Roofing Specifications

Since its foundation in the UK, Sika Sarnafil has continually developed a range of specification methods to suit client needs. Today these specifications are broken down into two clear categories:
Sarnafil job specific specifications and Sarnafil NBS Clauses

Sika Sarnafil prefers to provide specifications based on our own Standard Paragraphs, as they are updated more regularly than NBS Clauses and enable the specification to be written in a more detailed defined manner. Where an NBS specification is requested, Sika Sarnafil recommends the Sarnafil specification is attached to the NBS as an appendix.

Every Sika Sarnafil specification is unique and given its own unique Project Record Number, a number that stays with the project from specification, through order and all the way to sign off for final guarantee.

Sarnafil Specifications

Whatever specification format is used, Sika Sarnafil recommends that they are read carefully by all parties, as they are always project specific and every project is slightly different.

Each specification provides important tender information, which if missed or ignored, can prove costly to correct later. Key areas we would recommend close attention to are:

- Guarantee type and period
- Membrane grade and colour
- Any pullout test requirements on the substrate
- Substrate type (and assumptions)
- Preparation of substrate
- Sarnafil branded products required e.g. SarnaTherm insulation, for a Sarnafil Extended Product Guarantee
- Insulation fixing recommendations (where required)
- Insulation generic type, uniform or tapered (where required)
- Membrane fixing recommendations (and assumptions)

If in doubt about any item within a specification, the local Technical Adviser or the Sika Sarnafil Technical Services Department will gladly advise on any specification change and its consequences.

Sarnafil specifications are entirely bespoke, each using a unique Sarnafil Project Record Number, which should be quoted in any correspondence, whether new build or refurbishment. We recommend that you contact your local Sika Sarnafil Technical Adviser for a bespoke specification to ensure that best value and technical fit are achieved. Please click on the link for contact details of your local Sika Sarnafil Technical Adviser.

NBS Specifications

As a member of the RIBA NBS Plus service since 1998, we can provide the client with a bespoke NBS format Sarnafil specification for any project. When providing NBS format specifications we will always provide an accompanying Sarnafil specification, as this enables a more detailed project specific specification to be prepared

Sika Sarnafil recommends that when tendering using an NBS specification, a number of key details are checked in the Sarnafil specification, such as:

  • The Sarnafil Project Record Number
  • Guarantee type and period
  • Membrane grade and colour
  • Any pullout test requirements on the substrate
  • Substrate type (and assumptions)
  • Preparation of substrate
  • Sarnafil branded products required e.g. SarnaTherm insulation, for a Sarnafil Extended Product Guarantee
  • Insulation fixing recommendations (where required)
  • Insulation generic type, uniform or tapered (where required)
  • Membrane fixing recommendations (and assumptions)

If in doubt about any item within a specification, the local Technical Adviser or the Sarnafil Technical Services Department will gladly advise on any specification change and its consequences.

All specifications are issued using a unique Sarnafil Project Record Number, which should be quoted in any correspondence.

You can download examples of NBS Specifications click here.

Standard Details

Sarnafil AutoCAD and PDF format standard details have been created to assist the design of high quality roofing systems that maintain buildability on site.

The Sarnafil Accredited details for New Build, minimise air permeability and heat loss to provide designers and engineers with tools that enable the design of energy efficient new buildings and the improved thermal efficiency of existing buildings. Thermally modeled by the BRE these details support heat loss calculations, check condensation risk and reduce air permeability; providing best in class performance that assists the designer in meeting the requirements of Building Regulation Part L 2010 and beyond.

The refurbishment section contains details created for actual refurbishment projects and most details show a variety of ways to solve similar details.

Green roof (and ballasted) details are drawn, for simplicity, on a concrete deck for illustration purposes only. The layered format of the standard details enables the designer to alter the details for specification on steel and timber decks.

Standard Details can be downloaded in our Technical Download section.

Special Roof Designs

Over the past 50 years Sarnafil single ply membranes have been used to waterproof a wide variety of special roof designs, from stunning domes and curves to hyperbolic parabaloids and listed buildings; and whether a special roof design or a standard flat roof, Sarnafil membranes can be supplied in a wide variety of colours.

The extensive range also allows the ability to reproduce the appearance of traditional metal roofing systems such as Lead, Copper or Aluminium, at a fraction of the cost, by incorporating Sarnafil Decor or Batten profiles.

Decor Profile

Decor Profile reproduces the appearance of traditional metal standing seam roofing systems. It is available as standard in Lead Grey, Copper Patina, Copper Brown and Light Grey.

Batten Profile

The larger Batten Profile simulates the appearance of a traditional lead batten roll and is therefore available in lead grey only. Both profiles are suitable for use on projects of any design, shape and/or pitch.

General Design Considerations

When deciding on the type of waterproofing to be specified for a project, the designer has to take into account a number of general considerations regarding the performance and aesthetic requirements of the roof.

In this section, we are seeking to focus on some of the fundamental design considerations that are regular enquiries from designer and specifiers alike. For further information, please select the appropriate design consideration from the links provided below.

In all cases, we recommend that a Sika Sarnafil Technical Adviser is involved in a project as early as possible. This enables the Technical Adviser to advise on interfaces, compatibility with other systems and the optimum design for the given parameters. If consulted early enough in the design process, the Technical Adviser may also be able to assist in providing significant savings in the rainwater drainage system.

Wind Uplift Calculations

Acoustic Performance

Compatibility of Components




Fire Performance

Building Regulations

Wind Uplift Calculations

To ensure that every Sarnafil roofing system will stay on the building, no matter what the geographic location or height of the building, the Sika Sarnafil Technical Services Department calculates the wind load on every individual roof; ensuring the right method of attachment is chosen for the project.

Whilst many others leave the calculation of these critical factors to chance, or pass on liability to the roofing contractor, Sika Sarnafil use specialist software to design the roofing system to BS 6399: Part 2: 1997 code of practice for Wind Loads and also the new Eurocode, BS EN 1991-1-4: 2005 UK National Annex Method.

What’s more every Sarnafil roof benefits from suitable design insurance that indemnifies our work.

Acoustic Performance

All likely sources of unwanted external and internal noise should be identified in order to establish the degree of attenuation required to suit the building’s function. As acoustic performance is heavily dependent upon the selection of materials (especially the deck and thermal insulation), early identification of the requirement may assist the design selection process.

On lightweight, wide-span structures, noise from heavy rain is often overlooked in design. However, the inherent flexibility of single ply membranes, on the appropriate insulation, provides for good attenuation when compared with rigid sheet systems.

Compatibility of Components

Compatibility between the various elements of a roofing system is vital to the longevity of the system.

This rule applies to not only the components in the roof but also any elements interfacing with the waterproofing system, such as vertical cladding or rendering.

Sika Sarnafil aims to ensure the compatibility of all products interfacing in the roof build up, not only our own products included in our specifications. For example, any thermal insulation designed for use in an adhered system is required to undergo testing at our R & D facility in Switzerland, to ensure that it is not only a suitable adhesive bond, but also that none of our adhesives will affect the performance of the thermal insulant.


One of the key aspects of a product’s environmental impact is its durability. This is expressed in terms of anticipated life until renewal. In financial terms it is the period over which the depreciated initial capital cost and annual maintenance cost do not exceed the annual cost of a replacement roof.

The British Board of Agrément (BBA) assess the durability of single ply roofing membranes as part of the Agrément Certification process. Single ply membranes are typically given a life expectancy of:

  •  “at least 25 years”
  •  “in excess of 25 years (Sarnafil TCG/TCS has this)
  •  "at least 30 years”
  •  “in excess of 35 years or 40 years” (Sarnafil G/S has both)

Download BBA Certificates click here.


Generally environmental impact considerations range from Cradle to Gate (consumption of natural resources and energy during manufacture) to Cradle to Grave whereby consideration of installation through to removal, recycling and disposal are additionally considered. Many tools exist to assess these, among the most widely used of these are the BRE Green Guide and BREEAM tools. With these, realistic durability and maintenance input estimates are an essential pre-requisite of impact studies.

Thermal insulation performance generally has the most significant effect on the environmental impact of any roofing proposal and the environmental impact of a particular roof is specific to that design and must be calculated for accuracy. Many simplistic impact ratings for individual materials are available, but in reality the impact of a design is dependent upon the client’s selection of which environmental issues are most important.

Sika Sarnafil can provide a wide variety of information regarding the environmental profiles of our materials, including the provision of recycled products. Sika Limited base these profiles upon agreed national and international protocols, as they develop, not on our own claims.


BS 6229:2003 - Flat Roofs with Continuously Supported Coverings Code of Practice and the SPRA Design Guide both recommend that flat roofs should have a Design Fall of 1:40 to achieve a Minimum Finished Fall of 1:80.

Whilst Sika Sarnafil recommends that this guidance is followed we understand that it is not always possible. In such circumstances relevant parties should be notified of the risk of ponding water on the roof surface. Any ponding water will not be detrimental to Sarnafil membranes as standing water has no detrimental affect on their waterproofing integrity, life expectancy or guarantee.

When a roof is installed at less than the recommended fall, consideration should be given to the following;

  • Loading on the structure; the additional weight of ponding water must be accommodated
  • Aesthetics; risk of standing water, algae, moss, mosquitoes, etc.
  • Health & Safety; dangerous conditions may occur as the water freezes in winter or becomes slippery after rainfall 
  • Contradiction of all good practice guidelines including BS 6229:2003, SPRA, NFRC and NHBC guidance
  • Increased risk of leakage if the roof waterproofing gets damaged

Sarnafil Agrément Certification may be invalidated if no fall is provided as this is in contravention of good practice. This will depend on the circumstances and the interpretation by Local Authority Building Control.

Fire Performance

The Building Regulations Approved Document B requires that the roofs of certain buildings meet specified performance levels for exposure to fire from external sources, the minimum requirement being Ext, FAC. Insurance loss prevention consortia, such as Loss Prevention Council (LPC) Factory Mutual (FM), may specify performance in excess of the mandatory requirement.

External fire performance of roof systems is assessed according to BS 476: part 3: 2004, where the roofing system is subjected to fire exposure from a simulated external burning brand comprising a gas flame. The external fire performance is expressed as a requirement for the whole roof construction build up, including deck, vcl, insulation and covering, characterised by penetration of fire and spread of flame. Classification ranges from (highest) Ext. FAA (external flat, penetration rating A, spread of flame rating A) to Ext. FCD (lowest).
CEN TS.1187 test method 4 may also be used but does not include spread of flame as part of the test. Test results are classified in accordance with European Standard EN 13501-5 Fire classification of construction products and building elements - Part 5: Classification using data from external fire exposure to roof tests and should achieve a minimum classification of Broof(t4).

The Building Regulations Approved Document B: also makes fire resistance provisions for where fire exposure to the underside of the roof needs to be considered. These are listed in the approved Building Regulation document and apply: When the roof and its support structure are part of an escape route, including any opening within 3m of the escape route.

When the roof performs the function of a floor
Building designers should ensure that the client is aware of the current Fire Precautions (Workplace) Regulations wherein the employer is required to undertake the continual review of risk assessment to ensure that employees are not placed at risk from fire. It is recommended that a fire expert be consulted as part of this process.

Building Regulations

The 2006 revisions to the Building Regulations Approved Document L for England & Wales introduced the requirement to notify Building Control of any intended refurbishment work to a thermal element (Roof, Wall or Floor) of a certain size and to present the Building Control Body (BCB) with proposals for the upgrade of its thermal performance, to a required minimum standard.

The 2010 revision of Approved Document L requires a further flat 25% reduction in energy use, over the 2006 regulations for dwellings and an aggregate 25% reduction for buildings other than dwellings, based on a notional building type.
The requirements of the Scottish Building Standards Association (SBSA) Technical Handbook, Section 6 are very similar.

With 99% of the annual built environment consisting of existing buildings, significant carbon emission reductions can only be achieved by upgrading existing building stock (Saving around 150 million tonnes of CO², according to EU figures), the benefit to the building owner/operator will be a reduction in the energy costs associated with heating and cooling.

Can you avoid the extra expense?

Building Regulations Approved Document L (ADL) Part L2B: Conservation of fuel and power (Existing Buildings other than Dwellings) England and Wales requires minimum U-values to be achieved. The regulations state that when refurbishing a flat roof, a check on the insulation within that roof must be made to ensure that it meets the requirements of Approved Document L 2010.

The Exceptions

There are some exemptions, unheated buildings, some listed / heritage buildings or where the payback period (total cost of upgrading versus savings made) exceeds 15 years to achieve. Ultimately the BCB is required to make a judgement on what will be required, Sika Sarnafil therefore advises that Building Control is consulted early in the planning process of the project, as the potential financial consequences for non compliance could be significant.

These requirements are that:

  • A roof that already has insulation installed must achieve a U-value of 0.35 W/m²K
  • A roof with a U- value of less than 0.35 W/m²K must be improved to achieve a U-value of 0.18 W/m²K
  • A new build property / extension roof must achieve a minimum U-value of 0.25 W/m²K
  • A roof repair or overlay that accounts for more than 50% of the individual roof element or 25% of the entire building must comply with the new requirements
  • When an extension is built consequential improvements to the existing building are required and should be not less than 10% of the cost of the extension

Independent Verification

As the market leader in the supply of single ply roofing membranes Sika Sarnafil strives to ensure that it is in step with all relevant regulatory changes and that any advice provided to contractors and clients, regarding the impact of these changes, is accurate and current. However we appreciate that sometimes contradictory advice provided by others may cause confusion. Therefore, listed below are the contact details for some independent bodies that can be consulted on this key topic.


Single Ply Roofing Association
Telephone: 0115 914 4445



National Federation of Roofing Contractors
Telephone: 0207 436 0387



Local Authority Building Control
See local telephone directory or call Local Authority

Green Roofing Design Considerations

The installation of any green roofing system requires the consideration of additional factors to those considered for an exposed roofing system. The links below provide guidance on a variety of these factors, as this will both speed the specification process and minimise installation issues on site through effective planning.

Early consultation with a local Sika Sarnafil Technical Adviser will assist the correct system specification and construction, ensuring water storage, substrate formulation and nutrient levels are optimally balanced to sustain the plants for maximum year round performance and environmental benefit.

Building Physics

Design Loads


Planting Pitched Roofs

Site Considerations

Waterproofing Section

Building Physics

In a green roof, it is very important to calculate the building physics. This includes the dew point position, the effect of any water retention layer and the thermal impact of any additional layers within the construction, to ensure that the correct vapour balance is achieved.

With a flat roof, the most effective diffusion current under winter conditions in our latitude will be directed from interior to exterior. In the summer, there is only a small differential from interior to exterior, so for only a few hours during the year these conditions are likely to be reversed.

However, the usual vapour diffusion calculations do not take sufficient account of the moisture transport mechanisms of planted flat roofs, where standing water has a significant effect on the vapour diffusion properties.

With any system (including the SarnaVert system), water is stored above the waterproofing membrane to supply the vegetation’s needs during prolonged dry periods. The affect of this water on top of the waterproofing and/or protective layers (such as AquaDrain), prevents the moisture from being able to diffuse out, not only during condensation periods, but also during drying periods. For example, assuming that the water film acts as 100% humidity, under cold conditions, a vapour pressure differential arises from interior to exterior, as demonstrated in the following example:

Room air temperature 20°C /External air temperature 5°C
Relative Humidity (RH) 50% /Relative Humidity (RH) 100%
Vapour Pressure (p) 1169 Pa /Vapour Pressure (p) 872 pa

In the summer, the internal and external vapour pressure is practically balanced; as shown in the example below.

Room air temperature 25°C /External air temperature 18°C
Relative Humidity (RH) 70% /Relative Humidity (RH) 100%
Vapour Pressure (p) 2217 Pa /Vapour Pressure (p) 2064 pa

Sika Sarnafil therefore recommend the installation of a vapour control layer with a high diffusion resistance (at least 400 m²h Pa/mg), ensuring the absorbed vapour quantities are generally kept low. This means that during the life of the roof, even with impaired evaporation, only small acceptable quantities of moisture accumulate. In warm ballasted applications, a high quality vapour control layer (such as loose laid Sarnavap 2000E or Sarnavap 5000E SA) will usually be required due to the potential negative (downward) vapour drive. For this reason, Sika Sarnafil do not recommend the use of Inverted ballasted green roofing systems.

To ensure that designs are correct, project specific calculations will be made by Sika Sarnafil’s thermal insulation partners.

Design Loads

A SarnaVert green roof will create an increased load on the finished roof area. This needs to be included in the structural design of a new building, or thoroughly checked in a refurbishment application.

Suggestions of critical areas to check, together with indicative design loads, are given below:

  1. Check load bearing capacity and deflection of deck, new and refurbishment systems.
  2. Ensure calculations take into account the wet load per surface of greenery, soil and live load (wind, snow, etc).
  3. Check spot loads (trees, etc).
  4. Check the compressive strength of thermal insulation.
  5. When used on a pitched roof, the retention battens must provide sufficient resistance to the thrust of the build-up.

Substrate Media Design load per 10mm thickness

  • SarnaVert Growing Medium - 0.12 kN/m²
  • SarnaVert AquaDrain - 0.055 – 0.06 kN/m²
  • Pumice, swelling clay* - 0.08 kN/m²
  • Pumice, swelling clay* - 0.14 kN/m²
  • Gravel* - 0.18 kN/m²
  • Drainage layers* - 0.02 - 0.40 kN/m²
  • Soil - peat mixture* - 0.10 - 0.13 kN/m²
  • Soil - sand mixture* - 0.16 - 0.18 kN/m²

Greening Media Design Load

  • SarnaVert hydroplanting 0.05 – 0.15 kN/m²
  • SarnaVert plug planting - 0.05 – 0.15 kN/m²
  • SarnaVert Sedum Mat/Blanket - 0.25 – 0.30 kN/m²
  • Grass, herbs & moss* - 0.05 - 0.15 kN/m²
  • Shrubbery* - 0.10 - 0.30 kN/m²
  • Trees* - 0.60 - 1.50 kN/m²

* source, German FLL Guidelines, 1995

All figures provided are for guidence and should be confirmed by the Sarnafil Horticultural supplier for the specific green roof specification.


When designing a green roof, Sika Sarnafil recommend a reduction in the capacity of the drainage system, if it can be guaranteed that the greening system will remain in place for the life of the building. This is due to the fact that the green element retains water. If this has been taken into account in the drainage design, removing it may result in the drainage being unable to cope.

Regardless of the drainage option chosen, any standing water will have no effect on the Sarnafil membrane. However, it is always good practice to try and ensure that a roof drains as efficiently as possible. This prevents the accumulation of airborne dirt and dust and the risk of ponding water affecting plant life.

To assist the designer in the provision of a suitable gravity drainage system and in accordance with BS EN 12056 – 3: 2000 ‘Gravity drainage Systems inside buildings. Part 3 Roof drainage, layout and calculation and the Building Regulations Approved document Part H, Sika Sarnafil offer a drainage calculation service based on our standard rainwater outlet range. This has been tested to BS EN 1253 : 2000, to enable the accurate provision of these calculations. For siphonic drainage systems, Sarnafil can provide whole system solutions, with the assistance of Branded Product Partner Dallmer Ltd.

Planting Pitched Roofs

Extensive and Biodiverse roofing systems can be planted on roofs of varying pitches, with pitches of up to 35° generally being accommodated with standard systems. Pitches greater than this, all the way to vertical, can be ‘greened’, but will need bespoke system design to suit the application and may be easier to achieve using ground grown planting.

Pitched warm roofs are the ideal candidates for planting, as pitched cold roofs will require ventilation between the thermal insulation and the pitched roof decking, resulting in the need for a venting system at the ridge. This venting system will differ to standard pitched roof ridge ventilation, as it is required to be above the finished roof/plant level.

The first consideration when designing a green roof should be the desired slope of the roof. All SarnaVert green roofing systems can be installed on a flat roof, with other roof pitches requiring consideration, as shown below.

Extensive and Biodiverse Systems
1° slope or less Drainage layer needed
> 5° to 35° slope Movement/shear protection required
> 35° slope tailored to project requirements

In a pitched application, the position of the thrust battens to retain the growing medium must be calculated, taking into account the affect of thrust loads.

Intensive Systems
0 to 3° slope Water retention/irrigation needed
> 3° slope Not recommended

In all roofing systems, correct detailing is critical. This is even more vital in a green roofing system, where all parapet and upstand flashings should terminate a minimum 150mm above the level of the finished roof greening.

Health and safety is also critical, both in installation and for future maintenance. Either a parapet wall in excess of 1.1m high, an edge protection or fall arrest system should be provided, together with suitable safe access. Where a green roof is being installed to provide utility or amenity, suitable handrails or parapets should be provided.

The principle difference between planting on a flat roof and planting on a pitched roof, is the requirement for special measures to resist the thrust load and sliding of the planting system. This is generally achieved by using thrust battens, typically consisting of a 60 x 100mm cross section timber, with a maximum moisture content of 16% by weight. The thrust battens should be screw fixed firmly into timber inserts in the thermal insulation and weatherproofed, with cover flashings welded to the finished roof surface.

Depending on the roof pitch the thrust battens will be installed at the apropriate centres. The first thrust batten should be positioned approximately 500mm below the ridge line. The balance of the thrust battens will be installed at the required spacing, starting from the first batten. Each horizontal run of the battens should be broken every 6 metres, by leaving a 200mm gap to allow for drainage. Each run of battens should have the drainage gaps staggered in a brick bond pattern. The number and spacing of the thrust battens will vary, according to roof pitch and should be confirmed by the Sarnafil Horticultural supplier.

Site Considerations

Key to the successful installation of any green roof is the co-ordination of all members of the construction team.

Other Trades
Ideally the start time should be programmed early enough to ensure good access, but late enough to ensure that other site trades are not accessing and damaging the newly installed plant layer. Similarly, the installation of the waterproofing system should not be subjected to potential damage from other trades.


Ideally, the plant layer should be installed between September and April as, this virtually eliminates the need for an irrigation system.

Installation from May to August is likely to require the addition of an irrigation system, to assist plant establishment.


Water supply for installation irrigation is typically a ¾ mains take off with sufficient head pressure to reach all points on the roof. This is required to fill the 3000 litre ground sited tank. Use of a ½ pipe will delay the installation programme.

Hard Standing

Hard road standing for a silo tanker (if this method of substrate delivery is used), is typically roof areas over 300m².


If crane access is required for growing medium and Sedum mat/blankets (if used), the crane must be capable of delivering a maximum load of 1.3 tonne to any point on the roof.

Waterproofing Selection

In the absence of a British or European standard for green roofing, GRO and the British Board of Agrément recommend that any waterproofing system used in a green roofing system carries certification to the German FLL standard.

This standard, involves a 2 year test of a 1m x 1m x 1m test sample filled with growing medium and rhizone rooted plants. The sample is then observed to ensure no root systems are able to penetrate the waterproofing system or more importantly the joints. The additional requirement for the membrane to be resistant to humic acid, is the toughest waterproofing test in Europe, if not the world.

Sika Sarnafil G476-20 and TCG membranes have passed this stringent test and are certified by the British Board of Agrément, for use in green roofing applications without the use of a root barrier (root barriers are generally chemical or copper based materials that can contaminate the water course).