A first course block is the unit used in the bottom course of a wall, laid straight onto the foundation or the floor slab. Together those blocks form the levelling course — the first course, set dead level, on which the whole wall is then built.
The Dutch term is kimblok, from kim, meaning edge: the line where wall and floor meet. The term is used mainly in thin-joint work with large blocks and elements, such as calcium silicate units.
Why the bottom course is treated separately
- It takes up unevenness — a foundation or slab is never perfectly flat; the first course is bedded in a thicker mortar bed and levelled to the millimetre
- It sets the height — the top of this course decides whether the wall height works out in whole block courses, without cutting at the top
- It gives a solid bearing — these blocks are bedded in cement mortar even when the rest of the wall is thin-joint glued, because thin-bed mortar leaves no room for adjustment
- It carries the damp barrier — a damp-proof course against rising damp is often laid on or under this course
How it is done
- Mark out the foundation — set out the wall lines and find the highest point of the foundation; that is where you start
- Damp-proof course — where specified, lay a DPC membrane, EPDM strip or bituminous layer
- Mortar bed — spread a bed of cement mortar, thick enough to allow levelling
- Set the blocks — block by block, level and plumb, neatly in line
- Let it set — give the course at least a day to gain strength before stacking on it
- Build on — the rest of the wall follows in thin-bed or ordinary mortar
Points to watch
- Spend the time here — every millimetre of error in this course carries through the whole wall, in plumb and in height
- The right mortar strength — loadbearing masonry calls for a higher mortar strength class than non-loadbearing walls; follow the specification or the supplier’s advice
- In frost — at low temperatures special faster-setting mortars are used; ordinary mortar must not freeze before it has set
- The damp detail — do not let the damp barrier run out to the face unfinished; check how the detail is drawn
Related terms
- Levelling course
- Calcium silicate
- Thin-bed mortar
- Foundation beam
- Masonry
- Damp-proof course
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