Special Applications and Curved Surfaces
This chapter covers the techniques, principles, and regulatory requirements specific to installing lath and panels on non-planar surfaces, as well as special applications such as wrapped beams, columns, skylight wells, and curved partitions. For the Red Seal exam, you must master not only the technical skills but also the development calculations, tolerances, and references to Canadian standards.
1. Geometric Principles of Curved Surfaces
1.1 Development of a Curved Surface
Development is the flattening of a three-dimensional surface. For a lather, this means determining the exact shape of a panel or lath before cutting it, so that it fits perfectly over an arc, circle, or compound shape.
Radius (R): distance from the center of curvature to the surface. The smaller the radius, the more pronounced the curvature.
Chord (C): straight-line distance between two points on the arc.
Rise/Sagitta (f): perpendicular distance between the midpoint of the chord and the farthest point of the arc.
Rise formula (for a circular arc):
f = R − √(R² − (C/2)²)
Calculation example: For a vaulted ceiling with radius R = 3,000 mm and chord C = 1,200 mm:
f = 3,000 − √(3,000² − 600²)
f = 3,000 − √(9,000,000 − 360,000)
f = 3,000 − √8,640,000
f = 3,000 − 2,939.4
f = 60.6 mm
This rise of 60.6 mm represents the maximum depth of curvature at mid-span. It is used to lay out the arc on site using a bevel gauge or a trammel bar (beam compass) .
1.2 Development of a Truncated Cone
For a tapered column or flared shape, the development is a sector of an annulus (ring). The length of the outer arc corresponds to the circumference of the large base, and the inner arc to the circumference of the small base.
Calculating the development radius:
R₁ = (D₁ × L) / (D₁ − D₂)
Where:
D₁ = diameter of the large base
D₂ = diameter of the small base
L = slant height of the cone (hypotenuse)
Sector angle:
θ = (D₁ × 360°) / (2 × R₁)
Example: Tapered column with D₁ = 600 mm, D₂ = 400 mm, L = 1,500 mm.
R₁ = (600 × 1,500) / (600 − 400) = 900,000 / 200 = 4,500 mm
θ = (600 × 360) / (2 × 4,500) = 216,000 / 9,000 = 24°
You must therefore lay out a 24° sector with a radius of 4,500 mm to develop the lateral surface. In practice, plywood templates are often used to transfer this layout onto the panels.
2. Bending Techniques for Metal Lath
2.1 Metal Lath for Curved Surfaces
Metal lath is the most common support for curved plaster surfaces. Three types are used depending on the radius of curvature:
| Lath Type | Use | Minimum Recommended Radius |
|---|
| 2.5 mm (diamond) mesh | Flat surfaces and gentle curves | R ≥ 600 mm |
| 3.4 mm (diamond) mesh | Medium curves | R ≥ 300 mm |
| Rib lath (expanded metal) | Pronounced curves, vaulted ceilings | R ≥ 150 mm |
Rule of thumb: The smaller the radius, the smaller the mesh must be and the more flexible the lath needs to be. For radii less than 150 mm, fiberglass mesh or flexible drywall panels are used.
2.2 Lath Bending Procedure
38.Measure and lay out: Determine the exact radius and the developed length of the surface (arc perimeter).
39.Cut the lath: Cut to the developed dimension plus 50 mm overlap at each joint.
40.Pre-bend: For radii less than 300 mm, pre-bend the lath over a mandrel (cylindrical wooden or metal form) or by progressively bending it by hand.
41.Installation: Fasten the lath to the supports (furring channels or studs) starting at the center of the arc and working towards the ends. Use lath nails or self-tapping screws spaced no more than 150 mm apart.
42.Overlap: At joints, overlap the lath by at least 25 mm and fasten both thicknesses together.
Caution: Never stretch the lath to force it to conform to the curvature. This will distort the mesh and weaken the plaster bond.
2.3 Welded Wire Mesh and Special Reinforcements
For highly curved surfaces or complex shapes (vaults, domes), welded wire mesh of gauge 10 or 12 is sometimes used, combined with flexible corner beads. Reinforcing corners must be bent before installation, never after, to prevent cracking.
3. Gypsum Panels on Curved Surfaces
3.1 Flexible Gypsum Panels (Dry Bending)
Flexible drywall panels are 6 mm thick and designed for dry bending. They are used for radii of curvature as small as 300 mm.
Installation procedure:
50.Measure the developed length of the arc (perimeter).
51.Cut the panel to the exact width (typically 1,200 mm).
52.Pre-bend: Lay the panel flat, back face up, and lightly dampen the back face with a wet roller. Wait 15 to 30 minutes.
53.Installation: Lift the panel and fasten it to studs or furring channels spaced no more than 150 mm apart. Start at one end and screw progressively, following the curvature.
54.Fastening: Screw every 150 mm along the studs, slightly dimpling the screws without breaking the paper.
Important: Flexible panels must not be used for radii less than 300 mm. For these cases, use the wet method (see below).
3.2 Wet Method (Standard Panels)
For radii between 150 mm and 600 mm, standard 12.7 mm gypsum panels can be bent by wetting them.
Procedure:
59.Perforate the back face of the panel with a perforating roller at a rate of 20 holes per 100 cm².
60.Dampen the perforated face generously with clean water (no soapy water).
61.Stack the dampened panels flat, dampened face up, for 1 to 2 hours.
62.Bend progressively over a wooden template, increasing the curvature in successive passes.
63.Install immediately, before the panel dries completely.
Table of minimum radii by method:
| Panel Thickness | Dry Method (Flexible) | Wet Method (Standard) |
|---|
| 6 mm (flexible) | R ≥ 300 mm | — |
| 12.7 mm (standard) | Not recommended | R ≥ 150 mm |
| 15.9 mm (standard) | Not recommended | R ≥ 300 mm |
3.3 Calculating the Developed Length of a Curved Panel
The developed length of a panel conforming to an arc is equal to the length of the arc at the neutral axis (centerline of the panel's thickness).
Formula:
L = (π × R × θ) / 180°
Where:
L = developed length (mm)
R = radius of the neutral axis (mm)
θ = arc angle (degrees)
Example: 90° arc with an inside radius of 1,000 mm and a 12.7 mm thick panel.
Radius of the neutral axis = 1,000 + (12.7 / 2) = 1,006.35 mm
L = (π × 1,006.35 × 90) / 180 = (3.1416 × 1,006.35 × 90) / 180
L = 284,477 / 180 = 1,580.4 mm
You must cut the panel to 1,580 mm to cover the 90° arc exactly. An incorrect neutral axis calculation is a frequent source of error on the exam.
4. Wrapped Beams and Columns
4.1 Beam Wrapping
Beam wrapping involves covering a structural beam (steel or concrete) with lath and plaster, or panels, for aesthetic reasons and fire protection.
Regulatory requirements:
According to the National Building Code of Canada (NBC), steel beams must be fire-protected according to their required fire-resistance rating (Article 3.1.7.4).
The wrapping must have a minimum plaster thickness of 19 mm over metal lath for a 45-minute protection, and 25 mm for 1 hour (according to Table 3.1.7.4.A of the NBC).
Installation procedure:
86.Clean the beam: Remove rust, grease, and debris.
87.Install attachments: Weld or bolt steel clips to the beam, spaced no more than 400 mm vertically and 600 mm horizontally.
88.Install the lath: Fasten the metal lath to the clips with 18-gauge tie wire, ensuring a 25 mm overlap at joints.
89.Apply the plaster: Apply the plaster in two or three coats, with a total thickness conforming to the requirements.
Common pitfall: Clips must be galvanized or stainless steel, never bare steel, to prevent galvanic corrosion with the lath.
4.2 Circular and Rectangular Columns
Columns are vertical elements that require precise layout development.
Circular column:
Measure the column diameter (D).
Calculate the circumference: P = π × D
Add 25 mm for the lath overlap.
Cut the lath to the column height and to the width P + 25 mm.
Fasten the lath with steel bands or tie wire, spaced 300 mm vertically.
Rectangular column with rounded corners:
Develop each face separately, adding the length of the quarter circles at the corners.
Developed length of a rounded corner = (π × R) / 2 (quarter circle).
Example: 400 mm × 400 mm column with rounded corners of R = 50 mm.
Developed length of one face = 400 − (2 × 50) + (π × 50) / 2 × 2
= 300 + 157 = 457 mm
Total perimeter = 4 × 457 = 1,828 mm
4.3 Skylight Wells and Roof Lights
Skylight wells are openings in ceilings that require careful framing.
Key requirements:
The well angle must be at least 45° from the horizontal to ensure diffused lighting (NBC recommendation, Article 9.7.2.1).
Interior surfaces must be finished with a smooth, reflective material (smooth plaster or panel).
The lath must be fastened to a wooden or metal frame, with supports spaced no more than 300 mm apart.
Procedure:
113.Build the well frame with studs and furring channels angled at 45°.
114.Install the lath on the angled faces, cutting the corners at 45°.
115.Apply the plaster in two coats, with a corner bead at each angle.
116.Finish with a trim frame at the ceiling level.
5. Curved Partitions and Wavy Walls
5.1 Constructing a Curved Partition
A curved partition is constructed with bent metal studs or straight studs spaced more closely together.
Method 1: Bent studs
Use 25-gauge (0.53 mm) or heavier steel studs.
Bend the studs using a stud bender or manually with a template.
Space the studs no more than 150 mm apart (instead of 400 mm for straight partitions).
Fasten the studs to the top and bottom tracks, which have been pre-bent.
Method 2: Straight studs with curved furring
Install straight studs spaced 400 mm apart.
Fasten curved horizontal furring channels to the studs, spaced 300 mm vertically.
Install the lath or panels over the furring channels.
Comparison table of methods:
| Criterion | Bent Studs | Curved Furring |
|---|
| Minimum radius | R ≥ 1,000 mm | R ≥ 300 mm |
| Cost | Moderate | Higher |
| Strength | Excellent | Good |
| Acoustic insulation | Good | Average |
5.2 Wavy Walls (Free-form Shapes)
Wavy or free-form walls require careful planning:
133.Lay out the curve on the floor using a template or trammel bar.
134.Transfer the curve to the ceiling using a plumb bob every 300 mm.
135.Install the tracks following the laid-out curves, bending them progressively.
136.Install the studs perpendicular to the curve, spaced 150 mm apart.
137.Install the lath in vertical strips, with 25 mm overlaps.
Tolerances: According to industry standards, the flatness tolerance for a curved surface is 3 mm under a 1,200 mm straightedge, and 6 mm under a 2,400 mm straightedge.
6. Regulatory Requirements and Canadian Standards
6.1 National Building Code of Canada (NBC)
The NBC (2020 edition) contains several articles relevant to curved surfaces and special applications:
Article 3.1.5.12: Fire resistance of wall and ceiling assemblies — curved surfaces must meet the same requirements as flat surfaces.
Article 3.1.7.4: Protection of steel beams and columns — minimum plaster thicknesses according to the required fire-resistance rating.
Article 9.29.4.1: Metal lath must be fastened in accordance with the manufacturer's requirements and the NBC.
Article 9.29.5.2: Gypsum panels must be installed with a maximum screw spacing of 300 mm on studs, and 150 mm on curved surfaces.
6.2 Canadian Electrical Code, Part I
The Canadian Electrical Code, Part I (C22.1-21) applies to electrical installations in buildings. For the lather, the relevant points are:
Rule 8-200: Conductors and junction boxes must be accessible. Never cover a junction box with lath or plaster without leaving an access opening.
Rule 12-108: Cables must be mechanically protected when passing through metal studs. Use bushings at the penetrations.
6.3 CSA B149.1 — Natural Gas and Propane Code
CSA B149.1 applies to gas installations. For the lather:
Article 4.4.1: Gas piping must be accessible for inspection. Do not embed it in plaster without authorization.
Article 4.4.2: Embedded piping must be protected against corrosion and marked.
6.4 CSA Standards for Materials
CSA A82.30: Metal lath — specifications for mesh, gauges, and coatings.
CSA A82.31: Gypsum panels — dimensions, tolerances, and test methods.
CAN/ULC-S114: Test method for fire resistance of building materials.
7. Practical Calculations and Layouts
7.1 Laying Out an Arc on Site
To lay out an arc with a large radius (greater than 3 m) without a trammel bar:
161.Calculate the rise f at mid-chord.
162.Draw the chord AB on the floor or ceiling.
163.Mark the midpoint M of the chord.
164.Draw a perpendicular at M and mark the rise f (point C).
165.Use the method of intermediate points: divide the chord into equal segments (e.g., 4), calculate the intermediate rises with the formula:
fₓ = R − √(R² − x²)
where x is the distance from the center of the chord.
Example: R = 5,000 mm, chord = 2,400 mm.
f at x = 0: f = 5,000 − √(25,000,000 − 1,440,000) = 5,000 − 4,854 = 146 mm
f at x = 300: f = 5,000 − √(25,000,000 − 90,000) = 5,000 − 4,991 = 9 mm
f at x = 600: f = 5,000 − √(25,000,000 − 360,000) = 5,000 − 4,964 = 36 mm
f at x = 900: f = 5,000 − √(25,000,000 − 810,000) = 5,000 − 4,918 = 82 mm
f at x = 1,200: f = 146 mm (symmetrical)
7.2 Calculating Material Quantities
For a curved surface, the quantity of lath or panels is calculated on the developed surface, never on the projected surface.
Developed surface of a cylinder:
S = π × D × H
Developed surface of a sphere (dome):
S = 2 × π × R × h
where h is the height of the spherical cap.
Example: Hemispherical dome with a radius of 2,000 mm.
S = 2 × π × 2,000 × 2,000 = 25,132,741 mm² = 25.13 m²
Add 10% for waste and overlaps: 27.64 m² of lath required.
7.3 Calculating Screws and Fasteners
Number of screws for panels on a curved surface:
Maximum spacing: 150 mm on studs.
Number of screws per panel = (panel width / spacing) × number of studs.
Example: 1,200 mm × 2,400 mm panel on 4 studs.
Screws per stud = 2,400 / 150 = 16 screws
Total = 16 × 4 = 64 screws per panel.
Tie wire for lath:
Fastener spacing: 150 mm along supports.
Wire length per tie: 300 mm (double loop).
Quantity = (surface area / (0.15 × 0.15)) × 0.3 m.
8. Quality Control and Tolerances
8.1 Installation Tolerances
| Element | Allowable Tolerance | Measurement Method |
|---|
| Surface flatness | 3 mm under 1,200 mm | Straightedge |
| Partition plumbness | 3 mm per 2,400 mm | Laser level or plumb bob |
| Arc alignment | ± 5 mm from layout | Template or compass |
| Plaster thickness | ± 3 mm | Thickness gauge |
| Fastener spacing | ± 25 mm | Tape measure |
8.2 Inspection Before Plaster Application
Before applying plaster to a curved surface, verify:
200.Fastening: All fasteners are in place and tight.
201.Overlap: Lath joints overlap by at least 25 mm.
202.Sharpness: No sharp edges are protruding.
203.Alignment: The curvature matches the layout (tolerance ± 5 mm).
204.Moisture: Flexible panels are completely dry before plaster application.
9. Occupational Health and Safety
9.1 Specific Hazards of Curved Surfaces
Falls: Scaffolding for curved surfaces must be adjusted in height at each move. Use work platforms with guardrails.
Cuts: Cut metal lath has sharp edges. Wear protective gloves (CSA Z94.3 standard) and long sleeves.
Dust: Plaster and panel cutting generate silica dust. Use at minimum an N95 mask and a dust extraction system.
9.2 Handling Curved Panels
Flexible panels tear easily. Transport them vertically, with two people.
Never stack more than 10 flexible panels.
Store panels flat, protected from moisture, on supports spaced 400 mm apart.
Summary
Development of a curved surface is the calculation of its flat shape. The key formulas are the rise (f = R − √(R² − (C/2)²)) and the arc length (L = π × R × θ / 180°).
Metal lath is bent according to its type: 2.5 mm mesh for R ≥ 600 mm, 3.4 mm mesh for R ≥ 300 mm, rib lath for R ≥ 150 mm.
6 mm flexible panels can be dry-bent for R ≥ 300 mm. Standard 12.7 mm panels can be wet-bent for R ≥ 150 mm.
The neutral axis of a curved panel is the centerline of its thickness. Always calculate the developed length from this line.
Wrapped beams require galvanized steel clips and plaster thicknesses conforming to the NBC (Article 3.1.7.4).
Curved partitions use bent studs (R ≥ 1,000 mm) or curved furring channels (R ≥ 300 mm), with spacing reduced to 150 mm.
Tolerances for curved surfaces are 3 mm under 1,200 mm for flatness and ± 5 mm for arc alignment.
Material quantities are calculated on the developed surface, never on the projected surface. Add 10% for waste.
The Canadian Electrical Code, Part I (Rule 8-200) requires junction boxes to remain accessible. CSA B149.1 (Article 4.4.1) requires accessibility of gas piping.
Pitfalls to Avoid
225.Confusing projected surface and developed surface: For a dome, the developed surface is double the floor-projected surface. Calculate on the actual surface, never on the projection.
226.Forgetting the neutral axis: For a 12.7 mm panel on an arc of R = 1,000 mm, the difference between the inside radius and the neutral axis is 6.35 mm. Over a 180° arc, this represents nearly 20 mm of length — enough to miss the joint.
227.Bending the lath after fastening it: The lath must be pre-bent before installation. Forcing it once fastened distorts the mesh and creates weak points.
228.Using screws that are too long on flexible panels: 25 mm screws penetrate the 6 mm panel and protrude. Use screws of 19 mm maximum.
229.Ignoring the reduced fastener spacing: On curved surfaces, the maximum spacing is 150 mm, not 300 mm. Excessive spacing causes ripples.
230.Neglecting the protection of electrical boxes: Covering a junction box with plaster is a violation of the Canadian Electrical Code, Part I (Rule 8-200). Always provide an access opening.
231.Applying plaster to still-damp flexible panels: The panel must be completely dry, otherwise the plaster will crack during shrinkage.
232.Cutting skylight well angles at 90°: The angle must be a minimum of 45° from the horizontal for adequate lighting.
233.Using bare steel clips for beams: Galvanic corrosion with the metal lath compromises the fastening. Always use galvanized or stainless steel.
234.Not verifying the rise before laying out: A calculation error in the rise propagates across the entire surface. Double-check with the formula f = R − √(R² − (C/2)²).
