Roof Timber

Standards and Codes

BS 5268 should be adhered to when designing timber structures and structural elements. Eurocode 5 is just one of many structural design codes for building and civil engineering works designed to harmonise codes and standards across the European Union.

Timber species and stamps

In Ireland, the standard timber used in domestic construction is softwood. This timber is sourced both in Ireland and abroad. Spruce, fir, pine, redwood, whitewood, and larch are some typical species of softwood used in Ireland.

Timber is stamped to make identification easier on site. The stamp contains abbreviated forms of the species and source code as well as other information. Examples of abbreviated species and source code are:

WE/SG1

  • Western European (including Britain) Sitka/Norway spruce, pine and Douglas fir.

  • Species Group 1. Source – Ireland.

WE/SG2

  • Larch, Scotch pine.

  • Species Group 2. Source – Ireland.

EW, ER or EW,ER

  • Whitewood, redwood or whitewood and redwood combined.

  • Source – North and North Eastern Europe including Scandinavian countries and Russia.

Timber Strength Classes/Grades

Abbreviated strength classes are used to divide timber up in ascending order of its strength. The species and grade of timber determine the strength class of timber. C14, C16, C18, C22, C24 and C27 are the classes timber falls into.

Examples of strength grades are:

GS - General structural grade (visually graded) SS - Special structural grade (visually graded).

Timber should be marked with the strength class and grade in order to comply with the requirements of I.S. EN 1995.

Timber markings and moisture content

Markings require the following in order to comply with I.S. EN 1995:

  • Strength class and/or species or species combination.

  • Strength grade.

  • The standard to which the timber was graded. Illustrated in the example below.

Moisture content of timber

20% is generally the maximum moisture content allowed at the time of construction and, while in service, no individual element should have a moisture content of greater than 24%.

In situations where timber is being used in an area in which a higher moisture content may arise, account for the different moisture content in design.

Timber ceiling joists

According to BS 5268, the permissible clear span is the distance between wall supports. With traditional cut roof construction, however it is assumed that the hanger/binder connection provides support to the ceiling joists, thus reducing the span to the distance from the support to the binder connection.

Diagram D78 - Typical cut roof and junction details Diagram D78 - Typical cut roof and junction details

Note: The binder is fixed to every ceiling joist and to hangers; ensure hangers are not nailed to ceiling joists until the roof is fully loaded. Traditional cut roof construction assumes that the binder/hanger combination supports the ceiling joists.

Diagram D79 - Water cistern support Diagram D79 - Water cistern support

In order to ensure strength and lateral stability, all spreader beams and bearers should be fixed together with appropriate proprietary fixings. The spreaders should be fixed to the ceiling joists also.

Sizing

Example: Ceiling joist required to span 2.45 m. From the table above a 44 mm x 150 mm C16 joist @ 400 mm centres wall span 2.46 m and therefore would be suitable.

Table D3 - Ceiling Joist - with water tank

Measuring span

Diagram D80 - How to measure roof and truss spans Diagram D80 - How to measure roof and truss spans

Ceiling joist spans are measured between supports, the rafter spans are measured on the slope.

If design for uplift is necessary, an engineer should be engaged to design the roof appropriately in accordance with I.S. EN 1995. The engineer engaged should be qualified by examination, in private practice, and possess professional indemnity insurance.

Wind Speeds Extract TGD A

Map showing wind speeds in m/s - Extract from TGD A

The following tables of rafter, purlin, and flat roof joist spans adopts specific wind-loading criteria calculated based on the map reproduced above that is also included in the National Annex to I.S. EN 1991-1-4. Each table is designed for the 27 m/s wind speed region; the region or county in which the site is located does not matter.

Roof Rafters

SLC 1 (below) covers sites that are:

  • Greater than or equal to 5km from an open water shoreline.

  • less than 50m above mean sea level.

  • The height of the ridge is less than or equal to 10m.

Provide mid-span bridging where possible to rafters. Bridging should be less than 0.75 x depth of the rafter. Position the bridging so that 0.25 x depth gap is left at the top of the rafter. Ensure a 50 mm gap is provided to allow air to flow unobstructed.

If design for uplift is necessary, an engineer should be engaged to design the roof appropriately in accordance with I.S. EN 1995. The engineer engaged should be qualified by examination, in private practice, and possess professional indemnity insurance.

Roof rafters in heavy roofs – SLC 1 – 27 m/s with wind pressure q~p~(10)

Table D4 - Sizing of roof timbers - rafters

Example: A 36 mm x 225 mm C18 at 400 mm will span 4.19 m.

Roof Purlins

Diagram D81 - Purlin spans between supports

Diagram D81 - Purlin spans between supports

SLC 1 covers sites that are:

  • Greater than or equal to 5km from an open water shoreline.

  • less than 50m above mean sea level.

  • The height of the ridge is less than or equal to 10m.

Bearing is based on a butt joint and a maximum purlin width of 75 mm; where bearing is in excess of 105 mm must be provided, provide deeper strut or additional struts. Struts supporting purlins unless specifically designed should be at least 75 mm x 100 mm C16 section and less than 1.5 m in length.

If design for uplift is necessary, an engineer should be engaged to design the roof appropriately in accordance with BS 5268. The engineer engaged should be qualified by examination, in private practice, and possess professional indemnity insurance.

Roof purlins in heavy roofs – SLC 1 – 27 m/s with wind pressure q~p~ (10)

Table D5 - Sizing of roof timbers - purlins

Example: A purlin is required to span 1.95 m, supporting a 2.5 m maximum rafter span on the slope either side. From the table above, a 75 mm x 175 mm C18 purlin will span 1.97 m. This purlin will require 74 mm minimum end bearing each end.

It is important that the provision of lateral stability does not interfere with the ventilation of the roof space. Blocking at centres less than 30 times the breadth of the joist are required to provide additional lateral support where plasterboard ceilings are not fixed directly to the underside of the members. Ensure that blockings are securely fixed to walls.

Struts supporting purlins unless specifically designed should be at least 75 mm x 100 mm C16 section and less than 1.5 m in length.

Flat Roof Joists

SLC 1 covers sites that are:

  • Greater than or equal to 5km from an open water shoreline.

  • less than 50m above mean sea level.

  • The height of the ridge is less than or equal to 10m.

Lateral support should be provided to flat roof joists at supports and in the length of the span of the joists. Acceptable methods of achieving lateral stability include:

i) Building joists into masonry construction.

ii) Fixed end bridging between joists.

iii) Use of joist hangers that provide lateral stability.

Flat roof joists – SLC 1 – 27 m/s with wind pressure q~p~ (10)

Table D6 - Sizing of roof timbers - flat roof joists

Example: A flat roof joist is required to span 2.00 m. From the table above a 36 mm x 125 mm C14 at 300 mm centres will span 2.06 m and therefore would be suitable.

Preservation

In favourable conditions, timber will not deteriorate and will have a long life-span. The problem is that, in the majority of cases, timber is subject to attack by fungi or insects. Timber exposed to high moisture content is in danger of fungal attack. Insect attack can occur in fresh timber or in timber in service to a limited extent. Risk of attack increases when the moisture content of the timber exceeds 20%.

The purpose of preservation is to increase the durability of the timber against attack.

It has been shown through experience that if they are not at risk of getting wet and staying wet for some time, roof timbers (i.e. prefabricated trusses, rafters, joists, and purlins) in a standard, well-maintained pitched roof are usually safe from fungal attack without requiring treatment of preservative. It is stated that it is not required to treat tiling or slating battens with a preservative in the Irish Code of Practice (ICP) 2: 2002 Slating and tiling.

Risk of Attack

The hazard class system that categorises the risk of deterioration to which timber may be exposed is outlined in BS 5268. The grading is from low risk, 1, to high risk, 5. The hazard classes along with their end use situations are summarised subsequently.

The types of preservatives recommended provide an appropriate level of protection for the service lives listed. These service lives are just for guidance purposes and assume good design, proper construction, and normal conditions of use for timber.

Treatment processes

The most common methods of applying preservatives to timber are vacuum pressure processes. These are designed so that the preservative liquid penetrates the cross-section of the timber.

Level of preservative treatment required

It is strongly advised that, when preservative pre-treated timber is required, the timber provider or treater is consulted to ensure the treatment process and preservative type are correct for the desired service life and hazard class.

In cases where preservative pre-treated timbers must be cut on-site, the exposed ends should be coated in 2 heavy coats of colour-tinted preservative. The manufacturer’s recommendations must be followed with regards to PPE as required and also, if necessary, cleaning of equipment and disposal of residues.

Classes of hazards

The table provided below details the typical end use, the source of attack, the desired service life (measured in years), and the recommended preservative type for the three hazard classes.

Table D7 - Hazard classes