A levelling course is the first course of bricks or blocks in a wall, bedded dead level in mortar on the foundation right around the building. In Dutch it is called the kimlaag, from kim, the line where wall and floor meet — a word that originally meant edge.
The course has one main job: to absorb every irregularity in the substrate, so the rest of the wall can be built true and to size.
Why this course decides the rest
- Flatness — a foundation or slab is never exactly flat; the mortar bed of this course takes that up
- Height — the top of the course decides whether the coursing works out neatly at sills, lintels and the upper floor
- Line — the levelling course fixes the wall line, and everything above follows it
- Damp protection — a damp-proof course against rising damp is often laid on or under it: bitumen, DPC membrane or EPDM
How it is done
- Find the highest point — survey the foundation all round with a laser or water level and start at the highest point
- Damp-proof course — install it according to the detail on the drawing
- Lay the mortar bed — in cement mortar, even when the wall above is thin-joint glued: thin-bed mortar gives no room to adjust
- Set the units — level, plumb and in line; check every one
- Let it cure at least a day — only then build on, or the weight will squeeze the course down
- Carry on — the rest of the wall in thin-bed or ordinary mortar
Points to watch
- Mortar strength — the strength class depends on the wall: loadbearing masonry needs a higher compressive strength than a non-loadbearing partition. Follow the specification or the supplier’s advice
- Never rush it — a millimetre out of plumb in this course means centimetres at the top of the wall
- Levelling course and wall-floor joint — the levelling course is the first course of units; the wall-floor joint is the joint between slab and rising wall. In basements and bathrooms that joint is the leak point and needs its own seal
- Frost — do not lay this course in mortar that can freeze before it has set
Related terms
- First course block
- Wall-floor joint
- Masonry
- Foundation beam
- Damp-proof course
- Thin-bed mortar
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