The short answer
On most Zanzibar buildings: 6 inch for load-bearing and external walls, 4 inch for internal partitions, 8 inch where the wall retains ground, stands tall and exposed, or is specified structurally by the engineer.
That rule covers the majority of cases, but it is a rule of thumb — the structural drawings always take precedence over it.
What actually changes with thickness
- Wall mass and stiffness — thicker units resist lateral load and impact better
- Floor area — every extra inch of wall thickness is usable space you do not get back
- Foundation width — thicker walls carry through to wider footings and more concrete
- Laying rate — heavier units slow the mason down and raise the labour line
- Mortar consumption — bed area rises with thickness
- Service chasing — thin partitions limit how deep you can safely chase
Cost is not the unit price
Comparing block sizes on unit price alone is misleading. The honest comparison is cost per square metre of finished wall: units, mortar, labour, and the knock-on effect on foundations and floor area.
By that measure 4 inch is usually the cheapest wall and 8 inch the most expensive — which is exactly why 8 inch should be specified deliberately rather than used as a general upgrade.
Where projects get it wrong
- Using 4 inch for external or boundary walls to save money, then dealing with cracking and water ingress
- Using 8 inch throughout a house, inflating foundation and material cost with no structural benefit
- Mixing sizes without planning junctions, so partitions are butted rather than tied in
- Ordering one size for the whole job and cutting partitions from it on site
Choosing for a Zanzibar site
Exposure matters here more than in an inland climate. A tall boundary wall facing prevailing coastal weather takes wind-driven rain and salt that a sheltered internal wall never sees, and that argues for a heavier section and better detailing rather than the minimum that stands up.