NHBC Standards 2010

7.1 Flat roofs and balconies

SITEWORK STANDARDS

7.1 - S1
All sitework shall:
(a) meet the Technical Requirements
(b) take account of the design
(c) follow established good practice and workmanship
 

Construction that complies with the design and the guidance below will be acceptable for flat roofs and balconies.

 

IN-SITU REINFORCED CONCRETE

7.1 - S2
In-situ reinforced concrete flat roofs shall be constructed to ensure they achieve the required design, strength and durability
 

Items to be taken into account include:

(a) accuracy of formwork

The formwork should be constructed accurately.

Items to take into consideration are:

  • accurate location of holes
  • adequate support
  • proper allowance for placing of steelwork
  • cast-in features, such as drips and weatherchecks
  • surface finishes.

Where a metal deck is used as permanent shuttering, drying of the concrete will take place from the top surface only. A temporary roof should be provided to allow drying to take place. The permanent waterproofing should only be installed when the deck has fully dried.

(b) concrete grade

Reference should be made to Chapter 2.1 'Concrete and its reinforcement' for guidance on concreting. The design should have specified a concrete mix with low shrinkage characteristics.

 

PRECAST CONCRETE

7.1 - S3
Precast concrete flat roofs shall be constructed to ensure they achieve the required design, strength and durability
 

The supporting structure should:

  • be even and true
  • have a minimum 90mm bearing for the pre-cast units unless the design shows a lesser dimension.

Precast units should be:

  • installed to provide an even surface
  • grouted, where required by the design.
 

PROFILED METAL

7.1 - S4
Profiled metal flat roofs shall be constructed to ensure they achieve the required design, strength and durability
 

Items to be taken into account include:

(a) material type and protection against corrosion

Sheets should be checked for conformity with the design and specification when they are delivered to site.

(b) adequate storage

Profiled sheets should be stored to prevent :

  • rusting of cut edges
  • severe scratching of the galvanising
  • sheets being bent or deformed.

(c) fixings

The number, type and location of fixings should in accordance with the design and specification.

(d) installation

The supporting steelwork and purlins should be square, true and free from twists or sagging. Unless the manufacturer agrees otherwise, deck materials should be side stitched to ensure the deck performs as a continuous plane layer.

 
Assembly of the roof, end laps, etc. should be in accordance with the design; any variation from this should be approved by the designer. 

Protection sheets, e.g. plywood, should be used in areas of the roof deck subjected to construction loadings, including point loads imposed by foot traffic and storage of materials. Loads imposed by following trades should be taken into account.

Any deformed sheets should be stripped and replaced, before the waterproofing and insulation system is installed.

 

 

TIMBER

7.1 - S5
Timber flat roofs and (where appropriate) balconies shall be constructed to meet the required design, strength and durability
 

Items to be taken into account include:

(a) grades and sizes of joists

Materials delivered to site should be checked for conformity with the design and specification.

(b) the spacing and bearing required to achieve reasonably level support for firrings and deck

Timber joists should be:

  • level - where necessary, hard packing should be used, e.g. tiles or slates bedded in mortar. Loose or soft packing, including timber, should not be used
  • spaced at the centres specified on the drawing (not more than 600mm centres).


The use of regularised timber joists will help to achieve a level deck.

(c) strutting

Strutting should be one of the following:

  • herringbone type (timber 38mm x 38mm)
  • solid blocking (38mm thick timber x ¾ depth of joist)
  • proprietary steel strutting.

Strutting should be located as follows:

Joist span
[m]
Rows of strutting
Up to 2.5 none needed
2.5 to 4.5 1 (at centre of span)
Over 4.5 maximum 2.5m centres, Spaced equally along the span

In cold deck roofs, the strutting should not prevent free cross ventilation.

(d) holding down metal strapping

If the design specifies holding down straps to prevent the roof being lifted off the supporting structure, they should be at 2.0m centres (maximum).

Where straps are fixed to masonry, hardened nails 4mm in diameter x 75mm long or No 12 wood screws x 50mm long into plugs should be used.

The number of fixings should be in accordance with design requirements and the lowest fixing should be within 150mm of the bottom of the vertical strap.

(e) timber quality

Timber should be rejected if it:

  • is excessively bowed, twisted or cambered
  • is excessively wet
  • has large edge knots or shakes
  • has a waney edge more than half the thickness
  • has fissures
  • has any sign of rot.

Where preservative treated timber has been cut after treatment the cut edges should be re-treated with a coloured preservative.

(f) joist hangers

The joist hanger should be the correct size for the timber joist or trimmer and nailed in accordance with the design.

The masonry course to carry the joist hangers should be level and at the correct height. The walling should not be cut into.

 

STRUCTURAL DECKS

7.1 - S6
The structural deck shall be installed to form a satisfactory substrate for the waterproofing system
 

Items to be taken into account include:

(a) fixing of plywood and oriented strand board

Tongued and grooved boards laid either with the long edge at right angles to the joists or parallel to the joists should have the short edge supported on a joist or nogging.

Oriented strand board should be laid over supports in the direction indicated on the boards. The stronger axis should be laid at right angles to the supporting joists.

Roof perimeter edges of boards which do not coincide with joists should be supported on noggings.

Unless the design specifies closer centres oriented strand board and plywood fixing centres should not exceed 100mm. Movement gaps at abutment with rigid upstands should be not less than 10mm and gaps between square edge boards should not exceed 3mm.

Oriented strand board should be fixed with flat headed ring shank nails, 3mm in diameter at least 2½ x board thickness long and not less than 9mm from the edge of the board.

Plywood should be fixed with ring shank nails, at least 50mm long x 3mm in diameter.

(b) fixing of softwood boarding

Softwood tongued and grooved boarding should be closely clamped together.

Each board should be nailed with two ring shank nails to each joist or firring. Nail heads should be punched below the timber surface. End joints should be staggered.

(c) protection of structural deck

The deck should be installed in dry conditions and be protected from wetting until the roof is complete. The joints in sheet materials which are pre-felted or coated should be sealed immediately after fixing.

The area of deck installed in any working day should be no greater than can be quickly protected from wetting.

Damaged materials and materials that have been adversely affected by moisture should be discarded.

 

DRAINAGE

7.1 - S7
Flat roofs shall have effective drainage
 

Falls and gutters should be constructed in accordance with the design. Flat roofs and balconies should have a finished fall of not less than 1:80 except flat roofs with metal sheet roof coverings and green roofs which should have a finished fall of not less than 1:60.

Items to be taken into account include:

(a) falls on concrete roofs

CEMENT/SAND SCREEDS

Cement sand screeds should be 1 : 4, cement : sand. Minimum screed thicknesses should be as follows:

Location of screed Thickness [mm]
Bonded monolithically to insitu or pre-cast concrete nominally 40mm (25mm minimum)
 
Unbonded (on separating layer) nominally 70mm (50mm minimum)

 
Reference should be made to Chapter 8.3 'Floor finishes' for further guidance on laying screeds.

LIGHTWEIGHT SCREEDS

Lightweight screeds should be laid only by specialist contractors. Lightweight concrete screeds should have a topping of 1 : 6, cement : sand, 13mm thick.

SCREED FINISH

Screeds should be free from ridges and indentations. They should be finished with a wooden float to provide a smooth, even surface for the vapour control layer and waterproof finish.

(b) falls on timber roofs

Firring pieces should be used to form falls, unless the design shows sloping joists and ceiling. Where laid across the joists, firrings should be not less than the following sizes:

Joist Centres [mm] Minimum firring dimensions [mm]
  width depth
400 or 450 38 38
600 38 50

(c) tapered insulation systems

The manufacturer's specific design and layout drawings should be followed. The sequence of installation should ensure that boards are waterproofed and the roof sealed at the end of each day, or before the arrival of inclement weather. Successive roof layers should be laid with a minimum of delay, to avoid trapping water during construction.

(d) access to rainwater outlets

Rainwater outlets from flat roofs should be accessible. For green roofs an accessible and visible inspection hatch should be provided at every outlet.

 

THERMAL INSULATION AND VAPOUR CONTROL LAYERS

7.1 - S8
Insulation and vapour control layers shall be installed in accordance with the design
 

Appendix 7.1-A gives details of suitable materials for insulation and vapour control layers for the different types of roof designs.

WARM ROOFS

The design should indicate that the vapour control layer is below the insulation and that it is sealed to the waterproofing at the perimeter and at all penetrations through the roof, e.g. at outlets and pipes. In single ply roofing systems the vapour control layer is generally not sealed to the waterproofing.

Insulation boards should be kept dry at all stages to prevent:

  • lack of bonding
  • trapping of moisture.

The area of insulation laid at any time should be that which can quickly be covered by the waterproofing or protected from wetting.

Insulation should be bonded or mechanically fixed in accordance with the manufacturer's recommendations.

Composite decks require noggings under transverse edges. The joist width should be in accordance with the design to ensure the insulation has adequate bearing.

The foil underface of a composite deck should be sealed with two beads of sealant along all board joints to maintain the integrity of the vapour control layer.

INVERTED ROOFS

The insulation material for inverted roofs should be in accordance with the design, be suitable for external use and withstand any anticipated traffic.

GREEN ROOFS

Where the vapour control layer has been used as a temporary waterproofing layer any damage should be repaired using a full width section of membrane.

COLD ROOFS

Ventilation should be provided at both ends of each joist void, and should not be blocked by the thickness of the insulation.

A minimum of 50mm clear air space above the insulation should be maintained.

 

WATERPROOFING

7.1 - S9
Waterproofing shall prevent water entering the building
 

Items to be taken into account include:

(a) preparation of surfaces

The structure and the surface to receive the waterproofing should be checked and approved by the waterproofing contractor. All nails should be well punched below the surface, which should be even and dry.

The manufacturer's recommendations on priming upstands, roof outlets, etc. should be followed to achieve a satisfactory bond with the waterproofing.

(b) construction sequence

Waterproofing systems should be laid in accordance with the design and specification following the recommendations of the manufacturer. Some proprietary systems should be laid only by specialist roofing contractors approved by the manufacturer. See clause S9(d) for green roofs.

It is preferable for one contractor to lay the vapour control layer, the insulation, the waterproofing and the surface finish. The contractor should ensure that the deck and the insulation boards are waterproofed and the roof sealed at the end of each day, or before the arrival of inclement weather.

Successive roof layers should be laid so as not to trap water during construction.

Membrane laps near outlets should not impede drainage.

(c) weather conditions

The manufacturers' recommendations for conditioning, (unrolling in advance of laying, etc) should be followed.

Generally, sheet membranes should not be laid or handled when the air temperature is 5°C or below unless the manufacturer agrees otherwise. Some self-adhesive reinforced bitumen membranes should not be laid below 10°C. For such systems, the manufacturer's specific instructions should be followed.

Membranes should not be laid on damp or frosted surfaces or when any rain, sleet or snow is falling.

(d) green roofs

Green roofs should be laid in accordance with the design and the membrane manufacturer's recommendations, taking into account:

  • root barriers
  • height of upstand in relation to soil height and flashings
  • integrity of waterproofing prior to burying
  • protection, reservoir and filter layers
  • moisture control of the soil.

Green roof systems should be installed by operatives who:

  • are competent
  • are familiar with the system being installed
  • hold a certificate confirming they have been trained by the system membrane manufacturer or distributor.

(e) correct detailing

Appendix 7.1-C gives, for the different roof types, typical details of:

  • abutments
  • parapets
  • edge details
  • fixing of guarding
  • projections through the waterproofing
  • roof lights.
 

GUARDING TO BALCONIES

7.1 - S10
Guarding to balconies shall be of adequate strength and height to minimise the risk of people falling, of adequate durability and fixed securely
 

Items to be taken into account include:

(a) strength and movement of masonry balcony walls

Masonry balcony walls should be built in accordance with Chapter 6.1 'External masonry walls'. In particular:

  • walls should incorporate strengthening as required by the design
  • movement joints should be provided in accordance with the design
  • copings should be firmly bedded.

(b) fixing of balustrading and guard rails

Balustrading and guard rails should be fixed in accordance with the design details. Reference should also be made to Appendix 7.1-C.

 

PROTECTION OF MATERIALS FROM WEATHER

7.1 - S11
Moisture sensitive material shall be protected from wetting
 

Timber-based roof decking and insulation materials should be stored under cover to prevent wetting.

Timber-based roof decks that have been fixed in position should be temporarily covered to prevent wetting, unless the waterproofing is to be laid immediately.

Zinc coils and sheets should be stored in dry conditions before being installed.