Requisitos generales

The insulation layer is placed at ceiling level with an air space between the top of the insulation and underside of the deck, ventilated to external air on opposing sides and should only be considered for timber structural decks as detailed in the guidance diagram below. (note: due to the technical difficulties in achieving the required levels of insulation between roof timbers and the associated risks of condensation within the 'cold roof' and at thermal bridges, a flat roof with a 'warm deck' is the preferred option).

Waterproof covering to be either:

 3 layers of high performance felt (hot bonded together with bitumen) to a current

BBA Certificate in compliance with BS 8217

 Single layer system with a current BBA or WIMLAS Certificate

 Glass reinforced plastic (GRP) system with a current BBA or other approved

accreditation

 Rolled lead sheet fixed in compliance with the Lead Sheet Associations

publication 'Rolled Lead Sheet- The Complete Manual' obtainable from: www.leadsheet.co.uk

 Mastic asphalt fixed in accordance with the Mastic Asphalt Councils technical

guides and specifications obtainable from: www.masticasphaltcouncil.co.uk Waterproof covering to be fixed by a flat roofing specialist in accordance with manufacturer's details typically onto a timber deck as follows:

 22mm external quality plywood decking or similar approved laid to 1:60/80 minimum

gradient using firing strips at spacing to match joists, fixed onto:

 Timber flat roof joists constructed of kiln dried structural grade timber with sizes and

spacing suitable for the proposed clear span in compliance with independent span table below or see TRADA Span Tables.

 Flat roof covering (excluding lead and areas used for habitable use) to have a surface

finish of bitumen bedded stone chippings covering the whole surface to a depth of

12.5mm to achieve a class AA (or B (t4) European class) fire rated designation for surface spread of flame

 Restrain flat roof to external walls by the provision of 30 x 5 x 1000mm lateral restraint

straps at maximum 2000mm centers fixed to 100 x 50mm wall plates and internal wall faces.

 Insulation to be fixed between/under joists in compliance with the guidance table below

and is to be continuous with the wall insulation. Fix 12.5mm vapour checked plaster board (unless plaster board is integral with the insulation) and 3mm skim to underside of joists

 Eaves ventilation- provide cross ventilation to each and every void of the flat roof by

installing eaves ventilation on opposing sides (fitted with insect grills) equivalent to a continuous 25mm gap up to 5m span or 30mm gap for 5-10m span.

 Ventilated air space over roof- provide an unrestricted ventilated air space between the

top of the insulation and underside of the complete roof deck at not less than 50mm for up to 5m spans and not less than 60mm for 5-10m spans.

Flat roofs with unlimited access/habitable use

Flat roof with unlimited access to have proprietary non slip finishes/tiles in accordance with manufacturer's details and suitable protection from falling in accordance with guidance details and Approved Document K.

Guidance Diagram 37: Typical section through a flat roof with 'cold deck' (not to scale)

13mm diam stone chippings bedded in hot bitumen or solar reflective treatment

Welted drip

1:60 - 1:80 gradient

Water proof coverings in accordance with flat roof specialists specification 22mm external quality structural ply wood decking or other approved

Firing strips at 1:60 min gradient (or tapered insulation as manf details) 50mm min air gap (up to 5m span) or 60mm (5-10m span)

Thermal insulation fixed between/under joists as guidance tables continuous with wall insulation

Vapor contol layer + 12.5mm plasterboard & skim finish

Eaves ventilation equivalent to 25mm continuous air gap with insect screen (up to 5m span) or 30mm gap (5-10m span) Cavity walls closed at eaves

Flat roof joist sizes and spacing suitable for clear spans & loadings as guidance

Welted drip

Rain water gutters & facia boards etc as guidance

Eaves ventilation equivalent to 25mm continuous air gap with wall plate & strapping as

guidance

Cavity wall as guidance

tables _{wall plate & strapping as}

guidance

insect screen (up to 5m span) or 30mm gap (5-10m span) Cavity walls closed at eaves Cavity wall as guidance

Guidance Table 31: Examples of 'cold roof' insulation fixed between/under flat roof joists (Vented ‘cold roof’ achieving a U-value of 0.18 W/m².k)

Product K -Value Position in roof

Kingspan Thermapitch TP10 0.022 185mm (105 + 80mm) friction fixed between

joists Kingspan Thermapitch TP10 and Kingspan Kooltherm K18 Insulated Plasterboard 0.022 0.021

120mm friction fixed between joists and 37.5mm fixed under rafters, with integral vapour checked plaster board and skim finish

Celotex XR4000 0.022 185mm friction fixed between joists or

125mm friction fixed between joists and 25mm fixed under joists, with 12.5mm vapour

checked plaster board and 3mm skim finish

Jablite Premium Board 0.030 220mm friction fixed between joists or

150mm between joists and 50mm fixed under joists with 12.5mm vapour checked plaster board and 3mm

skim finish

Notes: 1. The joist depth must be sufficient to maintain a 50mm air gap above the insulation and cross ventilation to be provided on opposing sides by a proprietary ventilation strip equivalent to a 25mm continuous gap at eaves level with insect grill for ventilation of the roof space.

2. All specifications assume 50mm wide joists at 400mm centers with 12.5mm vapour checked plaster board and 3mm skim finish. 3. Insulation to be installed in accordance with manufacturer's details Source: A representative selection of values taken from Technical Note 10, U-values of Elements

(Approved Document L1B2010) produced by Hertfordshire Technical Forum for Building Control.