New Roof, Roof Replacement & Metal Roof Cost Guide

Find out the cost of a new roof, a roof replacement or a metal roof. We rounded up the costs & factors that will impact the cost of a new/replacement roof.

Normal household roofs come in many different styles and shapes and are made from quite a few different materials. The one thing they have in common is protection. No matter what they are made from or what they look like, their purpose is to protect the people, their belongings and the rest of the house by keeping the warmth in, shedding rain and snow, and protecting from all the other ravages of the outside world. So their design and construction will depend mainly on what kind of weather you usually have in your locality, what kind of wildlife is common and what is a traditional type of roof for your area.

Let us expand on these factors one at a time and see if we can understand why we have the style of roof that we have.

Rain. If you live in a place where rain is common, you will want to protect yourself from the rain and divert any shed water in a controlled manner away from the eaves of the house. This is accomplished by a system of rainwater guttering and downpipes diverting the water to either mains drainage, a soakaway pit or to storage. The roof must have a slope to allow the water to run off but without being too steep. If the roof is steep, the water will be travelling too fast when it reaches the eaves and will overshoot the guttering.

Snow. If you have a lot of snow in the winter, the snow will build up on the roof and add considerable weight to the already significant weight of the roof. The result may be that the roof timbers sag and eventually break. The answer to this is to have a steep angle to the roof with a large overhang at the eaves. This will allow the snow to slide down the roof and shed safely away from the house walls. A modern innovation is to incorporate a heating element in the roof so the snow does not get a chance to settle and stick in the first place.

Wind. If your locality experiences winds stronger than just a breeze, you will want a roof constructed in such a manner that strong winds will seat the roof more firmly than usual. Basically you don’t want your roof to be blown away. This depends on the pitch angle as well as the roof height. You need a roof that either sits firmer or that allows the wind to blow across with no obstruction. Alternatively if you can make a roof that will collapse ‘safely’ without crushing the occupants then that was acceptable as well. Tropical countries commonly afflicted with hurricanes or typhoons often use very lightweight roofing materials. These are relatively safe when blown down and easily rebuilt after the storm.

Cold. Even if you live in a warm zone, with little difference between the temperatures of summer and winter, you will find that night time is always colder than daytime and in some areas that do not have cloud cover; night time temperatures can reach almost freezing point. You therefore need a roof that will insulate the house interior against the cold. This is usually done by means of insulation blankets fitted in the roof-space, protecting the rest of the house.

Wildlife. Ideally you need a roof which allows free ventilation in the roof-space above the insulation to prevent damp and condensation from forming. At the same time you do not want to give free access to wildlife that will gladly take advantage of the ready-made, dry nesting area you have so kindly provided for them. Normal infestations in the roof include flying insects, rodents and birds. Controlled ventilation is therefore needed by means of baffles or ventilation mesh.

Materials. Traditionally the roofing material commonly used in an area was that which was easily found or made. In areas near rivers or with clay soil, roofing tiles would be baked in the same way as bricks. In areas with laminated bedrock such as slate, shingles made from thin laminations of slate were more common; this provided a lightweight, waterproof covering. In areas where certain types of trees were common, wooden shingles would be used to cover the roof. In areas where crop farming with a straw waste product was practised, thatch roofs were commonplace.

Thatch roofs have the added advantage of providing insulation but they can easily be colonised by birds, rodents and insects. Generally the traditional roof covering was whatever waterproof material was at hand.

Now we understand a little about why our houses have the kind of roof they have, we can move on. Today we are going to talk about the various cost implications between having a new roof installed, replacing and repairing your existing roof or installing a sheet metal roof.

Why do you need anything done to your roof anyway?

The first things to consider are whether you need a new roof or just have your old roof repaired. Why do you need anything done to your roof anyway? Roofs are like any other part of your house; they are there for a reason and need to be properly maintained to continue to fulfill their purpose. When they reach the time when the repairs become more frequent and more costly than is wanted, it is time to consider having a new roof. Let us just think about the various things that may go wrong with a roof and what can be done to remedy the problem.

Old & dilapidated roof. Usually most roofs have a maximum lifespan of about 25 years. If yours is nearing that age, or is experiencing problems (more about those later) then it is time to consider having a complete replacement roof. Having a new roof is certainly not cheap, either with materials or with labour, but it is probably one of the most important parts of your home to keep in good condition. Repairing over and over again with patch-up materials will probably end up costing more in the long term than having a new roof installed in the first place.

Major leaks. If your roof continues to regularly have leaks, it is vitally important to find out what is causing them.
It could be something as simple as blocked guttering overflowing behind the fascia boards. This is easily remedied and is usually caused by a fall of leaves from nearby trees or maybe moss washed down from the roof. You just need to remove the solids, followed by a rinse with clean water to wash away any lingering traces. It’s now as good as new.

An inside leak may seem like it is coming from the roof whereas it is in fact coming from rotten, wooden fascia boards. Fascias are the wooden (or PVC) boards that are fixed to the walls below the roofline to cover the gap between the wall and roof timbers. They are also used to carry the brackets for the rainwater guttering. Because wooden fascias are usually difficult to inspect and maintain on a two or three storey house, they tend to become overlooked and maintenance which should be done becomes postponed. Like all exterior wood it requires regular painting to ensure protection from the rain and sun. If the paint layer becomes old and cracked, it no longer protects the wood. Subsequently the wood starts to rot and allows water to enter the roof-space. (It also allows wildlife to enter too).

This is an easy fix for the professional but will need the use of scaffolding to provide a safe work platform at height. If there is just a small area of rot, then it may just need a fill and paint repair. If it is a longer length then the best thing is to replace a convenient length of fascia. If rot is found at various places in the fascia around the entire perimeter of the house, then it may be worthwhile removing the entire wooden fascia and replacing with maintenance free PVC fascia.

The roof covering of shingles, tiles or slates may be cracked or missing. This is caused by either the continual exposure to temperature extremes causing hairline cracks followed by water seeping into the crack and subsequent freezing, or it can be the result of strong winds causing the tiles to vibrate and knock against each other, resulting in eventual breakage.

The leak may be caused by old and cracked roofing felt. In houses built after the 1950s, the roofs were constructed with a waterproof layer of bitumen impregnated inorganic fabric (usually polyester or glass fibre, known as roofing felt or roofing paper) between the tiles and the roofing timbers. This provided an extra waterproof layer in case of tile breakage. As the roofing felt becomes old, it turns brittle and cracks appear followed by crumbling of the fabric. If your roofing felt has deteriorated, it must be replaced. This involves removing all the tiles, flashing and old felt. Replacing the felt and flashing with new followed by either replacing the old tiles or installing new depending on the condition of the tiles.

It may be caused by the flashing becoming cracked or otherwise deteriorating. In a roof, there are certain places which need more waterproofing than others. These places are around openings in the roof such as chimneys, skylights and vents. Flashing is also found in the internal corner joint between two surfaces of the roof, known as ‘valleys’. The purpose of flashing is to redirect rainwater away from these openings in the roof. Flashing is usually made from lead sheet or some kind of self-adhesive bitumen impregnated inorganic fabric. Both these kinds usually do the job well, but can be easily damaged by strong winds. Repairing flashing is an easy job for the professional. Not only roofing contractors are able to repair or replace lead flashing, often it is a job for plumbers. Lead flashing used to be the sole responsibility of plumbers. In fact the name ‘Plumber’ comes from the ancient Latin word ‘Plumbum’, meaning lead.

You have now checked your roof and have found that the leaks are not caused by the guttering, fascia or flashing. What do you do next?

Get someone qualified to have a look.

The unfortunate thing about water leaks is that water will always find the easiest way through. If this means having to travel a considerable distance, then water will do just that. Unfortunately just because you have a wet patch in the Northeast corner of the master bedroom ceiling, does not mean that the leak is directly above that. The place allowing access to the water can be literally anywhere on the roof. The only way to find that leak is for an experienced eye to inspect the roof, both inside and out, for any signs of water ingress.

An experienced contractor, if allowed access to your roof-space, will probably be able to find where the water is coming through the roofing felt. He then transfers his attention to the outside and searches around that area until he finds a likely cause of the problem. This might not always work but it is better than blindly guessing. While the contractor is in the roof-space, ask him to inspect the roof timbers as well. If the water has been leaking for a long time you can be sure that there has been damage done to the wood. If that is the case then not only does the leak need to be fixed but also the roof timbers must be repaired or replaced. A good roofer or carpenter will be able to advise you on the best course of action there. He may not even have to remove the damaged timber but may be able to add an extra rafter to share the load.

We have now covered the most common possibilities that might be causing your leak. At last we have to start thinking about the cost and the factors which will govern the amount.

You must consider all your options.

The bottom line is that everything hinges on the cost of materials and labour. These can be expanded to include other factors, which will have an effect and either increase or decrease the overall cost of your roof.

Area or square footage. Obviously if you have a larger roof you will use more materials and so have a higher price. Roofing contractors measure a roof in squares of 100 square feet (that is 10 ft x 10 ft). The way they do this is to measure the total area of the roof and divide by 100. This gives them the number of roofing squares. They will price the job by the square.

Materials. The type of material you choose to cover your roof will have the biggest say in the cost of your roof. There are many different modern materials available to choose from but remember that the locality you live in may have restrictions on the type of material you can use. The materials you can choose from include:

  • Composition shingles. The most common. These are shingles made from laminated bitumen impregnated inorganic fabric.
  • Wooden shingles. Usually made from cedar or redwood. Very weather and rot resistant, although they can break with mechanical impact.
  • Fibreglass shingles. Shingles made from laminated fibreglass mat and epoxy resin
  • Slate. Either natural slate or reconstituted slate made from slate dust and cement.
  • Tile. Earthenware material made in the same way as bricks.
  • Metal. Aluminium, copper or galvanised steel sheet metal.
  • Bitumen. Usually used on a so-called ‘flat roof’ (meaning a roof with a slope angle just large enough to ensure a flow of water). They are constructed from sheets of manmade engineered wood board covered with alternate layers of roofing felt and pitch. These were very popular up to the 1980s but were very prone to leaks.
  • Epoxy. After the 1980s flat roofs started to be made using an epoxy resin mix to replace bitumen and a fibreglass fabric to replace the roofing felt. Flat roofs made from this material are very strong and resistant to wear. They are even strong enough to be used as a balcony floor over a porch.

Roof profile. The profile of your roof will also affect the cost, not only with materials but labour costs as well. There are six common roof types currently in use for buildings.

  • Flat roof. Discussed earlier, this roof is has just enough angle (pitch) to allow rain to run off.
  • Shed roof. A very basic roof. This gets its pitch by having two parallel walls at different heights. The roof is usually a manufactured wooden sheet covered in roofing felt, although it is possible to have them covered in tiles or shingles. Basically this is one half of a gable roof.
  • Gable roof. This is the roof that children draw when portraying a house. The traditional roof made popular by the ancient Romans. It consists of two pitched slopes joined in the middle by a ridge board. The pitch can be steep or shallow depending on the required height and what you want the roof to do.
  • Hip roof. This roof has four slopes, usually of the same pitch, joining in the middle at a ridge board. At least two of the opposite slopes are triangles with the ‘Hips’ forming the sides of the triangles.
  • Gambrel roof. Also called a Dutch roof. This is a more ornamental roof shape. It is basically a gable roof but with two different pitch angles on each of its sides. The upper slope being shallower than the lower slope.
  • Mansard roof. This is a version of the Gambrel roof but with Hips. This means the roof has four faces each with two different pitches.

Each type of roof has its own reasons for being used and has its own disadvantages and advantages. One principle runs through all the types and that is that the higher and steeper the roof, the more difficult it is to install so the more expensive it will be. Likewise if the profile is complicated like the Hip, Gambrel and Mansard roofs, the more difficult it will be to construct and the higher will be the cost.

Pitch. The pitch of a roof is defined as the vertical height divided by the horizontal distance, or ‘rise over run’. The run is the horizontal distance from the outside of the perimeter wall to the centre of the house. The rise is the vertical distance from the top of the perimeter wall to the roof peak. Any roof with a pitch of less than or approximately 10 degrees is considered to be a flat roof.

Labour. It is possible to do a small roof repair or even a major roof repair as a DIY project, it all depends on your experience. Or you might decide to call in a contractor. Whatever road you go down, the cost of labour comes into it. Either the cost of someone else’s time or the cost of your own time. If you are going to hire a contractor then get at least three different estimates and see which is the most favourable deal. Don’t forget it’s not always wise to choose the cheapest option, just choose the best overall package.

Where you live. Yes, the location of your house is an important factor in deciding the type of roof you have and therefore the cost. The type of roof and the pitch will govern the amount of materials and the labour charges. If your area suffers from a lot of rain or snow then you need either a gable or a hip roof. Pitched roofs cope with bad weather much better than flat roofs. Pitched roofs tend to allow the rain to be shed with just enough speed to self-clean the roof surface and wash any solids into the guttering. Flat roofs have gutters as well but they tend to become clogged with debris more easily. The debris and pooled water adds weight to your roof, eventually deteriorating the covering material of the flat roof, causing leaks.

Removing the roof. Removingt, slates and shingles requires know-how. You can’t just barge in and take off many at a time. Each one has to be removed carefully so as not to cause damage. Each has to be removed singly and each has to be removed in a specific order, from the ridge to the eaves, one course at a time. The tiles have been fixed firmly with either nails or clips, so they have to be removed carefully to avoid cracking or otherwise damaging the tiles, felt, flashing or tiling battens. Of course if you are removing a roof in order to replace it completely with new, or if you are building a new roof, then you do not need to be as careful. Remember that even if you aren’t going to use the removed tiles again, you may be able to sell them on to someone else who is doing a repair job, so try not to damage them if you can help it.

The season. Like all people who work outside, roofing contractors always seem to be busier in the spring and summer, so they will be both harder to hire and more expensive in these months. But the mortgage always needs paying so they will be glad of some work in the slow months, autumn and winter. If you give them enough notice and let them fit your work in around other jobs, you may even get a discount. One thing though, if you find you have a leak in the springtime, do NOT wait until the following winter before you have it repaired. Get it done as quickly as you can.

So far we have mainly talked about roofing methods using the more traditional methods of earthenware, slate and wood shingles. What about the metal roofs, are they ok too?

Repairing and replacing a metal roof

Up until now we have concentrated on roofs covered in tiles or shingles. What happens if your roof is covered in a metal sheet? I know this sounds unusual for a domestic roof, usually confined to commercial or farm buildings, but it is in fact quite common in certain parts of the country. Metal roofing materials are available as shingles or as sheets. At one time the only metal roofing material would have been galvanised corrugated steel. It was cheap, easily fitted and had a reasonably long lifespan. Its disadvantages were that it was not that attractive, tended to rust with age and needed painting if you wanted to hide the metal. It was good for using on farm buildings and sheds, but not a lot else. These days, we have aluminium!

Aluminium roofing materials come in sheets or as shingles. The metal is lightweight, rustproof, easily fitted, easily moulded and easy to transport. Its weight means that a single person can carry many pieces at a time, the job gets done quicker and it added less weight to the building’s structure. It can be bent, stamped and moulded easily to provide ornamental features and can be supplied with a coloured plastic coat if needed. The shingles interlock using clips, making it very hard for water to find its way underneath. Many homeowners fit either aluminium shingles or sheets because of its durability and because of its heat reflection and absorption properties.

Its disadvantages include the fact that it is among the most expensive roofing materials on the market. They are also very smooth and slippery leading to hazards when people need to walk on them. Aluminium roofs can also be very noisy when cooling down and heating up, and during heavy rainstorms and windstorms. It is always worth installing a solid membrane beneath the aluminium so that any noise is reduced to a minimum.

I expect nearly everyone has seen a galvanised corrugated steel roof on a farm building at one time or another but these were also very common as roofs for prefabricated houses as they were quick to produce and fit. Over time, the protective coating deteriorates and the underlying steel starts to rust. If you want to keep your steel roof waterproof then it will need regular maintenance and painting. If the surface is badly rusted and pitted then this paint job will turn into something more extensive using rust removing chemicals, rust converting paints and special fillers.

This is a very labour intensive job to do and is one of the reasons why you see fewer and fewer corrugated steel roofs. If you want to continue using a metal roof (and there are very good arguments for using a large sheet rather than small tiles) then advances in modern technology and the increase in recycling has made aluminium sheet roofs more popular and more easily affordable. There has always been a certain amount of lead and copper sheet roofs (both very malleable and easily worked metals) especially on churches and public buildings, but these are very expensive and have always been beyond the reach of the ordinary domestic property. The advent of cheaply produced aluminium allowed the advantages of rustproof sheet metal roofs to be exploited in the domestic market. Aluminium is strong and lightweight, rustproof, easily moulded, takes plastic coatings well, and is easily painted. All these factors would make it ideal for the domestic roofing market if it wasn’t for the disadvantages;

  • Easily bent and deformed by strong winds.
  • Extremes of temperature causes expansion and contraction.
  • If not properly protected, moss, rain and seawater can corrode the metal.
  • Hard impacts can cause serious damage.

Aluminium sheet roofing is therefore usually used only in areas that do not contribute to the damage. If damage occurs then the repair or replacement of components is very easily achieved. Common sheet metal maintenance and repair jobs include the following:

Repainting. A new coat of appropriate paint will easily resolve corrosion and staining. Removing any existing corrosion using an abrasive such as sandpaper or wire wool also gives the paint some adhesion.

Repairing fixings. Screws and rivets may sometimes deform or work loose with the constant expansion and contraction of the metal sheet. Remove the screw or drill the rivet head using a hand drill. Replace with a new screw or rivet and add a dab of paint for protection.

Repairing dents, creases and scratches. After physical damage, the existing paint surface will become cracked or chipped. Immediately repair by sanding the scratch smooth and giving the paint a touch-up. This will stop any corrosion from becoming established.

Repairing cracks. Cracks will eventually appear as the metal contracts, expands and vibrates. Cracks can occur anywhere on the sheet but are more common around screw and rivet holes and at the edges. If you notice a crack, drill a small hole at either end to prevent the crack from lengthening. Patch the crack with a small sheet of metal using fixings as well as flexible silicon mastic to hold it in place.

Replacing complete sheets. If the crack is too large to repair properly or if you want a more attractive appearance, replace the complete metal sheet rather than attempt a repair. Drill out rivet heads and replace with new as well as replacing screws.

When handling metal roofing always remember that metal sheets can be very sharp. Always wear protective gloves and spectacles. If you aren’t sure what you are doing, hire a contractor.

Where do I buy metal sheets?

You can buy metal roofing from building merchants and online retailers. Metal roofing is usually available in sheets of 2ft x 3ft to make handling easier. You may be able to find a metal fabricator who can make the sheets to your custom sizes but they will be more expensive than ready-made sheets.

Should I hire a professional or DIY?

Roofing is a very dangerous and skilful job and to make a roof to be proud of, requires a lot of specialist knowledge. Having said that, if you are prepared to do your research, acquire the correct tools and take your time, it is possible to do it yourself. If you make a mistake, it is relatively easy to put right as you go along. The only problems are that most people have limited spare time and live in areas where the weather is unpredictable. These two factors combine to make DIY roofing sometimes impractical. It is at times like this when you should hire a professional.

Any extras on your roof?

While you are having work done on your roof and have gone to the expense of hiring scaffolding as a safe working platform, it might be worth considering adding extras to either supplement your energy bills or to help shed snow.
I won’t go into great detail on these extras as the technology is changing almost every month and any figures given now will probably be soon out of date. Let it just be enough to mention them and then leave you to find out more up to date information.

Photovoltaic cells. These are panels of special electronic cells that convert sunlight into electricity. They are placed on a south facing roof (North facing in the Southern Hemisphere) to catch maximum sun. The energy produced by the cells can assist in supplementing the bought electricity needed to run your house. Modern technology now allows the photovoltaic cells to be incorporated within the roof tiles themselves.

Solar panels. These are panels fitted to the south facing roof (North in the Southern Hemisphere), through which water is pumped. The water collects the heat from the sun and transfers it to a heat exchanger located within the house. This provides a way of heating water for showers and baths as well as for the central heating system.
Heating element. In areas of heavy snowfall, it can be sensible to install a heating element in your roof to warm the roof covering to just above the thaw temperature. This will prevent snow and ice from settling on your roof and putting an extra strain on the roof structure.

Costings. Materials & tools.

A selection of different types of roofing materials and their minimum and maximum prices are shown below. Remember these are average costs and can vary with location. The table shows very clearly the wide variation in material costs.

Roofing materialMinimum cost per 100 sq. ftMaximum cost per 100 sq. ft
Aluminium sheet$890 $1,200
Aluminium shingles$600$1,500
Cedar shingles$650$900
Clay tile$2,000$5,000
Fibreglass shingles$40$200
Galvanised metal sheets$300$1,800
Roofing felt$40$60
Slate shingle$190$280
Synthetic slate shingles$50$150
Terracotta tiles$5$15
Zinc roofing$1,000$2,000

If you decide to repair or change your roof covering, then you will need to incorporate the hire or purchase of the following into your list of costs. Their actual cost varies greatly but in general is a minute fraction of the major roofing costs.

  • Hammer
  • Drill
  • Shingle fasteners & galvanised roofing nails
  • Sealants
  • Ventilation materials
  • Flashing
  • Roofing felt
  • Ladders
  • Scaffolding

You will need to have access to the part of your house furthest from the ground, so even if you have just a one storey house, you will need some way of climbing to the eaves, working from the eaves and walking up the roof to work on the ridge.


An extendible ladder allowing you to reach the height of the eaves and the peak of the gable end.

A roofing ladder. This is a ladder, the same length as the roof height, with a large semi-circular ‘hook’ on one end that hooks over the roof ridge. The ladder rests on the roof from ridge to eaves and allows access to the full height of the roof.


Although it is theoretically possible to work from a ladder, it is not very safe. You will need a walkway placed at a height so the eaves are at knee height. You can do this using scaffolding. Remember you can either have scaffolding platforms assembled by specialist contractors or you can hire self-assembly scaffold towers. Each has its own advantages and disadvantages and depends on the size of the roof needing access. Scaffolding has the added advantage that it is possible to store the roofing shingles in convenient locations along the scaffold boards.

Working at height is dangerous and the steeper the pitch, the more difficult it is to manoeuvre on the roof. Likewise if the roof covering is smooth like aluminium shingles, it can be difficult to keep your footing. Do not try this if you do not know what you are doing. Leave it to a professional.

Costings. Roof installation & repairs.

As stated earlier roofs can be expensive to install and difficult to repair if you don’t know what you are doing. We have seen a few of the various factors that can alter the cost of a roof.

The following table give average prices for various tasks associated with installation, repair and maintenance.

TaskLower end (average)Higher end (average)National average
Roof installation$3,000$12,000$6,500
Painting a metal roof$150$9,500$2,500
Fabricated sheet metal roof$800$4,000$2,100
Roof sealing$60$5,000$1,100
Roof repair$60$2,100$650
Install a heating element$100$2,000$600
Roof cleaning$60$800$400
Roof inspection$60$700$210

Safety on roofs

No matter what work you intend to do on a roof, you will always be working at height. Working at height means difficult access and injury if you fall.

If you have a choice between using scaffolding or a ladder, always choose scaffolding. Not only is it more stable to work from, but it is also more convenient.

If you have to use a ladder, make sure it is fixed and tied at the foot and at the head.

Always use a ladder at an angle of about 75 degrees (or 4ft of height to every 1ft horizontal distance from the wall to the ladder feet).

If you intend walking up a roof, use a roofing ladder or crawling board.

When handling, cutting or drilling any roofing material use protective gloves and safety spectacles.

When using a ladder, always have at least three points of contact and keep your tools in a tool belt or pouch.

To finish up

Today we have talked about the different types of roofing materials available, the different styles of roof and why each is used. We have talked about the factors influencing the cost of having roof repairs or replacement done. We have also looked at the costs of using different roofing materials.

If your roof develops a leak, get it fixed as soon as possible. If the roof deteriorates or the leaks become more frequent, it is time to consider replacing the roof with a new one. Have your roof inspected to try to get some idea of the extent of the job. Even so, if the leaks have been ongoing for some time or the roof is about 25 years old, be prepared when you remove the roof covering, to find that the roofing felt or rafters need work as well. Roof replacement and major repair is not an afternoon job so be prepared to use tarpaulins or similar to provide temporary covering of your house.

Your roof is one of the most important parts of your home and needs maintenance to preserve its weatherproofing, so if possible use a professional roofer or carpenter to do the job. If you are tempted to make it a DIY project, remember the following points:

  1. Roofing repairs need to be done as quickly and as smoothly as possible, so they are not really suitable for weekends only.
  2. Some roof repairs need specialist knowledge or tools.
  3. Roof work is ALWAYS done at height.
  4. Working on the roof can be dangerous.
  5. It is always better to work with a friend who can help you lift heavy or awkward materials and who can help if you find yourself stranded on a roof.

Good luck.

Let us know what you think of the information provided in this article.