House in Herne Hill, London by Oliver Leech Architects. Photo by Jim Stephenson.
Flat roofs have become synonymous with modern architecture and gained popularity in the UK during the 1950s with the widespread availability of bitumen waterproofing. However, many flat roofs from that era suffered from poor construction and frequent leaks due to inadequate practices and substandard materials.
Since then, flat roof technology has advanced significantly. Today, we have a much better understanding of how to construct and insulate flat roofs effectively, utilising more robust materials. A well-constructed flat roof, when properly insulated, can last over 30 years with minimal maintenance.
Flat roofs are a good roof form if you want to maximise the internal headroom, create a roof terrace or achieve a modern aesthetic.
High Sunderland house by Peter Womersley (1957). Restored by Loader Monteith (2022). Photo by Dapple Photography.
Flat roofs are rarely entirely flat but are constructed with a slight fall, or 'pitch', typically not exceeding 10 degrees. This slight pitch helps ensure rainwater does not pool, with a minimum recommended pitch of 3 degrees for adequate drainage.
In domestic construction, flat roofs are typically built using timber. Joists span between load-bearing walls or steelwork at 400 to 600 millimetre centres, with noggins providing lateral stiffness. A structural deck – usually plywood or OSB sheets – is then laid on top to form a rigid structure.
To create the necessary pitch, joists can be installed on a slight slope, or more commonly, angled lengths of timber known as firrings can be secured on the joists before laying the structural deck.
Insulation can be positioned over the roof deck to create a warm roof or between the joists and over the roof deck to achieve a hybrid roof. More details on flat roof build-ups are discussed below.
Timber flat roof construction in progress. Timber I-joists can provide extra rigidity and stability. Image by Steico.
Until recently, domestic flat roofs were often built as 'cold roofs', where insulation was fixed between the joists with a ventilation gap above. However, we now know that this form of construction is thermally inefficient and prone to interstitial condensation.
Today, it’s recommended that a continuous layer of insulation is installed above the joists to eliminate thermal bridges and the need for ventilation, thereby creating a ‘warm’ roof build-up.
Warm Flat Roofs
A common warm roof setup involves installing a vapour barrier over the structural deck, followed by rigid insulation and a waterproof membrane to finish. The waterproofing can be bonded directly to the insulation or to plywood sheeting, depending on the type and manufacturer’s recommendations.
While warm roofs excel thermally and reduce interstitial condensation risks, they can have substantial depth requirements and expose roof coverings to extreme temperature variations.
Inverted Flat Roofs
Inverted roofs, sometimes referred to as ‘upside down roofs’, are a variation of warm roofs where the waterproof layer is installed beneath the insulation rather than above it. The insulation is loose-laid, and a ballast of gravel or paving is added to weigh it down.
The insulation and waterproofing act as a vapour barrier negating the need for a separate sheet layer.
This setup protects the waterproofing from temperature changes but can pose challenges for repairing leaks.
Inverted roofs are ideal for roof terraces and are often installed over a concrete structure to support the additional loads.
Hybrid Flat Roofs
Hybrid roofs are gaining popularity, with insulation installed both between and above the joists. This setup includes a vapour control layer (VCL) under the joists, full-fill flexible insulation between the joists, rigid insulated sarking boards over the joists (which also tie the joists together), and then the waterproof finish.
The main advantage of hybrid roofs is that utilising the space between the joists results in a thinner overall build-up, though they can involve greater technical know-how.
Flat Roof Build-up Types: A- Cold Roof; B- Warm Roof; C- Inverted Roof; D- Hybrid Roof. Materials key: 1- Waterproofing; 2- Insulation; 3- VCL; 4- Protection layer; 5- Ballast; 6- Insulated sarking board
Warm roofs commonly use PIR insulation for its high thermal conductivity and compressive strength, suitable for general access.
Inverted roofs benefit from EPS or XPS insulation for their high compressive strength and water resistance.
Hybrid roofs are best insulated with vapour-open and hygroscopic materials like flexible woodfibre or natural wool insulation between the joists and rigid wood fibre sarking boards above.
Approved Document L Volume 1: Dwellings (ADL1) of the building regulations outlines energy efficiency requirements for residential buildings in England, which vary depending on whether you are constructing a new house, extension, or upgrading an existing building element. The requirements for Scotland, Wales and Northern Ireland are similar.
ADL1 states that roofs forming part of a new build dwelling must achieve a U-value of 0.11 W/m2K.
Extension roofs have slightly lower thermal requirements and need to achieve a target U-value of 0.15 W/m2K to ensure compliance or a threshold value of 0.35 W/m2K if you can demonstrate that improvements can be made elsewhere that satisfy overall requirements.
To upgrade an existing roof the U-value target is 0.16 W/m2K while the threshold is also 0.35 W/m2K.
The table below shows some examples of how to achieve these U-values.
Approximate insulation thicknesses (mm) required to meet domestic U-values for flat roofs.
Please note that for hybrid flat roof constructions, the first layer of insulation thickness must align with the depth of the joists. The example in the table allows for 200 millimetre deep rafters at 400 millimetre centres full-filled with flexible wood fibre insulation and 60 or 80 millimetre insulating sarking board over the top.
Interstitial condensation – dampness caused by vapour condensing within the building fabric – can be a significant issue in flat roofs. Proper vapour control strategies are crucial.
Cold flat roofs are prone to interstitial condensation primarily due to thermal bridging that occurred through the joists. Warm roofs address many condensation issues by installing a vapour barrier (a low vapour diffusion VCL) and insulation above the roof deck only to prevent moisture ingress and cold air contacting the structural members.
Hybrid roofs require careful management of vapour control due to insulation placement between joists. A combination of vapour-open insulation and a high/ mid-diffusion vapour control layer – one which enables vapour to diffuse internally or externally depending on outside temperature and humidity levels – helps regulate moisture levels.
Ampadex DB90 vapour control layer as part of a hybrid flat roof build-up. Photo by Eco Homes Store.
The three main types of flat roof waterproofing used in the UK are bitumen felt, GRP (glass reinforced plastic), and single-ply membranes.
Bitumen felt is common but prone to defects. GRP offers a hard, waterproof surface but can be inflexible. Single-ply membranes (PVC, TPE, PIB, EPDM) offer the best overall performance as they are flexible and highly durable. EPDM, in particular, offers excellent waterproofing (it can be made-to-measure, free of joints) and sustainability.
Black rubber EPDM roof by Prelasti.
Green roofs can enhance the aesthetics and biodiversity of a standard flat roof.
Sedum blanket roofs, a type of 'extensive’ green roof, are lightweight and simple to install. They consist of drainage and protection layers underneath a 20-50 millimetre thick growing substrate for planting. Sedum blankets can be installed over warm or hybrid roof structures with a single ply membrane. They are designed to enhance biodiversity and sustainable drainage, not walking upon.
‘Intensive’ green roofs are intended for occupied spaces. The 'green' layer may include a variety of soft and hard landscaping elements like grass, trees, shrubs, and paving. The construction is similar to a sedum blanket roof but with a heavier-duty drainage membrane and a deeper growing substrate. An automatic irrigation system is often included to maintain healthy planting.
Note that intensive green roofs are only compatible with inverted flat roofs, and the roof-supporting structure must be designed to accommodate the additional load of the heavier build-up.
Green roof by Bere Architects.
If you're about to build a flat roof it makes sense to also to use it for outdoor space, subject to local planning policies.
For decked roof terraces, boards are fixed to battens, which are supported on adjustable cradles over a protective sheet layer. This setup typically involves an inverted roof build-up to protect the waterproof membrane.
Paved roof terraces can be constructed in a similar manner, with paving slabs slightly elevated on low-profile pedestals above a protection layer. In both cases, the construction supporting the terrace finish is not fixed to the roof; instead, it creates a ‘floating’ construction layer.
Roof terrace as part of a loft conversion by Will Gamble Architects. Photo by Building Narratives.
Standing seam zinc, aluminium, or copper roofing can be installed over flat roofs to enhance aesthetics.
In most cases, a ventilation gap is provided underneath a supporting layer of plywood to manage moisture, prevent corrosion, and control temperature, all of which contribute to the longevity and performance of the roof system.
A typical build-up consists of the standing seam metal finish on 18mm exterior grade plywood sheeting, supported by 50-100mm softwood battens to provide an air gap. This is placed over a breather membrane on a warm or hybrid roof construction, as noted above.
There are systems such as VM Zinc Plus that can be applied directly over a warm flat roof build-up without the need for ventilation beneath. However, hybrid flat roofs nearly always require ventilation under the metal roofing.
Kalzip aluminium standing seam zinc roof by Moxon Architects. A roof with a pitch less than 10 degrees is technically classed as 'flat'. Photo by Tim Soar.
Roof spaces are prone to overheating in summer and freezing in winter due to the poor thermal mass of timber structures.
Incorporating higher density materials, such as wood fibre insulation, into the construction helps absorb and slowly release excess heat, creating a more comfortable indoor environment. This also reduces reliance on heating and air conditioning (a common fixture now in loft conversions in southern England), improving energy efficiency and lowering utility costs.
To achieve high levels of thermal comfort in roof spaces, it's crucial to eliminate drafts.
As modern flat roofs don’t require a ventilation gap within the construction, unlike many pitched roofs, achieving airtightness is easier.
Ensuring there are no gaps in the general construction will eliminate many drafts. However, for optimal airtightness, installing an airtightness membrane, preferably one that acts as both a vapour control layer and air barrier (AVCL), will greatly improve performance.
Hybrid roofs offer the best scope for achieving high levels of airtightness, as the AVCL is positioned below the joists, making it far easier to lap into the wall construction and form a continuous airtight seal around the external envelope.
Cold roof construction is outdated, with warm and hybrid flat roofs being the preferable options.
Warm roofs are robust and straightforward, while hybrid roofs offer thinner build-ups and environmental benefits.
Consider factors beyond just thermal efficiency, such as thermal mass and airtightness, to achieve optimal thermal comfort.
Install a single-ply membrane for longevity.
A sedum roof will enhance biodiversity and visual appeal, while sheet metal roofing is a good choice for improved performance and a contemporary aesthetic.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, appoint a skilled residential architect.
About the author
Aron Coates, an architect with over two decades of experience, draws upon a wealth of residential architectural expertise, encompassing projects ranging from the restoration of historic structures to the design of contemporary homes.
