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Foul Drainage Calculator — Pipe Routing, Cover Depth & IC Positions
Plan foul drainage pipe routing with inspection chamber positions, minimum cover depth checks, gradient fall and material quantities.
Total pipe run from building to sewer
Bends >45° need access (AD H 2.49). Count each only once — don’t also count it below
Extra gradient/horizontal changes not already counted as a bend above
Where other drains join
Installed cost per metre
AD H Table 6 minimum 1:40 = 0.025 for 100mm to one dwelling
Ground level to pipe invert (calc converts to crown for AD H Table 10 cover check)
Trench excavation uses a typical width (~450–500mm) and a +100mm bedding allowance for estimating only — these are installation conventions, not AD H requirements. Actual trench width depends on pipe size, bedding class and any shoring; confirm against your bedding detail.
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How We Calculate This
This calculator plans your foul drainage system by determining inspection chamber positions, checking cover depths and calculating material quantities.
The formula
IC Positions = Bends + Direction Changes + Branches + 2 (head & end)
Per AD H 2015 para 2.49, an access point is needed at the head of the run, at each bend or change of gradient, at each junction and at each change of pipe size. Count each physical point once — a bend over 45° is itself a change of direction, so do not enter it as both a bend and a direction change.
Total Fall = Gradient × Run Length
Pipe Runs = ceil(Length ÷ 3m per length)
Minimum cover depths (uPVC / thermoplastic pipe)
From AD H 2015 Table 10 (thermoplastics, ring stiffness SN4), measured over the pipe crown:
- Garden / field: 600mm
- Light road / driveway: 900mm
- Main road / highway: 900mm
Rigid clayware or concrete pipe (AD H Tables 8/9) needs 1200mm under any road. Where cover falls below the table minimum, AD H para 2.44 allows the pipe to be protected with a reinforced concrete cover slab or surround rather than re-laying deeper.
Frequently Asked Questions
Foul drainage carries wastewater from toilets, baths, showers, basins, sinks and appliances to the public sewer or private treatment system. It must be kept separate from surface water drainage in areas with separate sewer systems. All foul drainage must comply with Building Regulations Part H.
For uPVC (thermoplastic) pipe, AD H 2015 Table 10 specifies 600mm under gardens and fields, and 900mm under any road — light or main. The deeper 1200mm road figure applies to rigid clayware or concrete pipe (Tables 8/9), not to flexible plastic pipe. Where the minimum cover cannot be achieved, AD H para 2.44 lets you protect the pipe with a reinforced concrete cover slab or surround rather than digging deeper. These limits are measured to the top (crown) of the pipe.
An inspection chamber is required at every change of direction, change of gradient, junction with a branch drain, and at intervals not exceeding 45m on straight runs. You also need a chamber within 12m of the connection to the public sewer for rodding access. Use our Inspection Chamber Calculator to work out sizes and components, or the Manhole Calculator for deeper access points.
AD H 2015 Table 6 sets minimum gradients only: 100mm pipe carrying less than 1 l/s (e.g. one dwelling) at least 1:40 (25mm fall per metre); 100mm pipe over 1 l/s with at least one WC at least 1:80 (12.5mm per metre); 150mm pipe at least 1:150. Part H does not set a maximum gradient. As a rule of thumb installers avoid runs much steeper than about 1:10, since very steep drains can let water run ahead of solids; where a steep drop is unavoidable a backdrop or rest bend is used. Treat the 1:10 figure as good-practice convention, not a Part H limit.
Two 45-degree bends used to create a gentle sweep may not require an inspection chamber, provided a rodding access point is available nearby. However, any change of direction greater than 45 degrees requires an IC. Always confirm with your building control officer.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.