Foundation Concrete and Drainage Specification Notes

Every set of PorthouseDean structural drawings includes a specification section for foundations. This article summarises the key parameters and explains what they mean on site.

Concrete Grade

C35 is the standard concrete grade specified for pad foundations and column bases (a minimum compressive strength of 35 N/mm² at 28 days). The full specification is:

– Minimum cement content: 330 kg/m³
– Maximum free water–cement ratio: 0.55
– Maximum aggregate size: 20 mm nominal

This grade is required for structural foundations carrying concentrated column or wall loads. General strip footings under lightly loaded walls may use C28/35 depending on the design, but where column loads are involved, C35 is the default.

Reinforcement

Pad foundations are specified with two layers of steel mesh:

– A393 mesh — one layer at the bottom face, 50 mm cover from the underside
– A393 mesh — one layer at the top face, 25 mm cover from the top surface

Where a slab forms part of the foundation system:

– A252 mesh at the bottom of the slab (structural spanning capacity)
– A142 mesh at the top of the slab (crack control)

Reinforcement grade is high-yield Grade 460 to BS4449; mesh is high-yield steel to BS4483.

Minimum Cover Requirements

– Slab top face: 25 mm
– Slab bottom face (and footing underside): 50 mm
– Retaining wall faces: 40 mm

These cover values ensure adequate protection against corrosion. On aggressive ground (sulfate-bearing soil or groundwater), an enhanced cover or sulfate-resistant cement may be required.

Foundation Depth Defaults

Unless otherwise noted on your specific drawings:

– Non-clay ground: minimum 450 mm below finished external ground level
– Clay ground: minimum 900 mm below finished external ground level
– On clay near trees: as calculated per species and distance assessment

Drainage

Do not load existing drains with new foundation concrete. The formation level of new foundations must be below the invert level of any adjacent drains. Where drains must be bridged, a lintelling or spanning detail is required — this will be shown on the drawings if applicable.

Columns must be positioned centrally on their foundation unless eccentricity has been specifically designed and noted.

Questions?

If any of the specification values on your drawings are unclear, contact PorthouseDean for clarification.