BC BUILD
COMPASSTHE FRIDAY DETAIL
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01 · ATTENTION

Water damage often begins before water reaches the building.

Part 3.3 is not a collection of isolated dimensions. It is a coordinated defence that moves surface water, subsurface water and stormwater away from the building.

The practical questionWhere will the water go when the site receives more rain than expected?
FALL AWAY

02 · INTEREST

Three water pathways. One coordinated response.

The drainage strategy becomes easier to review when it is separated into the three pathways identified by Part 3.3.

01Surface waterShape finished levels so runoff moves away.
02Subsoil waterRelieve moisture and pressure where site conditions require it.
03StormwaterCollect, convey and discharge without flowing back into the building.
SurfaceSubsoilStormwater

03 · DESIRE

Falls and slab heights must work together.

A compliant number does not rescue a site where water is trapped against the building.

25 mmover the first 1 m in the specified low rainfall or access conditions
50 mmover the first 1 m in other cases
50 to 150 mmminimum slab height, depending on the adjoining surface and site condition

Always confirm which condition applies. Do not select the smallest number by default.

BUILDING
1,000 mm50 mm FALL

04 · APPLICATION

Subsoil drainage is a site response, not a default detail.

Where subsurface water needs to be diverted, the drain requires a uniform fall of at least 1:300, a maintainable outlet and a considered relationship with the soil and footings.

  • Confirm why the drain is required.
  • Provide a cleanable silt pit or sump arrangement.
  • Set the outlet at least 50 mm below the inlet invert.
  • Consider reactive clay and long-term moisture change.
MIN. FALL 1:300

05 · SITE CHECK

Follow the pipe all the way to its legal discharge point.

The system must satisfy the appropriate authority and prevent heavy-rain overflow from returning to the building.

Location above 90 mm Class 6 UPVCMinimum cover
Soil100 mm
Paved or concrete area50 mm
Light vehicle area, reinforced concrete75 mm
Light vehicle area, paved100 mm

Cover is measured from the top of the pipe to finished ground level, or to the underside of paving or concrete.

FINISHED SURFACE
VERIFY COVER
APPROVED DISCHARGE

06 · KNOWLEDGE CHECK

Can you apply Part 3.3 on site?

Answer all five questions. You need four correct answers to complete the learning.

01 Which three water pathways are addressed by Part 3.3?
02 In a typical case, what minimum fall is required over the first metre away from a slab-on-ground building?
03 What is the minimum uniform fall for a required subsoil drain?
04 What minimum cover is required above a 90 mm Class 6 UPVC stormwater drain beneath soil?
05 A paved area is almost level and the stormwater discharge point cannot be identified. What should happen before work continues?

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