A question often raised by someone contemplating the use of timber frame for their new home is:
“How much will my timber frame house weigh?”
It’s not just the self-builder that wants to know this, but also her/his architect and foundations engineer. House builders and developers also find it a very relevant question because if the superstructure of the building above the damp proofing course (DPC) level weighs less than, say, a brick and block building, then the substructure and foundations could possibly be designed and engineered differently.
Potentially this could save them literally thousands of pounds and hours of work that are all too often needlessly poured into the ground.
The fact that, compared with most other materials, timber frame is a ‘lightweight’ structure, can therefore be very beneficial for any architect, engineer, or builder.
Let's "Cut to the Chase"
As calculated below the weight of a panelised timber frame superstructure above DPC level for an average rectangular two storey house of 150m2 (75m2 per floor) will be approximately 14 metric tonnes. For ease of consideration for other size 2 storey buildings this equates to around 0.0933 MT per square metre of floor space.
A similar brick and block house will weigh around 80 MT - 5.3 times heavier
Using timber frame as the structural inner leaf will save 40% of the weight
So, for a 2 storey 150m2 house: Brick & Block superstructure = 80 MT
Timber frame + brick skin = 48 MT
Timber frame + timber cladding = 15 MT
(circa 0.9333 MT/m2)
As noted below, this will be different for single storey buildings (Bungalows), 1½ storey room in roof buildings (Chalet Bungalows) and 2½ storey room in roof cottages. It will also be different for semi-detached and terraced houses. However, as a quick assessment it provides a reasonable guide.
Timber Weight –v- Performance
However, don’t let the fact that a timber frame structure is lightweight confuse you over its strength. Or, indeed, how the building works. Light Weight timber frame is anything but ‘lightweight’. It is stronger and performs better than most other forms of house building structure.
Timber is strong
While timber is a relatively lightweight material, it is STRONG –stronger in fact than either reinforced concrete or steel on a strength to weight ratio basis. While strength varies depending on species, it tends to be much lighter than steel or re-enforced concrete, for example. Engineered timber components, like glulam (Glue Laminated Timber), I-Beams, Metal Web joists, Plywood Box Beams, Ty Ynos beams, Inside-Out Beams, and so on, can predictably and significantly outperform steel and re-enforced concrete on a weight for weight basis. However, they may require larger sections to do so.
The tensile strength of most timbers parallel to the grain is three to four times the compressive strength, with there being a marked difference in all strength properties when measured parallel or perpendicular to the grain. The tensile strength parallel to grain can be thirty times as high as perpendicular to it, while for compressive strength, the ratio is of the order of six to one(2).
When compared to other structural materials, softwood timber has a low density, meaning it can offer lightweight structural solutions, e.g. reduced foundations, easy handling and lifting(3).
Strength-to-weight ratios for a range of timber grades, glulam and steel
A Natural Material
Timber’s strength comes from its natural properties and the way the tree grows. The trunk of a tree is composed of cells aligned axially along the trunk4. This is long in relation to their width. Timber joists and beams sawn from the trunk have these cells parallel to their length and this provides both the axial and flexural strength.
Compression, Tension & Grading
Parallel to the grain, the compression and tension strength of timber is much greater than that perpendicular to the grain. This is due to the natural characteristics of these cell structures. Although it compresses across the grain, it does not do so along its length. Timber is therefore an excellent, stable material for use in supporting structures and composites, like glue laminated timber – or ‘glulam’. Strength grading methods have been devised to classify timber using either visual strength grading or machine strength grading method.
Growth Regions of a Tree - Britannica (see footnote)
Strength of Different Species of Tree
The range of strengths between species varies as much as their densities. The strongest hardwood species (e.g. greenheart) is almost eight times as strong in bending and almost six times as styrong in compression as the weackest (e.g. balsa).
The strength of sawn timber is a function of species, density, size and form of member, moisture content and duration of applied loading, together with strength-reducing characteristics such as slope of grain, knots, fissures and wane (bark remnants).
The small, defect-free specimens of wood used for basic strength testing give average values for all of these strength indicators but larger pieces of wood inevitably suffer from imperfections which weaken them.
Calculating the Weight of a timber Frame 'Panel' House
A masonry built house will way around 5.3 times more than a properly designed timber frame panel house.
This assessment assumed that the timber frame will be constructed from industry standard CLS timbers and OSB 'ply' sheathing on the outer face, with solid timber joists and solid timber roof with cut rafters, purlins, etc. While the weight will be similar, and possibly lighter, for a SIP (structurally insulated panel) frame, it will be very different for a solid CLT (cross laminated timber) frame and a traditional Green Oak post and beam style frame. All of these will of course also depend on the type of external claddings used.
The calculations are also only for the superstructure above DPC level, and do not consider the weight of the foundations themselves, which may also be lighter if a lightweight timber frame is used.
Timber Frame with timber cladding
'Rule of Thumb'
A quick rough and ready basic guide line to 'guesstimate' the weight of your timber frame house is to multiply the total m2 floor area of a two storey house by 0.093kg/m2. This will give you an approximation of the weight in kg / metric tonnes.
Although there is no agreement on what a 'standard' house might be, this theoretical weight is derived from the 14 metric tonne calculations made below for a 150m2 hypothetically square two storey house with gable ends.
this weight increases to around 15+ MT when a lightweight external cladding is used.
By comparison a 'wet build' brick and block structure would weigh around 80 metric tonnes, i.e. 5.3 times more than a properly designed timber frame building.
Timber Frame with Brick Cladding
Part & Part Savings
Even with a full brick external skin, the use of a structural timber frame to replace inner skin and internal wall lightweight block walls can reduce the weight on the foundations by over 40%.
Caveat on Building Sites
Assuming various sizes of timber and forms of construction, this resultant weight would be different if, say, the building were a single storey bungalow, a 1 1/2 or even 2 1/2 storey house offering a different floor area. Likewise it would also change if larger or smaller timbers and other elements were used in its construction.
Possible Cost Savings by Using Timber Frame
Using timber frame can also enable you to save weight on other aspects of your building, possibly leading to costs savings in foundations and other construction aspects.
The Weight of Timber Frame
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 weight of softwood and hardwoods per metre cute (m3).
Source: Foresters Companion Book
Weight of Timber in Timber Frame
The seasoned softwood timber used for timber frame panels 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 38x39 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 = 2.03kg, and 188 x 1m lengths of 38 x 140, each weighing 600/295 = 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 44.8kg.
To this has to be added the OSB wall sheathing. Usually 9mm OSB3 is used for external wall caldding. 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 weights 5.76 x 2.4 = 13.824 kg per lineal metre of wall panel.
Together the timber and the OSV 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 full 2 storey house with a square floor plan and an internal floor area of 150,2 (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, possible with areas branching off into 'L' shakes, '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 the wall.
For most UK house designs, the ratio of external walls to interneal 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.
Being triangular these can weigh up to 1/2 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 time 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, suggest 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.) Trussed rafters are generally of much lighter timbers, e.g. 35 mm thickness and will therefore create an even lighter roof. This solid timber 'cut roof' structure should also allow for more 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 / mn2 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 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 140 x 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:
4.5 metric tonnes
3.0 metric tonnes
0.5 metric tonnes
Joist Frame & Floor Deck
2.5 metric tonnes
Roof timbers & Sarking
3.0 metric tonnes
13.5 metric tonnes
Allowing 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 x 140 timber frame kit for a 150 m2 two storey house will be around 14 metric tonnes.
For our '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 to aesthetic effect and literally prevent them from falling over.
The Timber Frame IS the structure
Here it must be appreciated that, in this form of construction, as noted in the section "Conventional" - v - Timber Frame Build, Timbner Frame Walls, it is the timber frame that is doing all of the structural work of supporting the building including preventing the masonry walls from falling over, or being subject to wind pressures that can push in or pull out brickwork, for example, the following anecdote is an interesting example of this.
A national volume builder was suddenly faced by a structural emergency on a development of several dozen new houses. They were all sold and occupied but during the winter following completion, it was discovered that the brickwork was being sucked out by negative wind pressure.
The reason was simple, but crucial. Their bricklayers had not inserted sufficient wall ties. While quite dramatic, the remedy was to take down all of the face brickwork around all the houses and replace this with the correct number of wall ties inserted. The work was accomplished without any problems for the house occupants because the timber frame supported everything.
Surface Area of Walls
As shown above, our hypotehtical two storey house with 150m2 internal floor area has 34.64 linealmetres run of external wall. Assuming an eaves height of 5m, thisgives a wall surface area of 173.20m2.
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. 600kg/m3, as shown in the table above. Thus a standard 20 x 125mm 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 puporses we will assume an average external weight of 75kg/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 bridcks 1-2mm x 73 mm x 215mm as having a density of circa 2400 kg / m3 bricks, i.e. 600 bricks / m3, each brick should weigh 4 kg.)
Lightweight aerated concrete clocks, like Thermalite Hi-Strength 10, measuring 100 x 215 x 440 (4"x8"x17.3") weight approximately 7.5 kg (16.53 pounds) each, i.e. 770 kg/m3, (source Hanson). At 12/ 200 x 400 blocks per m2 the weight is 90kg / m2.
Combined Weight of External Cavity Wall
This gives a combined external wall surface area weight of 162 (bricks) + 90 (lightweight blocks) = 252 kh - say 260 kg allowing for mortar, well tiles, 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.
To this has to be added the internal walls, floor 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 clocks 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 and Block House Weight
52.0 metric tonnes
22.0 metric tonnes
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)
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.
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 constuction 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.
Lightweight Brick Slip Cladding