If you are pricing a SIP garden room, office, studio, annexe shell or outbuilding, the roof is one of the areas that deserves proper thought. It affects weather protection, internal height, drainage, planning height, delivery size, panel count and the final finish of the building.
This guide explains the main roof options customers ask us about at UltraSIPs: flat SIP roofs, mono-pitch roofs, dual-pitch roofs, EPDM rubber roof coverings, kerbs, trims, roof falls and drainage. It is written for customers who want plain English before they commit to a kit or send drawings over.
Short version: SIP roof panels can be used for both flat and pitched roof designs, but the SIP panel is the insulated structural panel, not the finished weatherproof roof on its own. You still need the correct roof covering, edge detailing, drainage and any structural sign-off required for the project.
What is a SIP roof panel?
A SIP roof panel is a structural insulated panel used to form the roof deck or roof plane of a building. UltraSIPs panels are made using OSB faces with PIR insulation in the centre, creating a strong insulated panel that can reduce cold bridging and speed up assembly compared with building a roof from separate joists, insulation and boards on site.
In a SIP kit, roof panels are normally designed alongside the wall panels so the kit works as one shell. The roof panel layout depends on the building size, roof type, overhangs, openings, rooflights, transport limits and the fixing detail agreed for the project.
Flat roof SIP panels are not actually dead flat
When customers say "flat roof", they usually mean a low-profile roof that looks flat from the garden. In practice, a roof should normally have a fall so water can move towards a gutter or outlet. A completely dead-flat roof is more likely to hold water, show ponding and cause avoidable problems with the roof covering.
For a SIP garden room or outbuilding, a flat roof design usually needs:
- SIP roof panels sized for the building.
- A fall or designed drainage route.
- EPDM or another suitable waterproof roof covering.
- Adhesive, trims, edge details and finishing pieces.
- Gutter, outlet or drainage route.
- Kerbs or raised upstands if rooflights or penetrations are used.
The fall can be created in different ways depending on the design. Some builds use tapered firrings or a shaped timber detail above the panel. Some designs use a mono-pitch roof rather than a concealed flat roof. The right answer depends on the building, the required appearance and the site.
Flat roof vs mono-pitch vs dual-pitch SIP roofs
There is no single roof type that suits every SIP build. The best roof depends on head height, boundary position, planning constraints, the intended use, budget and the look the customer wants.
| Roof type | Best for | Things to check |
|---|---|---|
| Flat or low-slope roof | Modern garden rooms, offices, studios and lower-profile outbuildings | Fall, drainage, EPDM details, edge trims, overhangs and height near boundaries |
| Mono-pitch roof | Simple drainage direction, contemporary garden rooms and buildings where one high wall is acceptable | High side height, gutter position, wall heights and roof panel direction |
| Dual-pitch roof | More traditional appearance, extra internal height, vaulted spaces and some larger buildings | Ridge detail, support requirements, roof finish, panel spans and planning height |
For simple garden rooms, flat and mono-pitch roofs are popular because they are clean, efficient and easy to understand. For larger buildings, workshops, annexes or buildings where internal height matters, a pitched roof can make sense, but the support detail and final roof covering need more attention.
Can EPDM be used on SIP roof panels?
Yes, EPDM rubber roofing is commonly used on flat and low-slope roof builds, including garden rooms and outbuildings. It is a separate waterproof roof covering installed over the correct roof deck and detail build-up. It is not a replacement for designing falls, edges and drainage correctly.
When we quote EPDM as part of a SIP roof package, the quote should make clear what is included. Depending on the build, this may include membrane, adhesive, edge trims and relevant accessories. The exact parts depend on roof size, roof shape, whether there are overhangs, rooflights, upstands, outlets or unusual details.
EPDM can also be considered on some shallow pitched or mono-pitch details where the system and installer are happy with the application. For steeper pitched roofs, customers often look at lightweight tiles, fake slate, metal sheet systems or other pitched-roof finishes instead. The roof covering should be selected to suit the pitch, exposure and final appearance.
What are roof kerbs or curbs?
In UK construction, the usual spelling is kerb. Some customers and suppliers also write curb, especially when talking about rooflight curbs. In simple terms, a roof kerb is a raised edge, upstand or framed detail that helps finish and protect an opening or roof edge.
Kerbs are often relevant around:
- Rooflights or skylights.
- Flat roof penetrations.
- Parapet-style roof edges.
- Raised upstands where waterproofing needs to turn up.
- Openings where the roof covering needs a clean termination point.
For SIP roofs, the roof opening can be formed in the panel design, but the kerb/upstand and waterproofing detail still needs to be right. If a customer is supplying their own rooflight, the exact rooflight dimensions and manufacturer details should be checked before panels are cut.
Drainage: the part people often leave too late
A good roof plan should answer one simple question: where does the water go?
That means thinking about gutter location, downpipe route, discharge point, roof fall direction and whether there are any areas where water could be trapped. On a garden room near a boundary, the fall and gutter position can also affect the final look and height.
Before ordering, it is worth deciding:
- Which side should the roof drain towards?
- Will the gutter sit on the front, back or side?
- Is there enough access to maintain the gutter?
- Will the roof have an overhang?
- Are there trees nearby that may drop leaves onto the roof?
- Are rooflights or other penetrations being added?
These details are not glamorous, but they stop small design decisions becoming site problems later.
What SIP roof thickness do you need?
The correct SIP thickness depends on the use of the building and the performance target. As a general guide:
- 122mm SIP panels can be suitable for many garden rooms, sheds, studios and non-habitable outbuildings where Building Control is not involved.
- 172mm SIP panels are often the better starting point for habitable spaces, annexes, extensions or projects likely to involve Building Control.
- 222mm SIP panels can be considered where stronger thermal performance is required, especially for roof U-value targets.
If U-values matter for your project, use the UltraSIPs U-value calculator or ask us to check the intended wall, roof and floor specification. Do not guess roof thickness if the project needs Building Control approval.
Planning height: roof shape can matter
Roof type can affect whether a garden building falls within permitted development limits. The Planning Portal guidance for outbuildings says limits can include maximum eaves height of 2.5m, maximum overall height of 4m with a dual-pitched roof, or 3m for other roof types, with a 2.5m maximum overall height if the building is within 2m of a boundary. You should check the exact position and rules for your property before building.
Planning rules can vary with property type, designated land, conservation areas, listed buildings, previous extensions and local restrictions. For official guidance, see the Planning Portal outbuildings guidance.
Building Regulations: when roof specification becomes more important
Building Regulations may not normally apply to some small detached outbuildings if they meet exemption criteria, but they can apply where the size, use, sleeping accommodation, attachment to the house or other factors change. Planning Portal guidance states that small detached buildings under 15m2 with no sleeping accommodation are normally outside Building Regulations, and buildings between 15m2 and 30m2 may also avoid approval if they contain no sleeping accommodation and meet boundary or non-combustible construction conditions.
That is general guidance, not project approval. If your building is habitable, attached, used as an annexe, used for sleeping, part of an extension, or likely to be inspected by Building Control, roof U-values, structure, fire details and ventilation can all matter. See the Planning Portal Building Regulations guidance for outbuildings and speak to Building Control or a competent professional where needed.
What is included with an UltraSIPs roof quote?
Every quote should be read carefully because different customers choose different levels of supply. A SIP roof quote may include only the SIP roof panels, or it may include additional items such as fixings, timber pack, membrane, EPDM and trims.
As a practical checklist, ask whether the quote includes:
- SIP wall panels.
- SIP roof panels.
- SIP floor panels, if required.
- Splines.
- Fixings.
- Timber pack.
- Breather membrane or waterproofing membrane where specified.
- EPDM roof covering, adhesives and trims where selected.
- Roof overhangs.
- Rooflight openings and kerbs/upstands.
- Provisional physical support allowance if beams, lintels or goalposts are flagged.
- Delivery.
- Installation.
Unless it is specifically listed, do not assume windows, doors, cladding, rooflights, foundations, electrics, plumbing, internal finishes, Building Control fees or structural calculations are included.
Structural support: roof panels still need sensible spans
SIP roof panels are strong, but they are not magic. Roof size, span, openings, loads, pitch, supports and connection details all matter. Large openings, wide rooms, vaulted roofs, goalposts, ridge beams or glulam beams may need an engineer to confirm the final design.
The UltraSIPs configurator can show a provisional support allowance where the model flags likely beams, lintels or goalposts. That allowance is for physical support items only. It is not a structural calculation and it is not Building Control approval. If the project needs calculations, those should be confirmed separately by a structural engineer.
Flat roof EPDM details customers should not ignore
If you are adding EPDM to a SIP roof, pay attention to the details rather than just the square metre price. A good roof covering package needs to deal with edges, corners, gutter edge, upstands, penetrations and any rooflights.
Common items to check include:
- Membrane size and whether the roof can be covered in one piece.
- Deck adhesive and bonding adhesive requirements.
- Perimeter trims.
- Drip edges and gutter edge trims.
- Corner details.
- Primer or tape where required by the system.
- Rooflight kerbs or upstands.
- How the roof edge meets cladding, fascia or parapets.
For simple garden rooms, this is usually straightforward. For awkward shapes, rooflights, parapets or multiple roof levels, it is better to send the drawing over before assuming a basic EPDM allowance will cover everything.
Using the UltraSIPs configurator for roof choices
The UltraSIPs configurator is useful because it lets customers test roof shape, SIP thickness, openings, timber, EPDM, delivery and installation options before drawings are final. It helps show how roof decisions affect the overall kit price.
For best results, enter the building size, roof type, openings and delivery postcode as accurately as possible. If the model is unusual, has multiple roof slopes, has large rooflights, or needs a particular roof finish, send it to us before ordering so we can check the assumptions.
Roof questions to answer before ordering
Before you buy a SIP roof kit, try to answer these questions:
- Is the roof flat, mono-pitch or dual-pitch?
- What SIP thickness do you need for the roof?
- Is the building a simple garden room or a habitable/Building Control project?
- Where will the roof drain?
- Do you need EPDM included?
- Do you need rooflights, skylights or service penetrations?
- Do you need overhangs?
- Are you near a boundary where height matters?
- Will the customer, installer or UltraSIPs arrange the final roof covering?
- Are structural calculations required?
Final thoughts
SIP roof panels are a strong option for UK garden rooms, offices, studios, outbuildings and larger SIP builds, but the roof should be treated as a complete system rather than just a panel price. The panel, fall, covering, edge details, drainage and structural support all need to work together.
If you are planning a simple garden room, the configurator may be enough to get a clear starting price. If you are planning a larger building, a roof with multiple slopes, a rooflight-heavy design, or a project that may need Building Control, send the drawings or sketches to us and we can check the assumptions before you commit.
Next step: use the UltraSIPs configurator to test the roof type and package options, or contact us with sketches, dimensions and any roof covering preferences.