Aluminium Threshold Drain and Underground Rainwater Pipe Installation
Step-by-step guide made from Tidybricks patio restore's YouTube video.
Install aluminium threshold drainage channels along exterior house walls and doorways to prevent water pooling. Splice T-junctions and Y-junctions into existing underground rainwater pipes using slip collars, backfill with 10mm chippings, and bed the threshold drain channels and corner pieces onto a 50mm mortar base.
The full step-by-step Spetz Specs for this project (every part, size, diagram and step, free): Aluminium Threshold Drain and Underground Rainwater Pipe Installation Spetz Specs.
- Difficulty: intermediate
Safety
- Secure short lengths of PVC pipe firmly before beveling or cutting with an angle grinder—holding a small pipe steady with your boot next to a spinning diamond blade carries a high risk of kickback and foot injury.
- Wear eye, ear, and respiratory protection when cutting PVC pipe, aluminium channels, or masonry with an angle grinder.
Must know before you build
- Use a slip collar without a center ridge for splicing into existing fixed underground pipes.
- Connect only to rainwater pipes; foul sewer connections require a trap to prevent odors.
- Bevel cut all PVC pipe ends to prevent rubber ring seal displacement during assembly.
- Set threshold drain channels two courses below the damp proof course (DPC) finished height.
- Orient Y-junctions and swept T-junctions to flow with water toward the manhole.
- Excavate a trench wide and deep enough to maneuver slip collars and fittings easily.
Tools
- Spirit level (Box beam level)
- Shovel (Square-mouth shovel)
- Marker pen (Permanent marker, Sharpie) (optional)
- Rubber mallet (Soft-faced mallet)
- Angle grinder (Corded angle grinder with diamond cutting disc, Makita)
- Wheelbarrow (Single-wheel builder's wheelbarrow) (optional)
Materials
- Underground drainage pipes and fittings (PVC underground drainage pipe, T-junctions, Y-junction, 45-degree and 90-degree bends, 100mm)
- Aluminium threshold drain channels (Slimline aluminium channel drain with removable metal grating, Polychannel)
- Threshold drain outlet connector (Bottom outlet connector, Polychannel)
- 10mm chippings (Pea gravel / stone chippings, 10mm)
- Threshold drain 90-degree corner pieces (Internal and external 90-degree corner channel units, Polychannel)
- Soluble pipe lubricant (1 tub, Water-soluble pipe joint lubricant, OSMA, 0.5 kg, Part No. 4D391)
- Pipe slip collars (Double-socket repair coupler without center ridge, 100mm)
- Threshold drain connector clips (Channel joiner bracket, Polychannel)
- Cement mortar mix (Sand and cement mortar, 50mm bed depth)
- Threshold drain end caps (Channel end cap, Polychannel)
Step by step
Step 1: Splice into the first rainwater pipe and backfill
Cut into the existing underground rainwater pipe, install a T-junction and vertical riser using a slip collar and pipe lubricant, test-fit the threshold outlet, and backfill the trench with 10mm chippings.
- Position corner drain and cut existing rainwater pipe. Hold the corner threshold drain and outlet against the wall to mark the vertical drop location, then cut out a section of the existing underground rainwater pipe and bevel the cut edges.
- Tip: Smooth and bevel the cut end of the existing underground pipe as much as possible in situ so the new fitting slides on without displacing the rubber seal.
- Splice T-junction using a slip collar. Push the T-junction and angled bend onto the right-hand pipe, slide a ridge-free slip collar onto the cut pipe section, align the two pipe ends, and tap the slip collar back over the joint with a rubber mallet.
- Tip: Choose a slip collar without an internal center stop ridge so it can slide completely onto one pipe and then slide back to bridge the cut joint.
- Common mistake: Grabbing a standard double-socket coupler with a center stop ridge, which cannot slide past the pipe end to close a butt joint in situ.
- Fit vertical riser pipe and backfill with 10mm chippings. Insert a short vertical riser pipe into the top of the T-junction, test-fit the threshold corner outlet over the riser, and backfill around the underground pipework with 10mm pea gravel chippings.
- Tip: Backfill around the newly connected underground PVC pipes with 10mm chippings immediately after completing the joint to support and protect the pipework.
Step 2: Connect the Y-junction and second corner drainage pipes
Assemble and splice a Y-junction, angled bends, and a secondary T-junction into the rainwater pipes on the opposite side of the building using beveled pipe sections and slip collars.
- Dry-fit the Y-junction and angled corner riser. Attach a Y-junction and angled bends to the existing downpipe branch so the new vertical socket sits right beneath the building corner, and cap the open pipe end temporarily with a plastic lid to keep soil out.
- Tip: Place the plastic lid from the pipe lubricant tub over the open underground pipe end while excavating so soil and stones do not fall inside.
- Bevel the short connector pipe with an angle grinder. Cut a short bridging section of 100mm PVC pipe and use an angle grinder or rasp to chamfer the outer edges of both ends.
- Tip: Always chamfer the outer edge of cut PVC pipes with a grinder or rasp before sliding slip collars over them.
- Watch out: Hold short pipe pieces securely when beveling with an angle grinder to prevent the blade from catching close to your foot or hand.
- Splice the Y-junction and second T-junction and backfill. Fit the short bridging pipe between the Y-junction and the existing drain using two slip collars, mark the pipe center so the collars sit evenly across both joints, install the second T-junction between the downpipes, and backfill with chippings.
- Tip: Draw a marker line on the pipe to show where the edge of the slip collar should stop so you know the collar is centered evenly over the joint.
- Common mistake: Not excavating enough soil around and underneath the pipe ends before trying to slide the slip collars into place.
- Common mistake: Installing Y-junctions or T-junctions facing against the downstream flow direction.
Step 3: Bed and connect the threshold drain channels in mortar
Set the aluminium threshold drain channels, joiner brackets, end caps, and 90-degree corner pieces onto a 50mm mortar bed two courses below the damp proof course and align the outlets over the underground risers.
- Notch the first corner channel and set over the outlet pipe. Cut a drainage notch in the base of the threshold channel where it meets the corner unit, bed the channel on mortar over the vertical riser pipe, and cover any exposed pipe opening with plastic so mortar cannot fall inside.
- Tip: Cover any exposed gap where the wider 100mm underground pipe extends past the narrow threshold channel so cement mortar cannot fall into the drain.
- Join and level straight channels along the doorway. Lay a 50mm bed of cement mortar along the wall, clip the channel joiner brackets half-on and half-off each section, fit end caps before setting the channels down, and tap the channels level two brick courses below the damp proof course (DPC).
- Tip: Lay a 50mm bed of cement mortar underneath the threshold channels and set the finished height two courses below the damp proof course (DPC).
- Tip: Fit end caps and connector brackets onto the threshold channel sections before bedding them into the mortar.
- Common mistake: Forgetting to attach the end caps or joiner clips before setting the adjacent channel into mortar, forcing you to lift the bedded channel back up.
- Cap the end run by the downpipe. Attach a plastic end cap to the final straight length of threshold drain, press the channel into the mortar bed up against the rainwater downpipe, and level and straighten the run.
- Fit the 90-degree corners and cover the oversized pipe opening. Cut a 190mm filler piece to bridge between the straight channel and the 90-degree internal corner unit, bed the corner assembly in mortar, and place a flat concrete block piece with mortar over the exposed edge of the 100mm underground pipe.
- Tip: When cutting a threshold channel to length, aim the cut between the grate slot groups if possible so the grate looks uniform next to the corner piece.
- Tip: Bridge any exposed portion of the 100mm underground socket that sticks out past the narrow threshold drain with a flat piece of concrete or paving and seal around it with mortar.