Owners and managing agents often assume that roof work on a commercial building is a maintenance matter with no paperwork. Sometimes that is right. But the Building Regulations apply to non-domestic buildings as well as houses, and a re-roof can bring in structural, fire and energy questions that a repair never does.
This guide is about the regulations that touch a commercial roof: the approval thresholds, the thermal upgrade rules known as Part L, the weight of the new covering, and how an inspection works in the Stroud area. It is not legal advice, and where a rule depends on the building we say who to ask. For the building type that adds listing on top, see our guide to historic mill roofs. The wider service is on our commercial roofing page.
Where the national guidance comes from, and its limits
Much of the published guidance on re-roofing is written for houses. The Planning Portal's re-roofing pages, and Approved Document L Volume 1, which covers dwellings, are the clearest sources available. Buildings other than dwellings are dealt with in a separate volume of Approved Document L, and its numbers are not the same as those quoted here.
So where this guide draws on the dwellings guidance, it is to show how the rules work, and the text flags it each time. For your own building the question to put to building control is plain: "Does this roof work need an application, and what thermal standard applies?" Bring the survey report and the proposed specification, and ask for the answer in writing. A written answer protects you if the work is later questioned by a buyer, a funder or a landlord, and it gives every contractor pricing the job the same starting point.
Who you ask in Stroud District
Within Stroud District, the council's service is run jointly with Gloucester City as the Gloucestershire Building Control Partnership, and applications are made through gbcpartners.co.uk. Since April 2024 a private route also exists through a registered building control approver, listed with the Building Safety Regulator. Cheltenham and Tewkesbury boroughs pool theirs under one name, and the Forest of Dean runs its own team. Our commercial roof survey page explains what we record for that conversation.
The approval thresholds for re-roofing
The Planning Portal's guidance sets two tests. Normally no application is needed while less than 25 per cent of the building envelope is affected and less than 50 per cent of the roof is affected. Both limits have to be met. Beyond them the work needs approval, and the portal notes that further considerations may require work to the entire roof, including making sure the thermal insulation is sufficient.
The envelope is every external surface of the building: walls, roof, floor. On a large unit with a single big roof, the roof can make up a high share of the envelope, so a full re-cover will often exceed one threshold or the other. A small extension roof on a large complex may not.
Other triggers for approval
The Planning Portal lists further reasons an application is needed, whatever the percentages:
- structural alterations to the roof;
- a fire rating for the new covering that departs markedly from the existing roof's;
- extra weight of 15 per cent or more from the new covering.
A repair that stays well within the old roof's weight, material and structure is the kind of work that usually needs no application. Patching a few seams, replacing an outlet or renewing a short run of flashing generally sits in this group. When we are unsure, we ask building control before pricing, not afterwards. Where notification is needed we explain consent, and we handle the Building Regulations notification for you. Sequence and timing are covered in our guide to the time a commercial re-roof takes.
Part L: when a roof renovation triggers a thermal upgrade
Part L of the Building Regulations deals with the conservation of fuel and power. In Approved Document L for dwellings, a roof is a thermal element, and renovating a thermal element can mean bringing it up to a modern insulation standard. The document gives two examples that fit flat commercial roofs well: stripping a roof down to its structure and rebuilding it, and the renewal of a flat roof's waterproofing.
The 50 per cent and 25 per cent tests
In that volume, the element as a whole should reach the stated U-value when more than 50% of that element's surface is renovated. The same applies if the job counts as a major renovation, which means more than 25% of the external envelope.
Here, a U-value expresses how quickly heat escapes through a surface, so lower is better insulated. Regulation 23(2), as printed in the document, adds that when more than 50% of an element's area is replaced piecemeal, the whole must comply, so far as that is feasible in technical, practical and cost terms.
The dwellings volume also sets numeric U-value targets for roofs, but those belong to houses and should not be carried across to a shop, unit or office. Approved Document L Volume 2, for buildings other than dwellings, has its own tables, so the target for your roof is a question for building control, answered for your building and its use.
What that means on a flat roof
A new membrane laid over wet or thin insulation would not meet a modern standard. A strip-and-replace job usually offers the chance to add insulation at the same time, and we explain what that insulation adds before you commit. Where headroom or parapet heights restrict the thickness, the dwellings volume accepts a lower standard in some cases, and the decision is building control's. Our guide to overlaying a commercial roof covers how an overlay is treated.
Heavier coverings and the structure beneath
Approved Document A, which deals with structure, treats a significant increase in roof loading as more than 15%, and also asks you to consider if the covering coming off is the first one the building had. The Planning Portal's wording is "15 per cent or more". The sources differ by a hair at the margin, so we treat any change near that line as a question for building control and a structural engineer.
The document goes on to say that if the existing structure cannot take the added load, because members are overloaded or deflect until water collects, strengthening follows. On a commercial building the commonest trigger is a change of system. A lightweight metal sheet roof replaced by a ballasted single-ply or by a heavier insulated build-up changes the load on every rafter, purlin and truss. Likewise a stone slate or clay roof replaced by a lighter sheet changes the loads on the walls.
What the checks involve
A structural check normally means a visit from an engineer, a look at the existing members and a short calculation. We supply the weight of the proposed build-up per square metre, taken from the manufacturer's data. The engineer compares it with the old build-up and the state of the timber or steel. The result, if favourable, goes to building control with the application.
If a roof has ponding, sagging or visible deflection, mention it when you ask for a quote. It changes what the structure can take, and it changes what the survey must look at.
Listed buildings, conservation areas and planning
Building Regulations approval and planning permission are separate systems, and a commercial roof can need either, both or neither. A building can pass its Building Regulations inspection and still need consent from the planning authority.
Planning law excludes works that leave how a building looks from outside essentially unaltered (section 55 of the 1990 Town and Country Planning Act), so a faithful repeat of the old covering is not normally development. A listed building is treated separately: alterations affecting its special interest need listed building consent, even where planning permission is not required. Stroud District has 41 conservation areas, with Article 4 Directions in five. Those directions remove certain permitted development rights from some houses; for a commercial building, take any query to the planning team direct.
New rooflights and plant
The permitted development rights for roof windows are written for houses. A new rooflight on a shop, factory or office is a planning question for the council, not something to assume. Our page on commercial rooflights covers what we survey. The same goes for solar panels on a commercial roof: the structural and planning questions are covered on our page about solar on commercial roofs.
Asbestos and work at height
These sit outside the Building Regulations but touch the same jobs. HSE classes careful, unbroken removal of asbestos cement roof sheeting as non-licensed work. If the sheeting is substantially broken, the job becomes notifiable. Notification goes in before work begins, by HSE's online form. See our guide to fragile roofs and safe access for access arrangements.
Inspection, sign-off and the paperwork you keep
When an application is needed, building control inspects at set stages. For a re-roof these are typically the deck or structure before it is covered, the insulation as it goes down, and the completed covering. Stroud District Council says that where notice arrives before 15:00 on a weekday, its aim is an inspection the following working day if it can manage it. Forest of Dean District Council gives a similar arrangement for calls before 3pm. These are council aims, not guarantees, and we plan the programme with some slack.
What we hand over
- The survey report with photographs of the existing roof.
- The specification of the new build-up, with product data sheets.
- Copies of any approval or notification.
- Photographs of each layer before it is covered.
- A written statement of the work, under our 10-year workmanship guarantee.
Keep the paperwork with the building's records, in a place the next owner or managing agent will find it. A buyer, funder or landlord will ask for it, and a tenant faced with a dilapidations claim will want proof of what was done. Our guide to claims, reports and dilapidations explains why.
Cost
Application fees are set by the building control body and vary with the value and nature of the work. Your written quote shows our own price, and we tell you what the building control body will charge once it has the details. VAT on roofing work is normally standard-rated. Our commercial roof cost guide has the sourced national ranges.