Blockwork looks like the simplest trade on site, which is why it gets the least supervision. It is also the trade whose errors are the most expensive to hide, because every millimetre a wall is out has to be made up in plaster, and plaster is charged by the bag.
The first three courses decide everything
A wall that starts straight and level tends to stay that way; a wall that starts 15mm out gets progressively worse as the layer corrects by eye. So we spend disproportionate time on the base:
- Set out from established gridlines with a tape and a line, not from an adjacent wall that may itself be out.
- Check the top of the ground beam or slab for level and dub out the difference in the bedding mortar of the first course — not over three courses.
- Build the corners up first, plumb them properly with a spirit level on both faces, then run the line between them.
- Check diagonals on every room before the third course. A room out of square at course three is fixable; at course fifteen it is not.
Mortar
Mortar is not a place to economise, and it is not a place to over-specify either. A mortar stronger than the blocks it joins forces any movement to crack the block rather than the joint, which is the wrong failure. For the hollow and solid blocks we use in Uganda and South Sudan, a 1:5 or 1:6 cement-sand mortar covers most work, richer where blocks are exposed or below damp-proof level.
The site issues are the usual ones. Sand needs to be clean and reasonably graded — silty sand from a cheap source makes a mortar that never gains strength and shrinks badly. Mortar mixed in the morning and re-tempered with water at three in the afternoon has already set once and has lost most of its value; we mix in quantities the gang will use within about two hours.
Joints should be full. A layer working fast will butter the edges and leave the middle of the bed hollow, which looks identical from the face and gives you a wall with a fraction of the bearing area it should have.
Wetting the blocks — and not over-wetting them
Dry blocks suck the water straight out of the mortar before it can hydrate, leaving a weak, powdery joint. Soaked blocks float on the bed and the wall settles overnight. The right condition is damp but surface-dry: wet the blocks, then let them stand for a while before laying.
Curing walls
New blockwork needs curing exactly as concrete does, and it almost never gets it. Two or three days of light spraying, morning and evening, makes a measurable difference to joint strength and to shrinkage cracking at the corners of openings. It costs one labourer part of a day.
Openings, lintels and the crack at the corner
The diagonal crack running up from the top corner of a window is the signature of a lintel with insufficient bearing. Our rule is a minimum of 200mm bearing each end for a standard opening, more for wide spans, and the lintel bedded on full mortar rather than dry-set. Where openings are close together we cast a continuous lintel over both rather than two separate ones with a short pier between.
Why it is worth the supervision
On a medium-sized building, walls built plumb and to line reduce the plaster thickness required across the whole job. That saving alone usually exceeds the cost of the extra supervision time. The straight walls, square rooms and doors that close are effectively free.
OK Estate Construction and Supply Limited carries out masonry and finishing works across Uganda and South Sudan. Talk to our team.