SLS (serviceability limit state)
SLS is the engineering check against excessive deflection, vibration, and cracking under normal loads. Members must feel solid and not visibly sag. Pairs with ULS.
Ask Chalkline about this →SLS (Serviceability Limit State) is the engineering design check against excessive deflection, vibration, cracking, or other service-level performance failures under normal use loads. SLS is the “feels right” check; it pairs with ULS (Ultimate Limit State), the “doesn’t fall down” check. Both checks must be satisfied under AS/NZS 1170.0; passing one alone is not enough.
What SLS controls in residential:
| Failure mode | What occupant experiences | Typical SLS limit |
|---|---|---|
| Beam deflection | Visible sag, door frames bind, tiles crack | L/300 to L/500 of span (where L is the span) |
| Floor vibration | Bouncy floor, glasses rattle when walking past | Frequency under 8 Hz unacceptable |
| Wind sway | Building moves perceptibly in storms | H/500 of height typical |
| Crack control in concrete | Visible cracks in slabs, walls | Crack width under 0.3 mm typical |
| Acoustic transfer | Footstep noise heard between floors | NCC sound transmission requirements |
Typical SLS load combinations (residential, AS/NZS 1170.0):
| Combination | Description |
|---|---|
| G + Q | Permanent + variable (working loads, unfactored) |
| G + Ws | Permanent + serviceability wind |
| G + 0.7Q | Permanent + reduced live load for long-term deflection |
Note: SLS load factors are typically 1.0 (or lower for long-term effects), in contrast to ULS factors of 1.2 to 1.5. The structure is designed to handle these working loads gracefully, not just survive them.
ULS vs SLS in plain builder language:
- ULS: will the lintel break under the worst-case design load? (catastrophic outcome)
- SLS: will the lintel sag visibly under everyday roof load, or bounce when somebody walks across a floor above? (everyday-life outcome)
A 200 UB 25 lintel sized only for ULS might be technically strong enough to carry the roof but will deflect 30 mm at midspan. The 30 mm sag will crack the brickwork above and tilt the door frame below. The lintel passes ULS, fails SLS, fails inspection.
Why SLS often drives the design in residential:
- Residential floor live loads are modest; ULS rarely binds for typical span and depth.
- The eye is good at picking up visible deflection; the occupant sees 10 mm of sag on a 4 m beam and is unhappy.
- Tile and plasterboard finishes are intolerant of small movements; SLS deflection caps protect them.
The L/360 and L/600 conventions:
| Element | SLS deflection limit (common AU residential) |
|---|---|
| Generic timber lintel under roof load | L/300 |
| Generic floor joist | L/360 |
| Lintel carrying brick veneer above | L/600 or 5 mm absolute |
| Floor joist supporting tile finish | L/360 plus crack-control check |
| Wall sway under serviceability wind | H/500 typical |
A lintel under a brick veneer with L/600 deflection requires substantially deeper timber or LVL than the same lintel with no brickwork above; the wall above is what makes SLS bite.
Also known as: service-state design; serviceability state; in-service limit state.
Category: Structure.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.