Traditional Cut Roofs
In good traditional cut roof construction requires full triangulation of the roof, the rafters and the joists should meet at wall plate level as if they do not, there is an increased risk of the roof spreading. If there is a risk of roof spread, an engineer’s design is required. The design engineer appointed should be suitably qualified and possess the appropriate professional indemnity insurance.
Proper transfer of purlin loads either to specially designed joists or load-bearing walls is also important in good design. It is recommended that in complicated or large-scale cut roofs that an engineer is employed to design the roof. The design engineer appointed should be suitably qualified and possess the appropriate professional indemnity insurance.
Ideal roof construction
Illustrated below are typical junctions of well-designed cut roofs. Note that that the purlin load is transferred through struts to a load-bearing wall and also that the rafters and joists are triangulated at eaves level.
Diagram D38 - Section through a traditional cut roof
Diagram D39 - Junction A - Detailed of arrangement of junction of rafter collar purlin hanger and strut
Diagram D40 - Junction B - Strut arrangement at ceiling level
Diagram D41 - Junction C - Hanger and joist arrangement at ceiling level
In a cut roof, the hangers and binders support the ceiling joists. The binder must be securely nailed to each ceiling joist. The hangers are provided every 4 joists and are nailed to the ceiling joists after the roof has been loaded. Please note that if the steel also forms part of a floor in a dormer or one and half storey it requires at a minimum of 30 minutes fire resistance by way of intumescent paint on all four exposed faces.
Fixing of nails
Ensure all connections are securely nailed using nails that are at least 3.1 mm in diameter and 90 mm long. Nails should be flush with the timber surface. When using pneumatic driven nails or gas-fired nails, ensure nails are not over-driven. Strapping of the roof to the external walls should be done in accordance with the guidance outlined previously.
In situations where a steel beam is incorporated to provide support to a ceiling joist or floor joist in a dormer construction, continuity is lost where joists are notched into the web of the beam. In order to maintain continuity, strapping can be provided as shown below using straps that are at least 30 mm x 2.5 that are adequately fixed to the joists.
Diagram D42 - Typical fixing detail where a steel member is used*
The diagram above provides a view from the underside of beam and ceiling joists detailing a typical fixing detail. The beam and associated fixing detail is to be in accordance with engineer’s specification. The design engineer appointed should be suitably qualified and possess the appropriate professional indemnity insurance.
STRUCTURAL TIMBER Extract from www.woodtechnologyireland.ie
All structural timber must undergo an assessment of their strength reducing defects usually carried out by grading machines or trained visual strength graders. These activities are regulated by European and national standards; the main standard being I.S. EN 14081-1:2016 “Timber structures - Strength graded structural timber with rectangular cross section - Part 1: General requirements”; this specifies not only CE marking (applicable to all harmonised standards) but also ‘general’ marking which is separate to the CE marking. Visually graded timber is usually undertaken by national standards which should comply with EN 14081-1. The Construction Products Regulations requires that there must be a Declaration of Performance before a product can be CE marking.
Visually graded timber is usually undertaken by national standards which should comply with EN 14081-1. The national visually grading standard for Ireland is I.S. 127 “Structural timber – visual strength grading – Sawn softwoods with rectangular cross-sectional”.
EN 14081-4 “Machine grading – grading machine settings for machine controlled systems” has been withdrawn and machine settings are now given in Approved Grading Reports.
Each individual piece of visually graded timber using I.S. 127 must be marked. Timber graded using visually grading standards of other member states is acceptable provided the standard complies with EN 14081-41 and the timber has been put on the market legally and has a Declaration of Performance and is CE marked.
EN 14081-4 “Machine grading – grading machine settings for machine controlled systems” has been withdrawn and machine settings are now given in Approved Grading Reports. Each individual piece of machine graded timber must be marked.
The Construction Products Regulations requires that there must be a Declaration of Performance before a product can be CE marked.
I.S. EN 1995-1- 1:2004+A2:2014 “Eurocode 5: Design of timber structures – Part 1-1: General - Common rules and rules for buildings” along with the national annex is the main design standard for use in Ireland. Designs should use the appropriate loading standards and their national annexes.
SR 71:2015 “Timber in Construction – Span Tables and Guidelines” provides non- contradictory complimentary information (NCCI) on designs using Eurocode 5. There are load span tables for floor joists, ceiling joists, flat roofs, roof rafters and purlins. Advice is given on purlin supports and timber studding (but not timber frame). There is also a method to allow wind loads acting on a roof to be estimated.