Internal flush doors

A flush door usually has smooth, flat faces that appear to be formed from a single slab of material. Some modern designs of flush door have faces which are more unusual, such as faces with decorative inlays or panel effects, giving you a range of different options. Flush doors are most commonly used as internal doors, but some designs, with additional features, can be used as external doors. Flush fire doors can be used for doorways which lead directly from a house to a garage, but are usually classed as external doors.

For more information see: Fire doors

Although they appear to be made from a single slab of material, flush doors are actually constructed with a frame and veneer faces, usually containing a core material. The specifics of the construction and materials used to make them will depend on individual doors, and will affect their characteristics.

They provide a lot of choice of finishing options on their faces. Not only can they usually be painted, stained, or varnished: many other decorative features can easily be added, such as decorative veneers, laminated papers, inlays, and moulded or carved faces.

Flush doors can have their properties upgraded in various ways with modifications to the faces and core. They can be fire resistant, insulated, strengthened, or given sound reducing properties. If they have the right internal construction, they can also have glazed panels added to let light through, although this will usually be done during manufacture.

How is an internal flush door constructed?

Flush door faces 

The faces of flush doors are covered with a face veneer, or door skin, and that is the surface visible on the door's faces. Some flush doors will only have a single veneer layer, which would be the face veneer. Other flush doors have one or more additional veneers, called core veneers, or crossbands. 

Face veneers can be made from a huge choice of materials with many options for finishes, and will usually be made of a more expensive material than the core veneers or cross bands. Not only can they be moulded or carved for a surface pattern, they can be finished in various ways, such as painted, varnished, stained, or laminated.

The core veneers and cross bands add strength to the structure of the door, but not all internal flush doors have them. Special core veneers can be used to enhance the properties of a door, such as soundproofing and fire resistance, along with the core material.

Flush door frames 

Underneath the face of a flush door, there is a simple frame. This sets the basic shape and size of the door. Inexpensive woods are usually used for the frames because they are covered with the door faces. Wood frames can be easily reinforced with additions such as metal strips, although this makes the door heavier and is not usually necessary for internal doors. 

Extra support pieces are often added to the frame, for installing door hardware. For internal flush doors, this will usually include pieces for the hinges, and a "lock block" for the door handle. If the door has a solid core material they may not need these extra pieces. The door will usually be marked to indicate which side the handle should be installed on.

Edging strips are usually used to cover the visible parts of the frame: this is called the lipping. Not all doors have lipping - some have the frame exposed, which means it will need another finish. Lipping will usually match the face veneer, giving the finished door the appearance of being a solid slab.

Flush door core 

The core material used in a flush door may not be visible, but it will significantly affect the door's properties, particularly the weight of the door. The two main types of flush doors are either hollow-core or solid-core. Some hollow-core doors are completely hollow, but most have some supportive material to increase the door's stability. 

Occasionally, additional frame pieces are added between the door's faces for hollow-core doors, but usually, they are filled with a honeycomb structure. Hollow-core doors are a popular choice for internal doors as they are inexpensive and lightweight, which reduces the risk of the door sagging. But, they can be easily damaged if their faces are not tough enough.

Solid-core flush doors have a heavier and more dense core material. They usually provide a more substantial but more expensive door. There are various options for solid-core materials, depending on the properties. As internal doors don't need to face the same challenges as external doors, you have more flexibility with what properties you choose.

Popular solid-core choices for internal flush doors include various engineered woods, like chipboard, and lightweight materials, such as polystyrene.

What can internal flush doors be made of?

Flush doors are very versatile, and can be used with any of the basic operating mechanisms; they can be hinged, sliding, or folding doors. The array of different materials which can be used to make them means that there will be a flush door to suit most needs. Individual doors will be made up of different materials, check the specifications to see exactly what materials have been used for a particular door. 

The frames for flush door leaves are usually an inexpensive timber. They can be strengthened with metal, but this is rare for internal doors. The materials that really affect the properties of a door are those used to construct the face and the core of the door slab. 

Modern flush doors can also be enhanced with various features such as the addition of glazing panels. Trim can be fixed to flush faces, or moulded or carved faces can be added to achieve a look resembling panel doors, for a more traditional look, or other designs, for a more unusual design.  

What can internal flush door faces be made of? 

The face veneer, or door skin, is the part of the door's face you see, so this will affect the overall look of the door, and what finishes can be applied to it. Any core veneers and cross bands will be there to improve the strength and structure of the door. The face veneers will usually be made from more expensive, and more attractive, materials than the core veneers or cross bands.

Fibreboards

Fibreboards are engineered wood products commonly used for core veneers of a door, and occasionally as face veneers. They are usually classed by their density on a scale from medium-density fibreboard (MDF), to high-density fibreboard (HDF), although these definitions are not clearly defined by some manufacturers. 

Fibreboard is produced by breaking down wood and similar materials into fibres, mixing them with wax and resin, and pressing them into panels. It will have a more uniform surface than natural wood, as the panels will not have any knots, rings, or grains, giving it a consistent strength throughout the panel and reducing their tendency to warp.

MDF and HDF

MDF and HDF are fibreboards similar to chipboard, but denser and usually thinner. HDF is denser than MDF, making it stronger but more prone to warping, and usually thinner than MDF, but otherwise very similar to each other. They can both be specially treated to improve properties such as fire or moisture resistance.

The advantages and disadvantages of MDF and HDF

ADVANTAGES

DISADVANTAGES

• Usually cheaper than plywood or natural wood

• Less tendency to expand or contract than natural woods or hardboard

• Easy to paint, varnish, or attach a veneer or laminate to

• Have some flexibility for use on curved surfaces, particularly MDF

• Doesn't split easily

• Relatively strong and durable

• Can be cut or shaped in various ways to produce moulded or carved faces

• Usually, has enhanced performance 

• More sustainable than natural wood

• Usually heavier than plywood or natural wood

• If not sealed properly on all sides it will soak up any moisture and can swell, causing permanent damage

• Will shrink in environments with low humidity

• Can't be stained

• Doesn't hold screws as well as plywood or solid wood. Fixing in screws can cause it to split

• It's not very attractive unless it has a quality finish applied

• Contains urea-formaldehyde which can be dangerous when sanding or cutting and small amounts will leak out over time unless they're finished correctly

Hardboard

Hardboard is often confused with HDF, but it is actually processed in a slightly different way. It's compressed fibres are heated so the natural glue within the wood acts as the primary binder. It's usually produced in thin sheets which typically have one smooth face and one textured. 

It usually comes in flat, smooth sheet form, but it can be produced with various designs to produce moulded or carved faces. It's denser than other fibreboards, although it can be similar to the density of some HDFs. This makes it strong but not as tough. Tempered hardboard is treated with oil for water resistance and added strength. 

The advantages and disadvantages of hardboard

ADVANTAGES

DISADVANTAGES

• Very dense 

• Can be tempered with oil to make it resistant to denting and scratches

• Cheaper than plywood and natural wood

• Can easily be painted, varnished, laminated, or covered with a natural wood veneer

• More sustainable than natural wood

• Usually only produced in thin sheets, which can make it more prone to warping or breaking

• Relatively heavy so it can be difficult to handle

• More prone to warping than other fibreboards

• Will absorb water unless it's been specially treated

Plywood

Plywood is produced by gluing thin layers, or plies, of wood together with the grain direction being rotated up to 90 degrees for adjacent layers. The alternating of the grains, called cross-graining, increases its strength and makes it more consistent in all directions.   

Plywood is less prone to contraction or expansion than natural woods. It's available in different grades and thicknesses, with lower grades usually being used where it won't be seen, such as construction bases like subfloors. High-grade plywood is more attractive and smoother, often consisting of five thin layers, with the grain of each layer set at 45-degrees to the last. 

The advantages and disadvantages of plywood

ADVANTAGES

DISADVANTAGES

• Very strong throughout because each layer has its grain running in a different direction

• Not as susceptible to water damage as fibreboards. It will soak up water, but much slower

• You can stain plywood

• Holds screws tightly when fitted through the face

• For its strength, it's relatively cheap compared to other materials

• Resistant to warping and shrinking

• Usually, has enhanced performance 

• Usually more expensive than most fibreboards

• The layers will be visible on the edges of boards unless they are finished properly

• Difficult to cut with a smooth edge, so it often splinters

• The layers of low-grade plywood can peel away from each other

• The adhesives used in plywood need to be the correct type for the application, like being water resistant for bathroom doors

Natural wood

The most expensive wood products are usually solid natural wood, although this will depend on the thickness and quality of the wood being used. Face veneers are often made of natural wood, and attached to cheaper face materials, because of its attractive and traditional appearance.  

There is a huge range of different woods which can be used for face veneers, depending on the finish you want. Natural woods come in a variety of colours with different grain patterns. The appearance of the face will also be affected by how the wood is cut and joined.

Natural woods need to be treated for protection. They can be painted, stained, or varnished, depending on the finish that's required. These coatings need reapplying occasionally to maintain the wood or it will deteriorate. The quality of the adhesive used to attach the veneer will have a significant effect on the quality of the door and how durable the veneer will be. 

Natural woods are either hardwood or softwood. Hardwoods are generally harder than softwoods, but not always. A major difference between softwood and hardwood is that softwood trees grow much quicker and are therefore cheaper to produce. Softwoods come from evergreens and conifers. They are usually lighter in colour and have straighter grains than hardwoods. 

The advantages and disadvantages of natural wood 

ADVANTAGES

DISADVANTAGES

• Ideal for veneers - looks attractive

• Offers lots of options for finished looks, with differences in grain patterns, colours, and finishes

• Expensive natural wood veneers can easily be applied to cheaper face materials

• Can be stained, varnished, or painted

• Can be expensive

• Requires more maintenance than other face veneers

• Can be difficult to repair if damaged

• Can be prone to warping

• Can peel off if an appropriate adhesive isn't used

What can internal flush door cores be made of?

The core material of a flush door will have a significant effect on its properties and its weight. Flush doors are classed as either hollow-core or solid-core. Most internal flush doors are hollow-core as they don't need to be particularly durable.

Most hollow-core doors actually do have something which is used to maintain the structure of the door leaf. Some have additional pieces added to the frame, usually running horizontally across the door. Most hollow-core doors have a honeycomb structure within them.

The honeycomb is usually made of paper or card which is reinforced with resin and heat treatment, making it very hard and rigid. It supports the door's faces and offers a small amount of sound reduction while keeping the door's weight down. This reduces the risk of the door sagging in its frame, and should mean the supports of the door last longer.

Hollow-core flush doors

The downside to hollow core doors is they can be easily damaged if their veneers are not tough enough. Dents or holes in the hollow-core cannot be fixed easily, often resulting in the door needing to be replaced. Resizing or refurbishing an old hollow-core door can be difficult. They are limited to how much can actually be trimmed for resizing, usually to just a few millimetres. 

The advantages and disadvantages of hollow-core internal flush doors

ADVANTAGES

DISADVANTAGES

• The door should be light enough to be fitted by one person

• Inexpensive

• They fulfil basic privacy requirements

• Can be used with any face material

• They're difficult to repair or refurbish

• They can be easily damaged

• They offer little or no fire resistance

• They have limited thermal insulation and sound reduction

Solid-core flush doors

Solid-core flush doors have the same basic construction as hollow-core doors, but, they have a heavier and more dense core material. They provide a more substantial door which will usually be more expensive than hollow-core doors. There are various options for solid-core materials - engineered woods are commonly used, as well as other materials which can improve certain properties. 

The swing of a solid-core door will feel different - some people prefer the way a solid-core door feels. They should last longer than hollow-core doors and will be able to stand up to heavy use. They provide much more sound reduction and insulation than hollow-core doors. They are much heavier than hollow-core doors, which makes them harder to install and more at risk of sagging on their frame.

Blockboard

Blockboard, also known as lumber core or laminated timber, is and engineered wood often used as a substitute to plywood when boards need to be thicker than 12mm. This is because it’s relatively cheap and easy to produce in thick slabs, and it produces fairly strong panels. 

Blockboard consists of strips of natural wood laid parallel with each other and sandwiched between veneer faces, plastic laminate, or melamine paper. The solid wood strips are glued together to produce a stable slab core which is cheaper, but more resistant to warping, than solid natural wood. 

The advantages and disadvantages of blockboard cores

ADVANTAGES

DISADVANTAGES

• Less expensive than solid wood 

• Can be worked in the same way as natural wood

• Consistent strength throughout and resistant to splitting and warping

• Lighter than chipboard

• Usually, has enhanced performance 

• Can absorb moisture if not sealed properly

• Can be quite heavy

• Usually more expensive than chipboard, but not as dense

• Not as strong as the wood used to construct it

Chipboard

Chipboard, also known as particleboard, is an engineered wood made from tiny particles of wood bonded with an adhesive. It is classed as another type of fibreboard (like MDF). No grain is produced, so chipboards are equally strong along all dimensions, although they're usually weaker and less dense than MDF.  

Chipboard comes in different densities and is commonly used for things like furniture and kitchen worktops as it's relatively inexpensive. Chipboard doors need to be properly finished and sealed on all sides to prevent moisture absorption; if they get wet they will swell.

Flaxboard

Flaxboard is sometimes used as a door core material as an alternative to chipboard, and it has many similar properties. It contains at least 70% chipped flax stalk, which makes it naturally resistant to fire. Fire doors can be made with a flaxboard core, where chipboard would not be suitable.

The advantages and disadvantages of chipboard cores 

ADVANTAGES

DISADVANTAGES

• Relatively cheap - cheaper than blockboard

• Can be treated to improve properties such as moisture resistance

• Flaxboard is naturally fire resistant

• Not as strong as blockboard 

• Can crack and fall apart over time

• If chipboard gets wet it will swell and is likely to be permanently damaged 

• Can be heavy

• Normal chipboard is not fire resistant

Oriented strand board (OSB)

Another common type of engineered wood is OSB, made from strands of wood fixed with an adhesive and pressed into boards. It's quite distinctive as the different strips remain visible on the surface. The strips of wood are oriented in specific ways to increase its strength. 

OSB is very strong and dense, but this means it can be heavy to work with. It is usually treated to resist water, otherwise it is prone to swelling. It can't be cut cleanly; if it needs to be trimmed the edge will be rough. As it's produced in sheet form, it's possible to use OSB as a face material for a door.

The advantages and disadvantages of OSB cores

ADVANTAGES

DISADVANTAGES

• Strong and durable

• Can be treated to use in external applications

• Relatively inexpensive

• Can be heavy due to its density

• Can be prone to swelling and splitting if exposed to moisture

Polystyrene

Polystyrene is a very lightweight material, which will keep the weight of the door down, giving a more similar feel to hollow-core doors. It doesn't provide the structural support engineered woods do. It's a common type of plastic available in different densities. The denser the polystyrene, the more expensive it will get, but the denser it is the better sound reduction and insulation properties it will have. 

The advantages and disadvantages of polystyrene cores

ADVANTAGES

DISADVANTAGES

• Relatively inexpensive

• Very lightweight

• Good insulation properties

• Can melt if it reaches very high temperatures 

• Offers no structural support