Constraint recipe
A room clash assigns one room to two lessons in the same period. Specialist rooms make the fix harder, because a replacement must match type, capacity, and equipment, not just be free. Before moving anything, check whether one assignment is a duplicated row from an import; a data error and a placement error need different fixes.
Key takeaways
- One room cannot host two in-person lessons at the same time.
- A replacement room must match type, size, and equipment.
- Duplicate source rows can masquerade as scheduling clashes.
- Fixing the room can create a teacher or class clash; check the receiving slot.
Why room clashes are serious
Two classes cannot occupy the same physical room at once, and the problem is sharpest for labs, kitchens, and workshops, where eligible alternatives are few. Room availability is a primary cause of infeasible timetables, so a clash on a specialist room often signals a supply problem, not a one-off mistake.
Keep preferred rooms out of this. A clash is two lessons claiming the same space simultaneously; a teacher not getting their favorite classroom is a quality preference.
A worked example: two claims on the chemistry lab
Wednesday P3 shows 9A and 7C both assigned to the chemistry lab. 9A runs a practical that needs the fume hood; 7C has a theory lesson that happens to be roomed there. Only one of them actually needs the lab.
Chemistry lab occupancy, before and after
| Mon | Tue | Wed | Thu | Fri | |
|---|---|---|---|---|---|
| P1 | 7C | ||||
| P2 | 10B | ||||
| P3 | 9A + 7C | ||||
| P4 | 8A | ||||
| P5 | 10B | ||||
| P6 |
- Two lessons, one room
- Valid booking
Choose which lesson moves by constraint, not by convenience: the lesson that can run in more rooms is the one that moves. A free classroom in the grid can still be the wrong answer if it is too small for the group or lacks required equipment. See restrict rooms by subject for classifying must-have room types before generation, which prevents most of these clashes.
Diagnose before you rebook
| What you see | Likely cause | Fix |
|---|---|---|
| The same lesson appears twice with the same room | Duplicated or outdated room assignment in imported data | Delete the duplicate assignment in the source, then revalidate. |
| Two real lessons claim one specialist room | Too much parallel demand for a scarce room type | Move the less room-bound lesson, or stagger the two groups across the week. |
| A lesson that needs no room is holding one | A theory or off-site lesson was roomed out of habit | Release the room from the lesson that does not need it. |
Rebook without breaking the rest
- Identify the two lessons sharing the room and period, and trace both to the source data.
- Confirm each lesson's real room needs: type, capacity, and equipment.
- List the eligible alternative rooms for the less constrained lesson.
- Move it only if its teacher and class are also free in the target arrangement.
- Recheck specialist-room rules and revalidate after the move.
Rebooking mistakes
- Moving a practical into a room without the required equipment
- Choosing a replacement room that is too small for the group
- Fixing the room clash while creating a teacher or class clash
In Smootables
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Period
Room A
Conflicts detected
The following conflicts will occur if you proceed:
Room Lab B is already bookedChemistry practical at 13:00–14:30
A room clash means two in-person lessons need the same room at the same time. Generate timetable keeps each physical room unique on the grid. Lessons marked as remote teaching do not reserve a room, so they do not compete for lab or workshop space.
When you move a lesson manually, Conflicts detected flags the other class already in that room and period before the change sticks. Room no-conflict in the post-generation summary confirms the solved timetable respected the rule.
- Check room type, capacity, and equipment before moving a lesson to an alternative room.
- Remove duplicate room assignments in the period plan if the clash came from source data.
Quick answers
Is a room clash a mandatory rule?
Yes for one physical room and two in-person lessons at the same time. The timetable must change.
Can I use any free room as the fix?
No. The replacement must match the lesson's room type, capacity, equipment, and availability.