Raft foundation detailing: by how much is a raft (mat) slab usually projected beyond the outer face of the external walls for cover, protection, and construction tolerance?

Introduction / Context:Raft (mat) foundations distribute loads over a large plan area. A modest projection beyond the wall face is customary to ensure proper cover, shuttering, edge protection against spalling, and tolerance during construction.

Given Data / Assumptions:

• Typical building raft supporting walls and columns.
• Conventional reinforced concrete detailing and site practices.
• We seek a standard practical projection range.

Concept / Approach:A projection of about 0.25 m to 0.30 m allows adequate edge cover, room for shear keys or starter bars near the face, and manageable formwork. Smaller projections (5–10 cm) are impractical; larger ones (≥0.45 m) may be used in special cases but are not typical for ordinary walls.

Step-by-Step Solution:Match common detailing practice: select 25–30 cm.Confirm constructability: supports adequate edge cover and shuttering.Hence, choose 25 to 30 cm as the standard range.

Verification / Alternative check:Standard detailing guides and BOQs often specify 250–300 mm overhang from the external wall face for rafts, barring site-specific constraints.

Why Other Options Are Wrong:

• 5–10 cm: insufficient tolerance/cover.
• 15–20 cm: lean for edges and tolerance.
• 30–45 cm or 60 cm: possible but not “usual,” increases concrete usage without need.

Common Pitfalls:

• Ignoring waterproofing terminations which also need space at slab edges.

Final Answer:25 to 30 cm