A shared wall, or party wall, is one wall that serves two homes, so each home pays for only half of it, stays warm because both sides are heated, braces the building against sideways loads and can stop a fire from spreading when it is thick and masonry. These advantages hold only if the wall is built first, built solid and carried up through the roof.
Part of Multi-Family Housing Design
Structural advantages of shared walls are the savings and strengths a building gains when neighbouring homes use one wall in common.
One Wall, Four Jobs
A party wall between two homes does four jobs at once:
- Saves material. Two detached houses have two walls where the shared version has one, so each home pays for half of the shared wall. How much that saves overall is worked out in Material Savings.
- Saves heat. A wall between two heated rooms loses little heat while both stay warm (Thermal Efficiency (Party Walls)).
- Braces the building. Walls at right angles to the front and back walls hold the structure square against wind and earthquake (Structural Bracing).
- Stops fire. A thick masonry or plastered earth wall resists fire between units, provided the roof does not carry the flame over the top.
What You Need
- Materials that suit a thick wall — earthbag, adobe, stone, brick, or plastered straw bale
- A solid foundation under the wall line (Foundations and Stem Walls)
- A plan that builds the shared walls first
Method: Using A Party Wall Well
Step 1: Build the shared walls first. The multi-family guide calls them the structural backbone.
Step 2: Make them solid. Voids, flues and recesses cut into a party wall reduce its strength, its sound isolation and its fire resistance.
Step 3: Carry it above the roof or add a firestop across the roof void, so the wall can do its fire job.
Step 4: Finish both faces with plaster (Plaster as a Fire Barrier gives a tested rating for plastered bale walls).
Common Mistakes
- Treating the wall as a partition and cutting it up for pipes and flues.
- Different foundations under the two sides — they settle apart and crack the wall.
- Assuming a shared wall means no heat loss — if a neighbour’s side is cold, it loses heat.
Safety
A party wall is load-bearing
Never cut a doorway or large opening through a party wall without a lintel designed for the load above it (Doors and Windows), and never remove part of it to join two homes without propping the roof first.
What’s Next
- Learn next: Material Savings — the actual numbers
- Combine with: Row Houses — the form that uses party walls most