Understanding Foundations on Your Structural Drawings

Structural drawings do not always prescribe a single foundation solution. For many domestic projects, PorthouseDean’s drawings provide two options, and your builder chooses the most practical one for site conditions.

Option 1: Strip Footing

The strip footing option specifies a minimum width and depth for a continuous concrete strip running beneath each structural wall. The drawing will show dimensions like:

“600 mm wide × minimum 600 mm deep, C35 concrete at recommended bearing strata”

This is the traditional foundation form and is familiar to most builders. Excavation follows the wall lines; a continuous trench is dug, inspected by Building Control, and filled with concrete.

Option 2: Pad Foundation

For situations where columns or posts are involved, or where isolated point loads need to be carried rather than wall lines, the drawings may specify a pad foundation — a discrete concrete block:

“e.g. 800 × 800 × 300 mm pad with two layers of A393 mesh reinforcement”

A pad is used where the load is concentrated at a single point (a steel column foot, for example) rather than continuous along a wall. Pads require more precise setting-out but less total concrete volume.

Choosing Between Them

Where both options appear on the drawings, your builder chooses based on what works best on site. Factors include:

– The position of underground services (drains, ducts) that may conflict with one option
– Soil conditions — a continuous strip may be difficult where ground is variable across short distances
– Building Control preference — some BCOs express a preference for one approach on a given site

If the strip footing depth proves inadequate during excavation (because suitable bearing strata is not reached at the minimum depth), the builder can use lean mix concrete to fill the lower section of the trench up to the required level for the structural-grade concrete strip. This avoids having to revise the calculations — the lean mix fills the void while the structural concrete strip sits at the correct design level.

Reinforcement Requirements

Strip footings in Approved Document A scenarios are typically unreinforced concrete. However, pad foundations specified by PorthouseDean always include reinforcement mesh — typically two layers of A393 mesh, one top and one bottom, with 50 mm cover to all faces. This provides both flexural capacity and crack control.

Questions About Your Foundation Drawing?

If something on your foundation drawing is unclear, get in touch with PorthouseDean. We are happy to explain what is shown and why.