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The Weight of Timber Frame

Posted on November 29, 2011 by Tim Bertec There have been 0 comments

How heavy is timber?

The weight of all types of timber varies according to their water (moisture) content.

Unseasoned timber is referred to as ‘green’.  Air or kiln dried timber is referred to as ‘seasoned’.

The following table gives approximate weights in metric tonnes and kilograms for ‘green ‘, i.e. un-dried / unseasoned weights of softwood and hardwoods per metre cube (m3)

Softwoods ‘green’ 15% moisture Hardwoods ‘green’ 15% moisture
Pine 1.98 mt 600 kg Beech 0.97 mt 720 kg
Larch 1.20 mt 500 kg Ash 1.28 mt 700 kg
Douglas Fir 1.15 mt 500 kg Oak 0.94 mt 740 kg
Spruce 1.04 mt 400 kg

Source: Foresters Companion Book

Weight of Timber in Timber Frame

The seasoned softwood timber used for timber framing generally has around 15% – 18% moisture content (must not be above 20%), so for the purposes of this calculation the weight of Pine, the heaviest at 600 kg per cubic metre, has been used.

Today, the two most common sizes of timber used for timber framing are 38x140 for external wall panels and 38x89 for internal open wall frames.

One cubic metre of timber will, on average, cut into 295 x 1m lengths of 38x89, each weighing 600/295 = 2.03 kg, and 188 x 1m lengths of 38 x 140, each weighing 600/188 = 3.2 kg.

Window & Door Openings

These calculations ignore openings in external walls for windows and doors.

These will be the same size for any building, whether timber frame or brick and block (see below) with whatever windows and doors used also being the same.

However, the weight reduction for brick and block openings will be greater than for timber frame openings, per the calculations below.

Weight of External Walls

Taking the above figures and allowing for openings, lintels, cripple studs, cills, heads, corners, junctions and the like, with principle studs at 600mm centres, one lineal metre of external wall panel 2.4m high requires around 14 lineal metres of timber, weighing 44.8 kg

To this has to be added the OSB wall sheathing.  Usually 9mm OSB3 is used for external wall cladding.  This weighs circa 640kg per m3, i.e. 640/111 = 5.76 kg / m2 (source Norboard). Thus the OSB3 in a panel 2.4m high weighs 5.76 x 2.4 = 13.824 kg per lineal metre of wall panel.

Together the timber and the OSB therefore weigh 44.8 + 13.8 = 58.6 kg

Allowing for external breather membrane, nails and staples a figure of 60kg per lineal metre run of external wall is therefore assumed.

A 2 storey house with a square floor plan and an internal floor area of 150m2 (75 metres per floor) will have 4 external perimeter walls each 8.66m (the square root of 75) long on each floor, i.e. 8.66 x 4 walls x 2 floors = 69.28 lineal metres. Thus the total external wall weight will be around 69.28 x 60 kg = 4.157 metric tonnes.

However, hardly any houses are ‘square’ and as they become rectangular, possibly with areas branching off into ‘L’ shapes, ‘T’ shapes and so on, the length of wall enclosing such are increased.

To allow for this these calculations will assume an external panel weight for this hypothetical house of 4.5 metric tonnes.

Weight of Internal Walls

Internal walls made of 38x89 timber do not generally have OSB sheathing, except where restraint (e.g. wind) walls are used, but they do have horizontal noggins through the centre.  Assuming, therefore, the same length of timber as for external walls, the weight of these is assessed as 14 x 2.03 = 28.42 kg per metre run of wall.

For most UK house designs, the ratio of external walls to internal walls works out at about 45:65, indicating around 100 lineal metres of internal wall, weighing some 2.84 metric tonnes.

Again, this varies with shape, so for these purposes a slightly higher weight of 3 metric tonnes will be assumed as the weight of the internal walls.

Gable Ends

Being triangular these can weigh up to ½ the weight per metre run of external wall, i.e. 60/2 = 30 kg, depending on roof pitch.  Here we will assume gables to two walls of the square plan form, giving 17.32 lineal metres of gable at a weight of 30 kg = 0.519, say 0.5 metric tonnes.

Joists Frame & Floor Decking

This has to include rim beams to the whole perimeter, trimmers for openings like stair wells, and blocking between joists.  With joists at 600 centres this has been calculated at around 2.75 lineal metres per square metre of floor.  Thus, for the 75m2 floor area required in this example, a total of 206.25 lineal metres is required.

Assuming 47x220 deep joists able to cope with reasonable spans, circa 96 x 1m lengths of joist per m2 are required, giving a weight of 600/96 = 6.25 kg per lineal metre. Thus the 75m2 floor will have a joist weight of 206.25 x 6.25 kg = 1.289 – say 1.3 metric tonnes.

Usually a 22mm floor deck is laid on this joist frame, for which we will extrapolate a weight per m2 from the OSB3 weight of 22/9 = 2.44 times 5.76 kg/m2 = say 14 kg per m2 of joist / floor area.  This gives a floor deck weight of 1.05 metric tonnes.

While this gives a combined weight of 1.3 + 1.05 = 2.35 metric tonnes, allowing for possible irregular shapes, screws and glue, suggests a rounded up weight of 2.5 metric tonnes as not unreasonable.

Cut Roofing Timbers & Sarking

With the need to increase roof insulation and use more roof space for residential accommodation, all roof timbers (rafters, ceiling/floor joists, purlins, collars, ridge, etc. have been assumed as being 45 x 200mm for a cut, rather than trussed rafter, roof, with rafters at 600mm centres. This should also allow for most heavy weight tile, or slate, roof coverings. The length of timber required for these elements per square metres of roof coverage has been assessed at 5.42 m /m2.

One cubic metre of softwood can yield 111 x 1m lengths with a weight or 600/111 = 5.4 kg each.  Thus the weight of the roof timbers is 5.42 x 5,4 = 29.268, or 2.195 metric tonnes for the 75m2 roof.

To this it is recommended that at least 9mm OSB3 roof sarking should be added (possibly 18mm if at a low pitch, or in very exposed conditions) . Therefore, assuming a requirement of circa 1.5m /m2 plan area (depending on roof pitch) and a weight of, say, 7 kg / m2 (including breather membrane and counter batten, nails, etc.) a self-weight of 1.5 x 7kg = 10.5 kg/m2, or 10.5 x 75 = 0.7875 metric tonnes is indicated.

This gives a combined roof weight for timbers and OSB3 sarking of 2.195 + 0.7875 = 2.98 – say 3 metric tonnes for the total roof.

Summary of Timber Frame Kit Weights

In summary, therefore, an open cell (e.g. not insulated) basic 140x 38mm timber frame for a two storey house with an internal floor area of 150 m2 may be said to have a self-weight of:

  • External Walls .......................... 4.5 metric tonnes
  • Internal Walls ............................3.0 metric tonnes
  • Gable Ends .................................0.5 metric tonnes
  • Joist Frame & Floor Deck ........2.5 metric tonnes
  • Roof Timbers & Sarking ...........3.0 metric tonnes
  • Total ..........................................13.5 metric tonnes

llowing for weight of possibly multiple sole plates, roof trim (barge, fascia, soffit) and sundry items, one may assume that the overall weight for a 38 x140 timber frame kit for a 150m2 two storey house will be around 14 metric tonnes.

For rule of thumb calculation, say 0.093 metric tonnes per square metre of total floor area.

Comparing Timber Frame with Brick & Block

First it must be stressed that when bricks and blocks are used, it is they that carry the weight of the structure.

Conversely, with timber frame construction it is the timber that carries the weight of the building and any dynamic loads placed upon it.  The timber frame also supports any brick or block masonry cladding used for aesthetic effect and literally prevents them from falling over.

Surface Area of Walls

As shown above, our hypothetical two storey house with 150m2 internal floor area has 34.64 lineal metres run of external wall.  Assuming an eaves height of 5m, this gives a wall surface area of 173.20 m2.

Adding to this the gable end triangles of 17.32/2 = 8.66 x  2.4 (eaves to ridge) height = 20.78 m2, gives an approximate external wall surface area of 193.98.  Allowing a little for shape, etc, adjustments a figure of 200 m2 will be assumed for calculation purposes.

As noted above, window and door openings are being ignored since they are the same whatever form of construction is used.

Inclusive Timber Frame Weight per m2 External Wall Surface

While the 14 MT weight calculated for the timber frame indicates an inclusive weight of 70 kg per m2 (14000/200) external wall area for the timber frame house, this does not embrace the weight of the external cladding.

Such cladding could be full width bricks, or brick slips, render, tile hanging, render or timber boarding, e.g. Cedar.

For these purposes we will assume the same heavier timber weight for Pine, i.e. 600 kg / m3, as shown in the table above.  Thus a standard 20 x 125 mm board coverage, with 8 boards per m2, (400 metre long boards per m3), will add 600/400 = 1.5kg per m2.  With fixings, membranes, battens etc. assume 2kg / m2.

Since this weight will be greater for any of the other claddings, for these purposes we will assume an average external weight of 75 kg / m2 of wall surface as the inclusive weight of the timber frame shell.

This indicates a shell weight, ignoring window and door adjustments, of 15 MT

Inclusive Brick & Block Weight per m2 of External Wall Surface

Weights for bricks and blocks seem even more difficult to ascertain than those for timber.

Red Clay Bricks

According to Wiki Answers a single red clay brick weighs about 2.7 kg.  With 60 metric bricks per m2 that is 162 kg per m2.

(NB: Simetric give the weight of fired clay bricks 102mm x 73mm x 215mm as having a density of circa 2400 kg/m3 bricks, i.e. 600 bricks / m3) each brick should weigh 4 kg).

Lightweight Blocks

Lightweight aerated concrete blocks, like Thermalite Hi-Strength 10, measuring 100 x 215 x 440  (4”x8”x17.3”) weigh approximately 7.5kg (16.53 pounds) each, i.e. 770 kg/m3, (source: Hanson).   At 12 / 200 x 400 blocks per m2 the weight is 90 kg / m2.

Combined Weight of External Cavity Wall

This gives a combined external wall surface area weight of 162 (bricks) + 90 (lightweight blocks) = 252 kg – say 260 kg allowing for mortar, wall ties, etc. and the somewhat heavier weights of some types of material, but ignoring the heavier weight needed for concrete or steel lintels.

Thus the weight of the 200m2 external walls of a brick and block cavity wall house of the same size and shape as the theoretical timber frame house, is 200 x 260 kg = 52 metric tonnes.

If these external walls use facing brickwork for the outside walls, then no additional weight will be incurred. However, this will be the case if render or other claddings are applied to, say, a block-work outer skin of an exterior cavity wall

Internal Walls

To this has to be added the internal walls, floors and roof.

Taking the same 100m length of internal walls as for the hypothetical timber frame above, this gives an internal wall  surface area of 100 x 2.4 (height) = 240m2.  If these were built in the same lightweight concrete blocks as used in the outside walls, the weight would be 240 x 90 kg/m2 = 21.6 MT – say 22 metric tonnes.

Summary of Brick & Block House Weight

  • External Walls                          52.0 metric tonnes
  • Internal Walls                           22.0 metric tonnes
  • Gable Ends                                  0.0 included in external walls
  • Joist Frame & Floor Deck         2.5 metric tonnes (same as for TF)
  • Roof Timbers & Sarking            3.0 metric tonnes (same as for TF)
  • Total   79.5 metric tonnes

Allowing for additional weights of lintels, etc. possible roof trim (barge, fascia, soffit) and sundry items, one may assume that the overall weight for a brick and block 150m2 two storey house will be around 80 metric tonnes.

For rule of thumb calculation, say 0.53 metric tonnes per square metre of total floor area.

Timber Frame with External Full Brick Cladding

From the above calculations, the weight of a 200m2 of full external brick cladding adds 200 x 162 = 32.4 MT to the weight of the basic 14 MT timber frame for the hypothetical square 150m2 two storey house, making this 46.4 MT.

This is still a weight saving of 33.6 MT, or 42% of the weight of a full brick and block structure.

Conclusion

Allowing for the fact that a timber frame building may need exterior claddings that increase its overall weight (to 15 MT in the hypothetical example) a brick and block building weighing in at circa 80 MT may be some 5.3 times heavier.

This difference in weight can make a considerable difference in the design and construction costs of the foundations and some other elements.

Using a timber frame rather than block-work for the main structure can still save over 40% of the weight of a ‘wet build’ house.


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