metre. Use clips to maintain the gradient, especially where the pipe curves downward. A sagging section creates a water trap that will eventually collect debris and cause a blockage.
3. Avoid Kinking
MultiFlex has a minimum bend radius — typically 75–100 mm. Bending tighter than this kinks the pipe, restricting flow and creating a blockage point. If you need a tight 90° turn, use a rigid bend fitting and connect the MultiFlex to it.
4. Use Transition Fittings
Where MultiFlex meets rigid pipe, use the manufacturer’s transition fitting. Do not force flexible pipe directly into a rigid socket — the corrugated outer wall does not seal properly against a standard push-fit ring seal.
5. Keep Runs Short
MultiFlex works best on short runs (under 1.5 m). For longer distances, switch to rigid pipe for better flow performance and cleaning access.
6. Cut Cleanly
Cut MultiFlex pipe with sharp scissors or a fine-toothed hacksaw. A ragged cut on the corrugated surface prevents proper seating in the transition fitting. After cutting, check that the cut end is round (not crushed) and seats fully into the fitting.
7. Test Before Concealing
Run water through the system at full flow before boxing in or covering any MultiFlex sections. Check every joint for drips and every section of pipe for sagging. Fixing a leak in a concealed flexible run is significantly more difficult than on exposed rigid pipe.
Common Applications: Before and After
Under-Bath Connection
Without MultiFlex: Bath trap → 40 mm 90° bend → short straight → 45° bend → another short straight → wall penetration. Five components, four joints, assembled blind in a 150 mm gap.
With MultiFlex: Bath trap → MultiFlex flexible run → transition fitting → rigid pipe through wall. Three components, two joints, bent to shape by hand.
Behind Vanity Unit
Without MultiFlex: Bottle trap → short 32 mm straight → 90° bend → straight to wall. All assembled in a 100 mm gap between the vanity back and the wall.
With MultiFlex: Bottle trap → MultiFlex directly to the wall penetration. One connection, routed around whatever is behind the vanity.
Frequently Asked Questions
What is MultiFlex waste pipe?
MultiFlex is a flexible push-fit waste pipe system that uses corrugated PVC-U pipe which can be bent by hand to navigate tight spaces. It connects to standard rigid push-fit waste fittings and is used for short runs under baths, behind vanities, and through complex floor voids where rigid pipe would require multiple bends.
Is flexible waste pipe as good as rigid?
For short runs in tight spaces, flexible waste pipe is equally reliable. On longer runs (over 1.5–2 m), rigid pipe provides better flow performance due to its smoother bore. Best practice is to use MultiFlex for the awkward sections and rigid pipe for the main waste runs.
Can I use MultiFlex for a shower waste?
Yes. MultiFlex is particularly useful for shower tray connections where the limited clearance beneath the tray makes rigid pipe routing difficult. Run the MultiFlex from the shower trap to the nearest rigid pipe section, keeping the flexible run as short as practical.
Does flexible waste pipe block more easily?
It can, on longer runs. The corrugated internal surface creates slightly more friction than a smooth rigid pipe, and debris can catch on the corrugations over time. Keep MultiFlex runs short, maintain a good gradient, and ensure the fixture trap is accessible for cleaning.
Can I connect MultiFlex to standard waste fittings?
Yes, using the manufacturer’s transition fitting. MultiFlex pipe pushes into a transition connector that converts to a standard push-fit socket. This connects directly to any standard rigid push-fit fitting, tee, bend, or trap.
Related Kalsi Products
- MultiFlex Waste System — flexible pipe and transition fittings
- Waste Push-Fit System — rigid push-fit pipe for main runs
- Soil & Waste Range — complete system overview
- Installation Guides — step-by-step fitting instructions
- Technical Downloads — datasheets and product specifications
