Warm Pitched Roofs - Between & over rafters

Structural stability

The Rockfall system is suitable for most roof pitches. It is not, however, designed to add to the structural stability of the roof. Advice should always be sought from a qualified engineer or the truss manufacturer as to the requirements for any additional bracing etc. (see ‘Installation and Design details’).
 
The resistance to wind uplift and likely dead loads depend on factors peculiar to each project, such as roof geometry and geographical location. The effect of wind loading
should be calculated in accordance with BS 6399: Part 2:1997 and for snow loadings, BS 6399: Part 3: 1998.
 
Breather membranes

It is important to ensure that the breather membrane specified is suitable for the specific type of construction and has a BBA approval.

Installation of Underlay boards
 
Installation from above rafters

On completion of the rafter constructions, timber battens or alternatively renderer's quoin bead, (minimum bearing width 40 mm) are clout nailed to both sides of the rafters at a level to suit the thickness of the Rockfall Underlay insulation. The underlay is then fitted; it must be tightly butt-jointed between rafters.
 
Installation from below rafters

Alternatively, where the ceiling is to be attached directly to the underside of the rafters, underlay boards may be installed from inside the building. Where the insulation fully fills the rafters, it may be friction fitted. Should the Underlay board only partly fill the rafter depth, it should be supported using continuous timber battens or renderer’s quoin bead.

Regardless of whether the underlay has been installed from above or below the rafters, a vapour control layer should be installed on the warm side of the insulation prior to the attachment of the ceiling. Alternatively, this may be accommodated using foil backed plasterboard as the ceiling.

Installation of Overlay boards

Rockfall Overlay Boards are laid across the rafters, starting at the eaves. All boards must be close-butted and laid with staggered joints. The boards may be cut to length to suit the rafter spacing. A fully rebated Rockfall Board is also available and should be laid as above.

Ensure that the dense layer of the overlay board is used ‘face up’. This side is clearly marked with the product name ‘Rockfall overlay board’.

Counter-battens of 50 × 38 mm treated softwood, spaced at rafter positions, are mechanically fixed to the rafters using Helifix Inscew 600 or SFS IF1 fixings, holding the Rockfall insulation in place. Fixings along counter-battens should be at centres subject to construction type and fixing manufacturer’s advice.

A stop batten (similar thickness to Overlay board) fixed at the eaves position will assist construction and provide resistance against downward counter-batten loads.

The fascia height must be sufficient to accommodate the level of the roof finish due to the Overlay Board and Counter-battens.

The BBA approved breather membrane is laid across (or under) the counter-battens and lapped (150 mm) and detailed at other parts of the roof construction in accordance with good building practice. Tiling or slating battens are fixed in the normal way.

Mansard roofs

On steeper roof pitches, additional timber stop battens may be required to resist shear forces, as well as additional mechanical fixings.
 
Fixings

Information and advice on the type and frequency of fixing can be obtained from the relevant manufacturers. For timber rafter or timber deck constructions, Helifix Inskew 600 or SFS IF1 fixings of a suitable length are recommended. In the case of profiled metal decking SFS Stadler will advise as to the correct fixing specification. In either case, the manufacturer’s literature should be referred to regarding the minimum penetration depths.

Eaves details

A substantial timber stop batten should be fixed to the foot of the rafters. This provides resistance against the downward counter-batten loads, relieving the fascia board of any horizontally applied load.
 
A continuous plywood tilting board fixed to the top of the fascia carries the breather membrane over and into the gutter so that it does not sag and allow moisture to collect at the eaves position.
 
Note that there is no requirement for eaves ventilation in the Rockfall system.

Ridge detail

The Overlay Board should be mitred to make a tight butt joint at the ridge. Ensure that the breather membrane is continuous at the ridge position (minimum 150 mm overlap). Note that the system does not require ventilation at or near the ridge.