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Structural Timber

From a green standpoint the main thing to remember is that timber should be woodmarked and sourced as locally as possible, preferably in the UK.

Structural timber has the advantage over most other structural materials of being renewable and also a relatively good thermal insulation material.

Structural timber is usually of the following types

  • normal softwood sections such as floor and ceiling joists as specified in the building regulations.
  • larger sections of softwood or hardwood forming beams, trusses etc. and usually calculated by a structural engineer.
  • laminated structural timber
  • engineered timber such as I beams
  • and occasionally – roundwood

The normal softwood sections are usually graded as C16 or C24 (respectively replacing SC3 and SC4) and this will be stated in the approved building regulation drawings. It is often available from UK plantations

Larger sections of softwood are also available from the UK and it is often possible to get them from local sawmills.

The larger hardwood sections may be more of a problem to source sustainably and in the case of tropical hardwoods, only the woodmark can be relied upon. (However it is seldom that tropical hardwoods are used structurally in housing). There is a considerable amount of timber produced sustainably in the UK which is not woodmarked mainly because it is produced in such small quantities that the certification procedure would not be warranted. E.g. there is a constant supply of hedgerow ash (although this is now threatened by Chalara) and to a lesser extent oak and other species which is not woodmarked but which gets replaced. There are also organisations such as Woodlots which may be of help in sourcing local timber.

Laminated and engineered timber

(sometimes known as Glulam) is generally well sourced environmentally. However, due to the poor understanding of timber building culture in the UK it has been marketed mainly towards large structures such as offices, swimming pools, theaters etc rather than the housebuilding market, so it can be difficult to find merchants who are supplying off-the-peg structural members. The Glued Laminated Timber Association has a list of member companies. Also try Panel Agency Limited and Lamisell

3551063731_ed3e95f987_mEngineered timber such as Masonite beams represents a huge step forward in timber technology. Compared with traditional beams and joists Masonite sections are, for the same structural strength, much lighter, much more dimensionally regular and use considerably less timber. The dimensional stability with traditional beams and joists can be a major problem if they are not supplied at a moisture content of 12%, as shrinkage can cause considerable movement. It is not unusual to hear of 20mm movement over a two storey timber structure in the first year . They also help with the insulation because the webs, being much thinner, cause minimal thermal bridging. They do however run at about twice the price of solid timber sections with merchants such as Arnold Laver quoting around £4.80/m for 220 x 38 I beams. (supplied in 12m lengths).

Span tables for timber I beams are published by James Jones

Roundwood

Roundwood means using timber just as it comes off the tree. The point about roundwood is that it is extremely strong for its size and needs no lengthwise sawing. This is because the strength of timber is in its grain. Trees are naturally as strong as they can be to resist wind and gravity.
If you saw lengthways through a tree trunk (which is of course, not perfectly straight), you tend to cut slightly across the grain and reduce the strength. So the joy of roundwood is that you simply find a piece of timber and use it as it is without squaring it or sawing it lengthwise. This preserves its huge natural strength.

The drawbacks are that the timber may have a slightly odd shape and that many structural engineers in the UK don’t know how to calculate its strength. This may be a problem when the Building Inspector wants proof of its structural strength. It may also, for instance, be a problem in creating a flat level floor on roundwood joists or a flat level ceiling on roundwood joists. When you look at traditional timber beams in old barns you often see a compromise between sawn timber and roundwood where large timber sections have been roughly sawn to give them a certain amount of squareness at the same time retaining most of the structural integrity of roundwood.

This image shows the roundwood reciprocal trusses being incorporated in some of the buildings at Lammas. This is an extremely strong way of forming a roof but at the same time it is a structural engineer’s nightmare because all the timber beams are slightly different and vary differently along their length. Why does this matter. Well the Building Regulations state not only how strong a roof (or floor or ceiling) needs to be but how much deflection (sagging) might occur. While a roof might easily pass the strength test, the deflection test is more difficult because it is based on a cube law. Engineers need to work with standardised dimensions, thicknesses, widths, timber properties etc. to prove things work.

There is of course the argument that this sort of stuff is all over-standardised and why not have things a bit out of square and maybe the odd collapse. Well – who knows?

Strength and stiffness

Although the structural strength of timber sections is important, it is usually the stiffness which is the deciding factor when it comes to the Building Regulations. This is more to do with not getting cracks in your ceiling than the building actually collapsing. There tend to be three sources of information about the structural values of timber

  • span tables for traditional joists, beams, purlins etc. These are referred to in Part A of the Approved Documents in the Building Regulations (and see also the archived 1992 version which gives span tables for timber in floors, ceilings and roofs. These are not in the current version but are virtually unchanged)
  • manufacture’s tables such as James Jones figures for their engineered timber ‘I’ joists
  • calculations by a structural engineer

European Standards

On 1st April 2010 the new CEN Eurocode standards for structural timber came into force in place of the old BS standard. These are –

  • BS EN 1995-1-1 Eurocode 5: Design of timber structures. Part 1-1 General – Common rules and  rules for buildings
  • BS EN 1995-1-2 Eurocode 5: Design of timber structures Part 1.2 General – Structural fire design

Span tables

TRADA have published span tables in a new softback book called Eurocode 5 Span Tables: For Solid Timber Members in Floors, Ceilings and Roofs for Dwellings and various companies do online calculation software. However for practical purposes the self builder will still find the span tables in the old (archived) Approved Documents from 1992 to be useful in getting an idea of sizes for floor, ceiling and roof joists, binders, rafters and purlins. There is not a great deal of little difference between the old span tables and the new ones. The slight discrepancy is mainly in spans shorter that 2.4m.

Building regulations

Structure

The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

3 comments to Structural Timber

  • Fiona

    I am looking for a structural engineer who is willing to calculate the stresses and loads using round wood for a yurt style structure to pass building regulations.

  • kevin lea

    Hi, I would like to know the regulations and common rules to install floor joists to a first , second floor new build flats , using steel construction frame . is this possible or does it pose problems ? the job already specifies precast floors .

  • david

    hi can you tell me please what size of timber purllin do i need over a 4.000 mtr opening in wall to carry joist above at 400 centres thanks david

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