Key points
- ASTM C920: Type S single-component, M multi-component; Grade P pourable (self-levelling), NS non-sag (gun grade).
- Class is the movement tolerated: 12½, 25, 35 and 50 (in both extension and compression), and 100/50 (100% extension, 50% compression).
- Use codes: NT non-traffic, T1/T2 traffic, I continuous immersion, and M/G/A/O for the substrate (mortar, glass, aluminium, other).
- Tile movement joints must be filled with 100% silicone, urethane or polysulfide, with a foam backer rod behind.
A sealant is not a "filler" for gaps; it is a flexible material designed to stretch and compress with the building's movement. A sealant that can't take the expected movement will crack or debond within months.
1) Reading the ASTM C920 codes
The manufacturer determines the type, grade, class and uses of a sealant under ASTM C9201:
| Item | Code | Meaning |
|---|---|---|
| Type | S | Single component |
| M | Multi-component | |
| Grade | P | Pourable, self-levelling |
| NS | Non-sag (gun grade) | |
| Class | 12½ | Withstands 12.5% movement in extension and compression |
| 25 | Withstands 25% movement | |
| 35 | Withstands 35% movement | |
| 50 | Withstands 50% movement | |
| 100/50 | 100% extension and 50% compression | |
| Use | NT | Non-traffic areas |
| T1 / T2 | Pedestrian or vehicular traffic (by hardness) | |
| I | Continuously immersed | |
| M / G / A / O | Approved on: mortar / glass / aluminium / other substrates |
Example: "ASTM C920 Type S, Grade NS, Class 25, Use NT, M, G, A" = single component, gun grade, ±25% movement, non-traffic, and bonds to mortar, glass and aluminium.
2) Which material where?
General guidance; final choice depends on the data sheet and compatibility with the substrate and paint.
| Location | Common choice | Note |
|---|---|---|
| Around baths, basins and showers | Sanitary, mould-resistant silicone | Can't be painted. See resealing silicone. |
| Tile movement joints | 100% silicone, urethane or polysulfide2 | With a backer rod so it bonds only to the two sides of the joint2. |
| Around windows and doors outside | Silicone or polyurethane with a suitable movement class | Check approval on aluminium and mortar (A, M). |
| External floor and car park joints | Polyurethane, grade P, use T | Takes traffic. |
| Interior joints to be painted | Paintable acrylic | Limited movement; for light interior work. |
3) Joint shape matters as much as the material
- Backer rod: stops the sealant bonding to the bottom of the joint, so it works "like an accordion" between the two sides2.
- Clean, dry joint sides, and primer if the manufacturer requires it.
- Tool immediately after application to press the sealant against both sides.
4) Silicone vs. polyurethane
Silicone
- Excellent resistance to sun and heat, high flexibility.
- Suits glazing and bathrooms (sanitary grades).
- Not paintable if 100% silicone3.
Polyurethane
- Tough, takes traffic and abrasion.
- Usually paintable.
- More sensitive to UV depending on formulation; check the data sheet.
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1TB #45 — ASTM C920 Standard Specification for Elastomeric Joint Sealants: Summary — Pecora Corporation
- 2Why Do You Need Expansion Joints When Installing Tile? (TCNA EJ171) — Ceramic Tile Education Foundation (CTEF)
- 3Sealant Installation, Application, Tools, Tips — FAQ — GE Sealants