Detail Post – Flat Roof Details

The following post includes excerpts from our book Understanding Architectural Details – Residential.

Construction Detailing book residential copy

The book is now in its fourth edition, and was updated to reflect the changes to the Building Regulations in June 2022. All of the construction details in the book comply with the latest u-value requirements.
You can download all of the details featured in this post – just scroll down to find out more.

Introduction to Flat Roof Details

The roof is a key element of the building structure, providing protection from the elements and has a vital role in reducing heat loss from the building. Roofs tend to be either pitched or flat, and typically in residential construction, timber is the most common material used.


The Building Regulations in the UK provide guidance on roof design. You can find the Building Regulations here.



Functional Requirements:


The functional requirements of the roof are:

• Strength and stability

• Protection from the weather

• Durability

• Resistance to passage of sound

• Thermal insulation

• Resistance to air leakage

• Fire safety

• Security

• Aesthetics

Flat Roof Construction for Residential

A roof is considered to be flat when it has a slope of 10˚ or less to the horizontal, according to BS6229. A purposefully designed slope in the roof either through firrings or laying joists on a slope helps to evacuate water from the roof and avoid pooling.


A flat roof usually consists of the following:
  • Waterproof membrane to prevent water penetrating the structure and interior of the building
  • Roof deck, providing the base for the waterproof membrane, and in some cases the insulation
  • Thermal insulation
  • Load bearing or primary structure, usually constructed in timber in a residential construction.


The construction of the flat roof is similar to that of the timber upper floor. Joists are placed on edge, spaced at 400-600mm centres, supported by external and internal load bearing walls. TRADA and Approved Document Part A give guidance on sizes of joists for flat roofs relating to spans and loading. Strutting is fixed between the joists to provide lateral restraint.


Depending on the roof build up, a roof deck is then fixed to the joists over firring strips, to provide the fall of the roof. The roof deck material is usually chipboard or plywood. The firring strips allow the roof to have the slope required to enable water run-off to the rain water outlets. An alternative to this method sees insulation boards made into shallow wedge sections to provide the fall.

Insulating the roof


Similar to the pitched roof, a flat roof can be insulated as a cold or a warm deck. However, due to the flat nature of these types of roofs, ventilation can be problematic within cold roofs, causing condensation issues and therefore they are usually not recommended by manufacturers.


Cold Roof/Deck

A cold roof system sees the insulation placed immediately above the ceiling between the joists, with ventilation space above the insulation. It is difficult to provide suitable ventilation of the void above the insulation to prevent condensation. This roof system is rarely used nowadays due to the difficulties in ventilation, thermal bridging and risk of condensation and failure.


Warm Roof/Deck (Sandwich roof)

Warm roof systems benefit from the insulation sitting above the deck. This enables the temperature of the structure and the deck to be kept close to the temperature of the inside of the building (hence warm). The insulation is tapered to provide a fall to the roof to enable water to run to the water outlets.


With the warm roof deck, there is less likelihood of condensation forming in the warm roof space, so ventilation is not required. A vapour control layer is still installed to minimise any moisture movement. This system is probably the most common arrangement for residential flat roofs.


The disadvantage of this roof system is that the insulation is directly under the roof covering, so the covering suffers considerable temperature fluctuations. An inverted roof system avoids this issue.


Inverted Roof

The inverted roof puts the insulation above the roof covering. The insulation is then protected with a layer of chippings or concrete paving. This system is often seen in more commercial projects.


Both of the warm roof options prevent wasteful cutting of insulation and decrease installation time, therefore labour costs.


Flat Roof Detail Examples

The following images provide examples of flat roof details.


Flat roof eaves detail MR6B 2D

Flat Roof Detail – Masonry cavity wall, flat roof, insulation above joists


Flat roof eaves detail MR6B 3D

Flat Roof Detail – Masonry cavity wall, flat roof, insulation above joists


Flat roof with parapet detail MR8 2D

Parapet Detail – Masonry cavity wall, flat roof with parapet, insulation above joists


Flat roof with parapet detail MR8 3D

Parapet Detail – Masonry cavity wall, flat roof with parapet, insulation above joists


Timber frame flat roof detail TR6 2D

Timber Frame Flat Roof Detail


Timber frame flat roof detail TR6 3D

Timber Frame Flat Roof Detail


Download the Flat Roof Details!

Download this selection of flat roof details in PDF and DWG format.
Need a starting point for your flat roof detailing? Download this small set of details to add to your own detail library.

Notes from the Building Regulations on Roofs

General Requirements – Roofs


The following information is a partial list of requirements from the Building Regulations Approved Documents – for full and detailed explanations and requirements please consult the full publications.

PART A: Structure

(2004 Edition, incorporating 2004, 2010 and 2013 Amendments)

The building shall be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground.


PART B: Fire Safety Volume 1 Dwellings

(2019 Edition, incorporating 2020 amendments & 2022 Amendments)


Requirement B1 – Means of Escape

Where a flat roof forms a means of escape it should comply with the following:

  • It should be part of the same building from which the escape is being made
  • The route across the roof should lead to a storey exit or external escape route
  • The part of the roof forming the escape route and any opening within 3m of the escape route, should be of fire resisting construction (minimum REI 30).


A flat roof intended to form part of an escape route should be provided with guarding in accordance with Approved Document K.


Requirement B2 Internal Fire Spread (Linings)


To inhibit the spread of fire within the building the internal linings shall adequately resist the spread of flame over their surfaces and if ignited, have either a rate of heat release or rate of fire growth which is reasonable in the circumstances. (Internal linens applies to any partition, wall, ceiling or other internal structure).


Wall and ceiling linings should meet the classifications in Table 4.1 of Approved Document Part B Volume 1.


For rooflight requirements refer to Approved Document Part B Volume 1.


Requirement B3 Internal Fire Spread (Structure)

The building shall be designed and constructed so that , in the event of fire its stability will be maintained for a reasonable period.


Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken, to an extent appropriate to the size and intended use of the building, comprising either or both of the following:

  • Subdivision of the building with fire-resisting construction
  • Installation of suitable automatic fire suppression systems


The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.



To reduce the potential for fire spread cavity barriers must be provided to divide cavities and to close the edges of cavities.


Cavity barriers should be provided:

  • at the edges of cavities, around openings such as windows, doors etc,
  • at the junction between external cavity wall and every compartment floor and wall
  • at the junction between an internal cavity wall and every compartment floor, compartment wall or other wall or door assembly forming a fire resisting barrier

Refer to the regulations for full requirements.


Requirement B4

External Fire Spread


The roof of the building shall adequately resist the spread of fire over the roof and from one building to another, having regard to the use and position of the building.


Resisting fire spread over roof coverings The risk of a fire spreading from the building to a building beyond the boundary should be limited.


Refer to Approved Document Part B, Volume 1, Section B4 for limitations on roof coverings in relation to boundaries. Table 12.1, 12.2, 12.3.


The above notes do not contain guidance for flats, for full requirements refer to the regulations.


PART C: Site Preparation and Resistance to Contaminants and Moisture

(2004 Edition, incorporating 2004, 2010 and 2013 Amendments)


Resistance to moisture

The roof of the building shall adequately protect the building and people who use the building from harmful effects caused by; precipitation including wind driven spray, interstitial and surface condensation.


Resistance to moisture from the outside Any roof will meet the requirement if; it is jointless or has sealed joints and is impervious to moisture or, it has overlapping dry joints, is impervious or weather resisting and is backed by a material which will direct precipitation with enters the roof towards the outer face.


Resistance to damage from interstitial condensation

A roof will meet the requirement if it is designed and constructed in accordance with Clause 8.4 of BS5250:2002 and BS EN ISO 13788:2002.


To avoid excessive moisture transfer to roof voids gaps and penetrations for pipes and electrical wiring should be filled and sealed (especially in areas of high humidity such as kitchens and bathrooms). An effective draught seal should be provided to loft hatches to reduce inflow of warm air and moisture.


PART F: Ventilation

(2021 Edition)


There shall be adequate means of ventilation provided for people in the building.


PART H: Drainage and Waste Disposal

(2015 Edition)


Rainwater drainage

Adequate provision shall be made for rainwater to be carried from the roof of the building.


The flow into a gutter depends on the area of surface being drained and whether the surface is flat or pitched.  Refer to Approved Document Part H for calculation tables of drained areas and gutter/ outlet sizes.


Refer to Approved Document Part H Diagram 1 for rainfall intensities for design of gutter and rainfall pipes.


Gutters should be laid with any fall towards the nearest outlet. Gutters should also be laid so that any overflow in excess of the design capacity, caused by conditions such as above normal rainfall, will be discharged clear of the building, reducing the risk of overspilling of rainwater into the building or structural overload.


Rainwater pipes should discharge into a drain or gully, but may discharge to another gutter or another surface if it is drained.


PART K: Protection from falling, collision and impact

(2013 Edition)


K2 Protection from falling Pedestrian guarding should be provided to any roof that people have access to.


PART L: Conservation of fuel and power, Volume 1: Dwellings

(2021 Edition)


Reasonable provision shall be made for the conservation of fuel and power in buildings by:

  • limiting heat gains and losses through thermal elements and other parts of the building fabric,
  • from pipes, ducts and vessels used for space heating, space cooling and hot water services.


New roofs must achieve a U-value of 0.11W/m2K

Upgrading existing roofs must achieve a U-value of 0.15 W/m2K


(Refer to Approved Document Part L for full requirements for new elements in existing dwellings, extensions, renovating elements in existing dwellings and necessary U-values.)



Insulation should be installed tight to the structure, without air gaps, and should extend to the wall insulation. For roofs insulated at ceiling level, the long term protection of the insulation layer should be considered: boarded areas should be provided above the insulation to give access for maintenance.


Thermal Bridging

Continue insulation across the wall-to-eaves and wall-to-gable junctions. Wall insulation should be installed to the top of the wall plate, in some cases this may be above the cavity closure or barrier. Roofs insulated at ceiling level; loft insulation at the eaves should extend beyond the wall insulation without any reduction in thickness due to the pitch of the roof. At gables and party walls, insulation should extend to the wall; if the space between the wall and joist is less than 100mm, perimeter insulation may be required.


Part 0: Overheating

(2021 edition)

Reasonable provision must be made to:

  • limit unwanted solar gains in summer;
  • provide an adequate means to remove heat from the indoor environment.


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