How to Pour Concrete Around Drainage or Service Pipes

✔ Step-by-Step: How to Pour Concrete Around Pipes or Drainage

1. 1

   Plan Pipe Positions Before Setting Mesh or Rebar

   Pipes and services must be in place before any mesh or reinforcement is fixed. Mark their locations on the formwork and sub-base so you can cut or bend rebar around them if needed. Use spray or chalk lines to trace pipe runs. Don’t bend pipework around mesh — design your reinforcement around the services. Planning this early keeps the layout tight and reduces problems during the pour.

2. 2

   Sleeve or Wrap Pipes for Movement

   Pipes expand and contract — especially plastic. Always sleeve them using a larger pipe, insulation wrap, or flexible ducting. This allows slight movement without stressing the concrete. For 110mm drainage, a 160mm sleeve works well. The sleeve should extend at least 50mm above and below the slab. If you’re pouring up to an inspection chamber or fixed collar, mask it with formwork or foam to avoid bonding solid to the structure.

3. 3

   Support Pipes So They Don’t Float or Shift

   Pipes must be fully supported before pouring. Use concrete blocks, sandbags, formwork, or suspended ties to hold them at the correct height. Avoid fixing them to reinforcement mesh — it causes shifting when concrete flows. For vertical pipes (like soil stacks), use formwork or timber to keep them plumb. Don’t assume the concrete will “hold them still” — it won’t if they float up or tilt mid-pour.

4. 4

   Reinforce Around Openings and Edges

   Concrete is weakest where it’s interrupted — and pipes create stress points. Reinforce around them by cutting mesh to fit and tying extra rebar around each pipe. For larger ducts or grouped services, form a rebar frame to resist cracking. Around floor drains or gulleys, use a light cage or box to brace the area. Never leave an unreinforced void in the middle of the slab — it’ll crack clean through under load or thermal change.

5. 5

   Seal Where Concrete Meets Services

   Once the pipe or chamber is placed and supported, use expanding foam, clay, or compressible filler to lightly pack the gap between it and the formwork. This creates a soft zone to absorb shrinkage and stops the concrete bonding too tightly. After curing, this joint can be sealed with flexible sealant (e.g. silicone or Sika Flex) for water/damp control. This is especially important around drainage runs or external services.

6. 6

   Pour with Care Around Openings

   During the pour, slow down when working near pipes or drains. Use a shovel or trowel to guide concrete around the base of services and vibrate gently to avoid voids. Don’t blast full pressure from the chute at these areas — it’ll shift formwork and supports. Once poured, check levels around each pipe to ensure no dip or bulge forms. Finish smoothly around exposed collars to avoid water pooling or tight buildup.