The most common blockwork defects in Tanzania are weak or badly cured blocks, poor mortar, walls out of plumb or line, missing bonding at corners and junctions, missing or short lintels, cracks where walls meet columns, damp rising from the ground, and walls built too fast without curing. Nearly all of them come from material quality and site workmanship, not design, which means they are preventable with good supervision and a few simple checks. Fixing a bad wall after plaster and paint costs several times more than building it right.
1. Weak or poorly cured blocks
Blocks with too little cement, too much sand, or no curing crumble at the edges and crack easily. Signs: corners break when handled, the surface scratches easily with a key or nail.
Prevention:
- Buy from a reputable maker, or make blocks with a proper mix and machine.
- Cure blocks with water for at least 7 days, then let them dry before use.
- Spot-check: drop a block from waist height onto hard ground. Good blocks mostly survive.
- For larger projects, test block strength in a lab.
Our guide on block making explains good mix ratios and curing.
2. Poor mortar
Mortar with too little cement, dirty sand or too much water gives weak joints that crumble. Common mortar mixes for blockwork are around 1:4 to 1:6 cement to sand, depending on the specification.
Prevention: use clean, sharp sand, measure by gauge box not by shovel, mix only what can be used within about an hour, and never re-mix mortar that has started to set.
3. Walls out of plumb or line
Walls that lean, bulge or wander make plastering thicker and more expensive, and in bad cases weaken the wall.
Prevention: set out with string lines, check plumb with a spirit level every few courses, use corner profiles or gauge rods, and keep joints an even thickness (around 10 to 12 mm).
4. No bonding at corners and junctions
Walls that simply butt into each other without overlapping courses or ties crack along the joint.
Prevention: stagger joints so each course overlaps the one below by about half a block, bond corners properly, and where walls meet columns, use wall ties or starter bars fixed into the column.
5. Missing or short lintels
Openings without a proper lintel, or lintels too short to bear on the wall, cause diagonal cracks from window and door corners.
Prevention: use a reinforced concrete lintel or ring beam over every opening, with a bearing of at least 150 to 200 mm each side, as the engineer specifies. See how buildings carry loads.
6. Cracks at wall and column junctions
Concrete frames and blockwork move differently as they dry and as temperature changes. Without ties or a movement detail, a vertical crack appears along every column.
Prevention: tie walls to columns, leave the frame to dry before walling where possible, and use mesh or expanded metal across the junction before plastering. Our guide on plastering walls covers this.
7. Damp at the base of walls
Walls without a damp-proof course (DPC) draw water from the ground, causing peeling paint, salt stains and crumbling plaster.
Prevention: lay a continuous DPC at the right level, keep ground and paving below it, and drain water away from the walls. See rising damp and DPC.
8. Building too high, too fast
Laying many courses in one day before the mortar has set can make a wall squeeze, bulge or crack. In hot weather, blocks and mortar can also dry out too quickly.
Prevention: limit the height built per day (often around 1.2 to 1.5 m), dampen blocks slightly in hot weather, and keep new work damp for a few days.
A quick blockwork check for owners
| Check | Good sign | Bad sign |
|---|---|---|
| Block edges | Sharp, hard | Crumbling, sandy |
| Joints | Full, even, 10 to 12 mm | Gaps, thick and uneven |
| Plumb | Level bubble centred | Visible lean |
| Courses | Straight lines | Wavy lines |
| Openings | Lintel in place, good bearing | Blocks over opening with no lintel |
| Base | DPC visible | Wall straight on soil |
Block sizes and where they go
| Block | Typical use |
|---|---|
| 230 mm (9 inch) hollow or solid | External walls, load-bearing walls, boundary walls |
| 150 mm (6 inch) | Internal walls, partitions |
| 100 mm (4 inch) | Light partitions, some infill |
| Solid blocks | Foundation walls below DPC, areas with heavy fixings |
Use solid or filled blocks below ground and wherever heavy items such as water heaters, cabinets or gates will be fixed. Hollow blocks hold screws poorly.
A daily blockwork check for supervisors
- Blocks on site are cured, dry and undamaged.
- Mortar is mixed in measured batches and used within its working time.
- String lines are set and walls checked for plumb every few courses.
- Ties are built in at every column and wall junction.
- Lintels are in place before blockwork continues above openings.
- No more than the agreed height built in one day.
- Finished work is kept damp in hot weather.
Ten minutes of checks each day prevents weeks of repairs later. See our guide to site supervision.
Frequently asked questions
What is the best mortar mix for blockwork?
Commonly 1:4 to 1:6 cement to sand, depending on wall type and the specification. Load-bearing walls use stronger mixes.
Why do cracks appear between walls and columns?
Because concrete and blockwork move differently. Ties, mesh across the joint and good timing reduce it.
Are hand-made blocks acceptable?
They can be, if the mix and curing are right, but strength varies more. Machine-made blocks are more consistent. See blocks vs bricks.
How do I know if a wall crack is serious?
Wide, growing or diagonal cracks need an engineer. See which cracks are serious.
Should hollow blocks be filled with concrete?
Where the engineer specifies it, for example at corners, around openings or in load-bearing walls with reinforcement. Otherwise hollow blocks are used as they are.
Want walls built right the first time?
Vika Construction supervises every stage of blockwork on our sites. See our quality approach or contact us.
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