Sharing walls saves less building material than it first seems, because the shared walls still have to be built: for square units in a row, exterior wall length falls to 62.5% of detached for a row of four and 58% for a row of six, but total wall length, party walls included, falls only to 81% and 79%. Larger savings of 30-40% need long rows of narrow, deep units, so state which wall you are counting before quoting a saving.
Part of Multi-Family Housing Design
Material savings in shared-wall housing are the reductions in wall, roof and floor material that come from units sharing walls instead of each having four of its own.
Two Different Walls: The Skin And The Total
The multi-family guide states that shared walls save about 30-40% of wall material, and its comparison table shows wall material of 62% for a row of four and 56% for a row of six. Those two statements, and the table, count different things. The geometry shows which.
Let each unit have frontage f and depth d, with n units in a row.
- Detached houses: each has perimeter 2(f + d), so n houses have 2n(f + d) of wall.
- Row, exterior walls only: front and back are each n × f long, and the two ends are d each, so 2nf + 2d.
- Row, all walls: the exterior plus (n − 1) party walls of length d, so 2nf + 2d + (n − 1)d = 2nf + (n + 1)d.
Worked Example: Square Units 5 m By 5 m
Inputs: f = d = 5 m; n = 4, 6, 8; wall height equal in every case, so ratios of length are ratios of area.
- Detached walls: 2n(5 + 5) = 20n m. For n = 4: 80 m; n = 6: 120 m; n = 8: 160 m.
- Row exterior walls: 2n × 5 + 2 × 5 = 10n + 10 m. For n = 4: 50 m; n = 6: 70 m; n = 8: 90 m.
- Ratio exterior to detached: 50 ÷ 80 = 62.5% (n = 4); 70 ÷ 120 = 58.3% (n = 6); 90 ÷ 160 = 56.3% (n = 8).
- Row total walls (all): 2n × 5 + (n + 1) × 5 = 15n + 5 m. For n = 4: 65 m; n = 6: 95 m; n = 8: 125 m.
- Ratio total to detached: 65 ÷ 80 = 81.3%; 95 ÷ 120 = 79.2%; 125 ÷ 160 = 78.1%. For rows of four and six that is about 81% and 79%.
So the guide’s table values of 62% for four units match the exterior-wall length of square 5 m units. The table’s 56% for six units is close to the 58% here and not equal to it; its 50% for an eight-unit longhouse cannot be reached with square units, where the exterior ratio never falls below 56% at n = 8. The values in the table depend on a unit shape the guide does not state.
The total wall saved for square units is 19-22%, not 30-40%. What makes a larger saving? Narrow, deep units: total ratio = (2nf + (n + 1)d) ÷ (2n(f + d)). As n grows large this tends to (2f + d) ÷ (2(f + d)).
- f = 4 m, d = 6 m: (8 + 6) ÷ 20 = 0.70, a 30% saving at best.
- f = 3 m, d = 8 m: (6 + 8) ÷ 22 = 0.64, a 36% saving at best.
So a 30-40% total wall saving needs long rows of units 1.5 to 2.7 times as deep as they are wide.
Count the faces as a cross-check. Six detached units have 6 × 4 = 24 faces. A row has 6 fronts + 6 backs + 2 ends = 14 faces, a reduction of (24 − 14) ÷ 24 = 42%, which matches the 58% remaining in the exterior-length arithmetic above. The parent article claimed 8 faces and a 67% reduction when this page was written; 8 is not reachable for six units in any arrangement, and the parent has since been corrected to 14 and 42%, so the two now agree.
Roofs, Floors And Fuel
The roof and floor of each unit are not shared in a row, so their area per unit is unchanged; only stacked units share floors and ceilings. The guide’s roof and fuel columns (85%, 82%, 75% and 60-70%, 55-65%, 50-60%) cannot be derived from this geometry and are not repeated here; the heat-loss side is worked in Thermal Efficiency (Party Walls).
What You Need
- Unit frontage and depth for each proposed row
- The wall height and thickness (to turn length into volume)
- Which walls are party walls and which are exterior
Method
Step 1: Compute detached, exterior and total wall length with the three formulas.
Step 2: Multiply by height and thickness to get volume, and by density for mass if needed (Dead Loads).
Step 3: Quote the right ratio for the right purpose: exterior ratio for heat, total ratio for material.
Step 4: Compare shapes by changing f and d before fixing the plan.
Common Mistakes
- Quoting the exterior ratio as a material saving.
- Counting an end unit as if it were interior.
- Using a short row — savings level off but the first few units give most of it.
Safety
A saving can become a hazard
A long row of deep units is also a long fire path and a deep dark plan. Do not trade away the fire spacing, the separate flues or the through-roof party walls to reach a material saving (Fire Breaks and Building Spacing).
What’s Next
- Learn next: Thermal Efficiency (Party Walls) — what the exterior ratio means for fuel
- Combine with: Structural Bracing — the structural use of those walls