Writing a re-roof specification: what the tender must say
Updated 23 October 2026 · SEO Dons Editorial
A specification is the document a tender is priced against. Where it is thin, each contractor fills the gaps differently and the client meets the differences later as variations. A tight specification protects the client more than any single line of a price.
Why does the specification matter more than the price?
Under JCT's Minor Works form, the employer must provide drawings, a specification or work schedules to define the quantity and quality of work. If the contractor is to design specific parts, JCT says the version with contractor's design must be used. Either way the client's document sets the scope. NBS notes that refurbishing a flat roof is made more difficult by different roof levels, hazardous materials, access and the large range of waterproofing choices, which is why each should be settled on paper first.
The contract terms that sit around the document are in JCT contracts, retentions and defects periods, and the checks on the firms that will price it are in how to choose a commercial flat roofing contractor.
What should a re-roof specification fix?
| Item | What to state |
|---|---|
| Condition evidence | The survey report and any core or moisture results, so bidders price the same roof. NBS calls a detailed survey of vital importance and says an infrared survey may be needed. See the survey guide. |
| System and certificate | The named system, manufacturer and certificate number, and whether alternatives are allowed. See how to check a BBA certificate. |
| Build-up and insulation | Each layer, the vapour control layer, insulation type and thickness, and the U-value target and who calculates it. The building regulations guide covers the thresholds; the warm deck page covers the build-up. |
| Wet insulation | A measured provisional item for removing it. NBS says high moisture content may mean full or partial removal before the new waterproofing. |
| Overlay compatibility | Where overlaying, written manufacturer confirmation; NBS says compatibility guidance should be sought from manufacturers. |
| Falls and drainage | The design falls and how they are achieved. See why flat roofs pond. |
| Details | Upstands, outlets, rooflights, penetrations and plant supports; NBS notes isolation products may be needed under plant on new insulation. |
| Safe working | Access, edge protection and fragile-surface arrangements, and whether hot works are permitted. See work at height and hot works. |
| CDM and building control | Who holds the CDM roles, the percentage of roof renewed and the sign-off route. See CDM 2015 on a re-roof. |
| Guarantee | Type, issuer, term and conditions. See the guarantees page. |
| Programme and handover | Phasing for an occupied building, working hours, weather protection, and the handover documents required. |
How do NBS clauses structure a roofing specification?
NBS says its Chorus platform lets teams write specifications using structured clauses aligned to recognised industry standards, with technical guidance and suggested values displayed alongside the clauses. The designer still has to decide the values. NBS also says CAWS is no longer maintained as a classification system and that more projects are choosing Uniclass, which Chorus uses natively.
NBS's article on refurbishing flat roofs classifies roof waterproofing along product lines, further divided by the position of the insulation. These are the entries it lists for warm and inverted constructions:
| Covering | CAWS section | Uniclass, warm | Uniclass, inverted |
|---|---|---|---|
| Mastic asphalt | J21 | Ss_30_40_30_55 | Ss_30_40_30_52 |
| Liquid-applied | J31 | Ss_30_40_30_43 | Ss_30_40_30_42 |
| Reinforced bitumen membrane | J41 | Ss_30_40_30_72 | Ss_30_40_30_71 |
| Single-layer polymeric sheet | J42 | Ss_30_40_30_80 | Ss_30_40_30_79 |
The codes are as listed in that article, dated 13 October 2025 and checked on 22 October 2026; NBS omitted cold roofs from its listing. NBS says it notifies subscribers when Uniclass tables are revised, so ask the designer which classification the tender uses and which version of the clauses.
What happens when the specification is thin?
For example, take a 2,000 square metre warehouse roof tendered on a one-line description: "strip and recover with single-ply and insulation to Part L". One bidder assumes the existing insulation is dry and prices only the new layers. Another allows for removing wet boards. The first bid looks cheaper, and the difference reappears after award as a variation for the wet areas, with the client's programme already committed. This is a hypothetical illustration, not a real project. A specification that names the survey evidence and prices wet insulation as a measured provisional item puts both bidders on the same footing.
How should alternatives and substitutions be handled?
Decide before award, not on site. Require each alternative to be priced as a separate option and to arrive with its certificate number and scope, its fire classification, the guarantee type and term, the effect on the programme and, where it overlays an existing roof, the manufacturer's written confirmation of compatibility. An alternative that cannot supply those is not like for like, whatever its price.
Who writes it, and what goes out with it?
The client's designer, such as an architect or surveyor, usually writes the specification. Under CDM 2015, regulation 4 requires the client to provide pre-construction information to every designer and contractor appointed or being considered, and HSE says that includes contractors bidding for the work. The regulations define it as information in the client's possession or reasonably obtainable that is relevant to the work and proportionate to the risks. Where more than one contractor will work on the project, regulation 5 requires the client to appoint a principal designer and a principal contractor in writing.
A complete tender pack therefore holds:
- the specification and drawings, with the survey report;
- pre-construction information, including asbestos results and existing risks;
- the form of contract and its particulars: retention, liquidated damages and rectification period;
- the evidence bidders must supply: insurance certificates, scheme records, certificate numbers, references, method statements and a draft construction phase plan;
- a price format with measured rates for provisional items, and a rule that any alternative system is priced separately with its own certificate;
- programme constraints for the occupied building.
If you want to talk through a specification for your roof, you can send an enquiry through the form on this page. The refurbishment guide explains the overlay and recover options a specification must choose between.
Sources
- Refurbishing flat roof areas, NBS (checked 22 October 2026). Page shows: dated 13 October 2025.
- Uniclass, NBS (checked 22 October 2026).
- NBS Chorus, NBS (checked 22 October 2026).
- Minor Works Building Contract, JCT (corporate site) (checked 22 October 2026).
- Minor Works Building Contract (MW 2024), JCT (online store) (checked 22 October 2026).
- CDM Regulations 2015, regulation 2, legislation.gov.uk (checked 22 October 2026).
- CDM Regulations 2015, regulation 4, legislation.gov.uk (checked 22 October 2026).
- CDM Regulations 2015, regulation 5, legislation.gov.uk (checked 22 October 2026).
- Commercial clients: roles and responsibilities, HSE (checked 22 October 2026).
Common questions
What is the difference between a specification and a quote?
A specification states what is to be built and to what standard; a quote or tender prices it. If the specification is vague, prices cover different scopes. Under JCT's Minor Works form the employer supplies the drawings, specification or work schedules that define the quantity and quality of work.
Is NBS the only way to write a roofing specification?
No. NBS publishes structured clauses and says Chorus uses Uniclass natively, but the test is whether the document is complete and every bidder prices the same one. Ask the designer which clause set and version they have used.
Should the specification name a manufacturer?
It should name the system and its certificate. If alternatives are allowed, require them to be priced separately with their own certificate, and where overlaying, with the manufacturer's written confirmation of compatibility, which NBS says should be sought.
Who provides pre-construction information for a re-roof?
The client must provide it as soon as practicable to every designer and contractor appointed or being considered. HSE says it goes to contractors bidding for the work. Include the survey, asbestos information and access constraints.