7.2 Pitched roofs
MATERIALS STANDARDS
| 7.2 - M1 | Materials standards |
| 7.2 - M2-M3 | Structural timber |
| 7.2 - M4 | Restraint straps and holding down straps |
| 7.2 - M5 | Roofing materials |
| 7.2 - M6 | Reclaimed materials |
(a) meet the Technical Requirements
(b) take account of the design
Materials that comply with the design and the guidance below will be acceptable for pitched roofs.
Materials for pitched roofs shall comply with all relevant standards, including those listed below. Where no standard exists, Technical Requirement R3 applies (see Chapter 1.1 'Introduction to the Standards and Technical Requirements').
References to British Standards and Codes of Practice include those made under the Construction Products Directive (89/106/EEC) and, in particular, appropriate European Technical Specifications approved by a European Committee for Standardisation (CEN).
STRUCTURAL TIMBER
Structural timber should be specified according to the strength classes in BS EN 338. Roof members are usually C16, C24 or TR26.
Timber specifications when using the BS 4978 grading rules should be accordance with BS EN 1912 or strength class specified.
Structural timber should be pre-treated with preservative where specified by the designer. Chapter 2.3 'Timber preservation (natural solid timber)' (each section) recommends methods of preservative treatment.
Use of reclaimed materials is covered in Clause M6.
RESTRAINT STRAPS AND HOLDING DOWN STRAPS
Lateral restraint straps should have minimum cross section dimensions of 30mm x 5mm. Vertical holding down straps should have minimum cross section dimensions of 30mm x 2.5mm.
Mild steel straps and fixings should be protected against corrosion in accordance with Tables A.1 and A.2 of BS EN 845-1 (see Appendix 7.2-A). Fixings and straps should be compatible. Sheradizing is not acceptable in Northern Ireland and the Isle of Man.
Straps should be ordered to the correct length and with the correct number of bends and/or twists required by the design.
ROOFING MATERIALS
Items to be taken into account include:
(a) roof coverings
The following roof coverings are acceptable:
- clay tiles and fittings to BS 402
- concrete tiles and fittings to BS EN 490 and BS EN 491
- fibre cement slates and fittings to BS EN 492
- natural slates to BS EN 12326 (see Appendix 7.2 - G).
- shingles should be of Western Red Cedar and pre-treated with CCA preservative to BS 4072 and be Grade 1 to the Canadian Standards Association. Care is needed in the selection of flashing materials and nail fixings to avoid corrosion. Follow the recommendations of the supplier.
Natural stone should be used in accordance with established custom and practice.
Thatch should be as recommended by the Thatch Advisory Service or other appropriate authority in accordance with Technical Requirement R3.
Use of reclaimed materials is covered in Clause M6.
Proprietary coverings should be assessed in accordance with Technical Requirement R3.
(b) fixings
Clout or slate nails for fixing slates and tiles should be one of the following and at least 38mm long:
- aluminium to BS 1202 : Part 3
- copper to BS 1202 : Part 2
- silicon bronze.
Galvanized steel nails are not suitable for fixing slates and tiles because of the risk of damaging the galvanizing but may be used to fix battens and underlay. Nails for fixing battens should be at least 30mm longer than the batten thickness. Ring shank nails should be used when specified by the designer.
Tile clips should be of aluminium or stainless steel.
(c) flashings
The following are acceptable:
- milled lead sheet to BS 1178. Flashings, gutter linings, etc should be at least Code 4, soakers may be Code 3
- aluminium and aluminium alloys to BS 1470 (0.6mm to 0.9mm thick) and protected from contact with mortar by a coating of bituminous paint
- zinc alloy to BS 6561 and 0.6mm thick
- copper to BS 2870, 0.7mm thick is suitable for gutters, 0.55mm thick fully annealed is suitable for flashing, soakers and saddles.
To prevent electrolytic action where metal items may be in contact, eg flashings and soakers, these should not be of different metals.
Proprietary flashings should be assessed in accordance with Technical Requirement R3.
(d) underlays
Where the underlay is exposed, such as at the eaves, a type 5U felt should be used. A type 1F felt may be used for the remainder of the roof.
To minimise the risk of condensation in the case of a fully supported underlay above rigid sarking, underlays should have a low vapour resistance, preferably less than 5.7MNs/g. Underlays with a higher vapour resistance may need increased ventilation to the roof space and between the underlay and sarking.
Proprietary underlays should have been assessed in accordance with Technical Requirement R3. Manufacturers' recommendations should be followed.
(e) rigid sarking
The following materials are acceptable:
- tongued and grooved or square edged boarding to BS 1297
- bitumen impregnated insulating board to BS 1142 : Part 3 (sarking and sheathing grade)
- WBP or CBR plywood to BS EN 636, durability Class G
- type P5 chipboard to BS EN 312
- oriented strand board type OSB3 to BS EN 300
- proprietary products which have been assessed in accordance with Technical Requirement R3.
(f) battens and counter battens
Battens and counter battens should be to the sizes specified in the design. Timber used for battens and counter battens should be as listed in BS 5534 : Part 1.
Preservative treatment is generally required for battens to vertical tiling and slating and is recommended for roof battens in severe exposure areas. Reference should be made to Chapter 2.3 'Timber preservation (natural solid timber)' (Materials) for guidance on the timber types and classes requiring treatment.
(g) insulation
Thermal insulation should be to the design specification.
The following materials are acceptable:
|
to BS 5803 : Part 1 |
|
to BS 5803 : Part 2 |
|
to BS 5803 : Part 3 |
|
|
Insulation of water pipes should be in accordance with Chapter 8.1 'Internal services' (Materials).
(h) fascias, bargeboards and soffits
Timber used for fascias, bargeboards, soffits, etc should be pre-treated with preservative. Reference should be made to Chapter 2.3 'Timber preservation (natural solid timber)' (Materials) for guidance on preservative treatments.
The following materials are also acceptable:
- WBP plywood to BS EN 636, durability Class M or better
- high density fibre reinforced calcium silicate board that meets the performance requirements of BS 3536
- glass fibre reinforced cement (GRC) board that meets the performance requirements of BS 3536
- proprietary products which have been assessed in accordance with Technical Requirement R3.
(i) fire-stopping and cavity barriers
Cavity barriers in boxed eaves should be wire reinforced mineral wool blanket, at least 50mm thick. Ordinary mineral wool quilt is acceptable as fire-stopping above separating walls.
(j) protection to ventilation openings
Ventilation openings where the least dimension exceeds 10mm should be protected to prevent the entry of birds, etc.
Acceptable protection of openings can be provided by:
- rigid fabrications with width of opening greater than 3mm and less than 10mm (no restriction on length)
- rigid fabrications with round holes greater than 3mm and less than 10mm in diameter
- square or rectangular mesh where the clear opening size is greater than 3mm and less than 10mm.
RECLAIMED MATERIALS
(a) of the type, size and quality required in the design
(b) suitable for re-use
Materials recovered from older buildings, such as timber, slate or tile, may be re-used only with the prior agreement of the NHBC. Independent certification of suitability may be required.
