Quick answer: dead load is the permanent weight a roof carries all the time. On a flat roof that normally includes the roof deck, insulation, waterproof covering, ceiling and other fixed parts of the build-up.
If you are looking at a joist span table, dead load is usually shown in kN/m². The easiest way to picture it is as an approximate weight spread over each square metre of roof.
What does 0.25 kN/m² mean?
| Dead load | Approximate weight |
|---|---|
| 0.25 kN/m² | about 25.5 kg/m² |
| 0.50 kN/m² | about 51 kg/m² |
| 1.25 kN/m² | about 127.5 kg/m² |
So a dead load of 0.25 kN/m² is roughly 25 kg of permanent roof weight spread across every square metre.
Use the kg figures to understand the scale. When using a span table, keep working with the stated kN/m² figures and its assumptions.
What counts as dead load on a flat roof?
Typical permanent loads can include:
- OSB, plywood or another roof deck
- PIR or other fixed insulation
- EPDM, felt, GRP or another fixed roof covering
- Plasterboard or another ceiling finish
- Firrings, battens, fixed finishes and permanent services
The total build-up matters, not one layer on its own.
Do the joists count?
The structure itself has weight, but you must follow the assumptions of the specific span table you are using.
The BSW Timber 2025 domestic flat-roof span table used in our joist guide states its dead-load bands exclude the weight of the joist. Do not add it again unless the table or calculation tells you to.
Dead load vs imposed load
Dead load is the permanent weight of the roof build-up.
Imposed load is variable load associated with use of the roof. Span tables state their imposed-load assumptions separately, so check the whole table heading rather than using a span figure on its own.
Why does dead load change joist span?
The heavier the permanent roof build-up, the less span the same joist may be allowed to cover.
For example, in the BSW C24 flat-roof table, a 45 × 145mm joist at 600mm centres has a maximum clear span of 3.19m at a stated dead load up to 0.25 kN/m². At 0.50 kN/m², the same joist and centres reduce to 2.94m.
That is why there is no single answer such as “6x2 spans 3m”. The roof build-up is part of the calculation.
How do I choose the right dead-load band?
Add together the permanent layers in the roof build-up using the relevant product weights or material data, then compare the result with the bands in the span table you are using.
If the result is close to the next band, do not round it down just to gain a longer span.
For joist sizing, see our Flat Roof Joist Sizes & Spans guide. Once the timber size and grade are established, you can shop our structural treated timber or browse the full Build a Flat Roof collection.
