Construction Criteria

The basic criteria for all carpentry tasks

Whilst there are many tasks that we are expected to be able to carry out as carpenters, as well as many reasons as to why the specific criterion for each individual task is as such, there are some basic criteria that apply to all construction processes and components. These criteria define the expected desired outcome of all components and serve to guide us in the right direction. When approaching unfamiliar scenarios, overcoming obstacles, or rectifying errors, these overarching criteria define how we should approach the task at hand. As we move forwards from this section to look at all of the specific carpentry tasks, we’ll see how all of the basic criteria play a major role throughout the construction process.  

Square

Though there are some design-based exceptions to this rule, most buildings in the UK, particularly residential properties, are built with a square or rectangular footprint; an efficient shape for the arrangement of buildings within close proximity to each other. Extensions that are carried out on existing buildings also generally follow this pattern of square corners and perpendicular walls. As we look closer at each carpentry task that is undertaken, we can see that this scenario appears again on a smaller scale. Floor joists are installed at 90 degrees to the exterior walls, square door linings are installed into square openings, and structural openings in the exterior skin of a house are square and perpendicular to the horizontal plane.  

Studs are fixed perpendicularly to the top and bottom plates of a stud wall, for maximum strength in their load bearing capacity. On flights of stairs, treads present themselves at 90 degrees to the risers, and the desired level plane of any floor system presents itself at 90 degrees to the walls that surround it.  

There are many layout and measuring tools that we can use in the pursuit of keeping construction components square. A range of different sized squares, from small speed squares to larger folding squares can be held into or over internal and external corners respectively to ensure that they meet at 90 degrees. Simple measuring tricks following basic mathematical principles can also be employed to ensure that components are perfectly square with one another.  

If the footprint of a building is set out with perfect perpendicular corners, and continued upwards in the same fashion, then we can avoid any complications as the building is constructed. If the wall plates are “out of square” by the time that we reach plate height, the straightforward process of assembling the roof can become confusing and complicated. If internal components are not installed square, then the finish of the building begins to suffer from a lack of precision, presenting as unprofessional and lacklustre in appearance. 

In reality, due to the size and scale of a standard residential house, in combination with the selection of different operatives that work within it at varying points of the construction, not every component is going to end up perfectly square. The corners of rooms can be out of square by a few degrees, though this falls within the allowable tolerance. Every component is going to have an allowable tolerance, however the closer we can get to perfect in the first place at every stage of the project, the better the outcome once the structure is completed.  

Plumb

Plumb is a very important descriptor in construction, referring to a component, usually a wall, partition, or post, presenting itself in line perpendicular to the horizontal plane. This can be visualised by hanging a plumb bob on the end of a string from a height. When the plumb bob stops swinging, the string will represent the natural vertical axis of “plumb”.  

The modern use of plumb as a word comes from the roman word “plumbum”, meaning lead - the soft malleable metal. Due to its density, plumb bobs, the instrument for finding the natural plane perpendicular to the centre of the earth, were traditionally made of lead, thus over time leading to this axis being known as plumb. The modern word plumber also traces its name from this same derivation, with traditional plumbers often working with lead.  

It's important in construction for walls and posts to be plumb, as this ensures that any load being supported is being distributed directly downwards through the centre line of the support. If the wall or post is leaning “out of plumb”, then the downwards force they are bearing can cause unnecessary stresses - leading to potential failure of the components.   

Even in terms of non-structural components, it's important for internal components to be installed plumb. The jambs of door linings should be installed plumb, so that the door that hangs within it is able to swing in one plane. If the jambs were not plumb, the door would tend to always swing open or shut under the force of gravity, depending on which way the jamb is leaning.  

Installing workpieces so that they sit plumb is simple when using the right tools, and the process of ensuring components are plumb soon becomes second nature to a skilled worker. Spirit levels, plumb bobs, and lasers are all common pieces of equipment that are used to check if an item is plumb.  

Level

Level is a descriptor that is equally as important as plumb and describes the natural horizontal axis that is perpendicular to the vertical plane of gravity. Many components within construction must adhere to strict tolerances in relation to the horizontal plane. From a safety perspective, floor surfaces are required to be installed with a maximum deviation of 3mm per metre from level. This is to ensure a consistent underfoot feel and prevent trips and falls. In a similar instance, the treads, or the overall horizontal plane of a flight of stairs are required to be installed level.  

In order for a building to remain true in all criteria as it grows upwards, it's important for the top plates of stud walls, as well as each course of brick or block laid, to remain level. The wall plates that create the foundation for a roof to be constructed upon should be level so as to ensure the correct geometric presentation of the finished roof. The head of a door lining should be installed level, so as to ensure an even margin around the door once it is installed. 

Thin fluids like water will assume the space within a vessel that it is placed inside of. Due to the even forces of gravity exerting itself upon such a liquid in this state, water will always find its own level if left undisturbed, a fact that has been observed for millennia. Before the invention of precision measuring equipment, the master builders of antiquity utilised this rule by which water abides to construct their buildings in a similar particularity to how we do in modern times. A bucket with a line drawn parallel to its bottom is a simple tool that can be used to find the plane of level. 

The libella - a Roman surveying tool - was also used in the past to find the horizontal plane of level. This piece of equipment consisted of a plumb bob on string hanging from the apex of a small A frame. The A frame was made as a 90-degree triangle, and a mark was notched into the exact centre of the crossmember of the frame. When the libella was placed on a surface, if the plumb line hanging from the apex lined up with the centre mark of the cross member, then the surface was level.  

Geometrically speaking, where a perfectly plumb surface meets a perfectly level component, the resulting angle is 90 degrees. This ties back into the criteria of components being square to one another that we have just looked at.  

Straight

Straight refers to the continuous position of a workpiece in terms of maintaining a specific plane along its length. Regardless of whether the component is plumb, level, or presenting at any angle in between, from a professional and aesthetic perspective it should be straight along its entire length, unless it is intentionally curved. 

When selecting timbers for use as wall plates or sole plates, it's important to sight them down their length to ensure that they are straight. A timber that isn't straight will affect the finished result of the task at hand, as the other components such as the studs or rafter will follow the profile of this bowed timber. If there is a solid fixing point available, a bowed timber can be pulled back into a straight plane.  

Another example of this would be the incorrect installation of a fascia backer board on a roof. It's very rare that these timbers are perfectly straight from the merchants, and so some adjustment is required during installation. The straight plane can be checked using a taut string line, or by eyeing the length of timber to observe any irregularities. Failure to straighten components such as these can lead to a poor-quality finish. 

In terms of observing a true straight line, lasers are a fantastic modern layout tool that can be utilised in many different applications. Whilst lasers can make many tasks so much more accessible, nothing can quite replace the age-old string line. A length of string pulled taut between two points will always display a straight line, provided it is unaffected by wind or other movement. The only exception to this rule would be a slight downwards sag over a long distance due to gravity. This usually doesn’t affect the work being carried out though.  

Flat

When it comes to two dimensional surfaces, the desired outcome in most situations is known as “flat”. Both floor, ceiling, and wall surfaces are all components that are intended to be flat. These surfaces are composed of only two planes - length and width, or length and height respectively. Provided that the materials used to construct these components are of good quality, if both of the planes are straight along their length, then the total surface can only be flat. A truly flat surface is free from deviations across all of its critical planes, resulting in a perfect finish. Realistically, based on the materials that are available to use, there are allowances in how flat a surface can be. When it comes to stud walls, as well as floor joists, 99% of the individual timber components will possess a noticeable crown. In arranging these components so that the crown faces the same way, we can reduce the amount by which the resulting surface is thrown out of flat.  

If the wall plates and outer studs of a stud wall are installed perfectly flat across their two straight planes, this forms a solid basis for the wall to be erected perfectly flat. That being said, the crown of the studs, when installed in one consistent direction, will create a slight belly on one side, with a slight hollow on the other. 

The same can be said for a floor joist system. Even if the masonry is perfectly level in both directions, the upwards crown of the installed joists will create a slight upwards belly in the floor surface. These are expected and allowable tolerances within the criteria of a flat surface.  

Issues occur when timbers are incorrectly crowned, effectively doubling the deviation by which the surface is not flat. Additionally, if the two starting planes are not straight, square, level, plumb etc, then the resulting surface will possess some type of twist, effectively throwing the total flatness out of the window. 

Parallel

Ensuring that important elements of our work are parallel is another quality criteria that is only achieved through careful layout. Within the parameters set by the previously discussed criteria such as square, we find that many components should also be parallel to one another. When we think of a building that is erected with walls at 90 degrees to one another, ideally the opposite walls are parallel with one another. When it comes to installing internal partitions, we can use these parallel reference surfaces to pull equal measurements at each end of the partition, ensuring that the partition is also parallel with the external walls of the building. 

When rolling joists, the standard specified layout calls for the timbers to be spaced at 400mm centres (Approx. 16” centres). Through proper layout and the utilisation of effective methods of work, a skilled carpenter will be able to install all the joists perfectly parallel to each other. Any sideways bow in the joists is also able to be eliminated during the installation of the solid blocking. This results in a finished product consisting of a row of perfectly straight, perfectly parallel joists.  

Similarly, during the installation of a cladding system, every row of cladding should be installed parallel to the last, with small acceptable tolerances allowed. If every row is installed parallel to the last, the finished product will result in a run of cladding that looks straight and professional.  

Flush

Flush refers to the state of presentation between two components. Specifically, it describes two objects meeting each other so precisely in alignment, that a distinction cannot be observed in the transition from one to the other by touch alone. In this instance, the joint would be described as flush. There are many instances where two components being flush with each other comes into play, and it is dependent on the outcome of the task as to how we approach this.

When installing joists on a wall plate, ideally the joists should be cut off square to length so that they sit flush with the back of the wall plate. When studs are installed between two plates, due to the slight discrepancies in the thicknesses of the timbers, one side of the studs should be kept flush with the plates. When installing trimmers between joists, either the upper or lower face of the timbers should be kept flush together, depending on the criteria of the task.  

For reference, in the context of upstairs ceiling joist trimming, the timbers should be kept flush on their underside, to provide a flat surface for the ceiling to be installed upon. This is due to the fact that the top side of the joists are in the roof space, and any discrepancies here are allowable. On the flip side, first floor joists should be kept flush on their top side to provide a flat surface for the installation of the floorboards. This may cause a slight unevenness in the flatness of the ceiling below, though this is more preferable to an uneven floor surface.  

Tight cuts

Ensuring tight clean cuts are present on any workpiece is another skill displayed by quality craftspeople. Whether the cuts are square stud ends meeting perfectly into a plate, a plumb cut in full contact with a ridge, a bird’s mouth sliding snuggly into a plate, or a halving joint coming together perfectly flush, the ability to consistently produce high quality cuts reeks of professionalism.  

By maintaining accuracy during our layout through the use of tools such as squares and bevels, as well as using sharp cutting tools, we can strive to achieve a perfect cut on every fitted component. When starting out within the trades and learning how to handle the tools, this criterion can be challenging to achieve. Even experienced operatives may not achieve a perfect cut the first time, every time. That being said, the important element here is when we choose to install our components. If a cut doesn't fit perfectly the first time, do we ignore the sloppy fit and fix it in place regardless? I think not. Taking the time to ensure our work is neat and tidy is a sign of respect for the trade, and for the client whose property we’re working upon.   

Solid

Almost all structural elements that are assembled within residential construction are not intended to move freely, though they do have an allowable tolerance for settling under expansion and contraction. By this initial statement, it is understood that all structural workpieces should be solid/rigid/devoid of movement when touched. But it isn't just structural components that follow this rule. All components installed within a house, unless they are specifically designed to move such as a swinging door, should be rigid to the touch.  

A stud wall shouldn't wobble or deflect when pushed or lent against, floor joists should not noticeably deflect beyond the specified tolerance, a flight of stairs should not bounce when walked upon, and all of the internal trims and mouldings should not simply fall off when touched. Components that are intended to be fixed solid, should not move. When movement in these components occurs, it is due to an incorrect selection of materials, improper installation, or inadequate attention devoted to the installation process. Too often are components slapped together with haste by poor craftspeople, resulting in occurrences as just mentioned. Not only are these instances not meeting their criteria, but they can cause a wide range of issues moving forward. Deflection of components within a building will cause plaster to crack, floorboards to creak, unnecessary vibration and sound transference, as well as component failure in extreme cases.  

True

Now that we’ve looked at all of these individual criteria, we can begin to see how they might all come together to give us guidance when tackling irregular tasks that don't have an easy approach. The element of bespoke carpentry that occurs when modifying or renovating buildings can often leave us pondering for a moment on how best to tackle a task. With these criteria in mind, it is easy to begin to formulate a plan in order to achieve an end result that is up to par.  

When a building meets all of these criteria together, we can refer to the construction elements as being “true”. This is an all-encompassing phrase that is used to describe perfect components that fall within the range of allowable tolerances. Ideally, every component installed should be true.