How to Design Exposed Timber Beams in Modern Extensions (UK Guide)

Exposed timber beams - whether within traditional old buildings in Oak or modern buildings as glulam - provide a fantastic opportunity to give character and personality to your interior space.

In a recent project in Lewisham, London, we used exposed timber beams to create a contemporary yet tactile interior.

In many ways this type of ceiling is very cost-effective, especially as it is very similar to a normal roof in all but a few ways. In this guide we'll walk you through the benefits of exposed timber beam ceilings, how to get the most out of them from a design standpoint and how to mitigate any increase in cost when specifying one on your project.

Sean Payne

Director / Architect

Why Choose Exposed Timber Beams?

First it's good to ask - In what cases would you actually want an exposed timer beam ceiling in your house or extension?

The great thing about this type of ceiling is that it can be used in whole rooms, whole buildings or even mixed in with traditional plasterboard ceilings.

The practical benefits to this type of ceiling and roof include:

- Adds extra depth to any ceiling.
- is a simpler roof type to maintain in the long run.
- A simpler roof type to spot problems with in the long run.
- Will typically be more energy efficient with less thermal breaks.

The aesthetic benefits include:

- Offsetting minimal interior spaces with texture for a more 'Scandinavian' feel.
- provides a 'barnhouse' or 'stables' feel for more traditional interiors.

With all this in mind exposed timber beam ceilings provide a lot of benefits, so long as you consider some important things we'll cover later in this guide.

Types of Timber Beams Used

Typically for a standard flat or pitched roof construction in the UK, structural engineers will specify C24 sawn timber.

As long as fire protection under building regulations is provided - there's nothing stopping the use of standard C24 timber beams from being exposed and used in this type of design feature. However the main issue comes down to the finish.

As a minimum - the timber beams should be at the very least a planed joinery grade softwood while also providing the structural capabilities for the floor or roof it's holding up. On previous projects - having the beams painted is also a good way of ensuring a premium finish with standard timber products.

This being said, other common alternatives will typically include.

- Glulam (Glue-laminated timber) - This is the most common exposed timber in modern extensions, given it's strong, stable, long spans, refined look and fire resistance. This is typically the easiest solution aesthetically and for building regulations compliance, but will be slightly more expensive.

- Solid oak beams - these provide a more traditional aesthetic, but can often form movement cracks due to their expansion and contraction, this was something that had to be considered on the Chorley Barn Conversion project that was finished last year.

Other hardwood solid beams provide their own pros and cons, generally these will be most costly than standard timber roof beams.

- Engineered timber (LVL) - Like standard C24 timber, has a particular aesthetic that needs to be managed if exposed, but can be utilized if good design principles are used to make for a successful interior.

Each of these options can be used, it mainly comes down to budget, the design vision and making the most of the pros while mitigating the cons of any one product. Sourcing of the products is also a key consideration.

Building Regulations For Exposed Timber Beams

When it comes to exposed timber beams, there are two main considerations when it comes to UK building regulations, those being:

- Part A (Structure)

This relates to the exact size and grade of the timber, in almost every case it should be specified and confirmed by a structural engineer.

- Part B (Fire)

With the timber beams being exposed, fire resistance plays a more active part in the compliance with building regulations, there are typically a 2 main solutions to this:

- Oversizing timber for charring allowance - This is the simplest way of ensuring the timber structure is fire resistant for the required amount of time, effectively accounting for how much the fire will char the outer layers of the timber in the case of a fire while retaining it's structural integrity.

- Intumescent coatings - these are coatings that expand and provide a layer of fire protection to the timber.

Other items such as thermal performance (part-L of building regulations) is relatively simple with exposed timber beam ceilings as the insulation will almost always run continuously above the beams. This actually makes the ceiling and roof much easier to maintain, and avoids thermal bridging at beam penetrations.

Common Design Approaches

When it comes to design approaches and when to use exposed timber beams, we'll usually find a successful design falls into one of the following categories.

Full Exposed beam runs in a room - This is the simplest option, providing a clear consistent ceiling type with interruptions, this is particularly effective with a simple ceiling or roof area where the structural spans are under 4 metres.

Clear exposed sections within a split - This approach will normally be favourable where there's a more complex structure and when working from an existing building into a new extension.

This requires more thought but allows for some clever interaction between the old-building and the new extension.

Feature beams - Feature beams can be a great way to accent the interior in a more subtle way. This is normally most effective as a transition piece between room uses, especially in open plan areas.

Vaulted ceilings - Vaulted ceilings with beams can further accentuate and dramatize the feeling of height and openness within a tall room. This can be best seen within the Kent Country House Extension we completed a few years ago.

Each approach has pros and cons - which again - should be considered and worked with in order to get the most out the feature for your interior.

In the recent House extension project in Lewisham, London, we used Clear exposed timber sections that was split up with a more traditional plasterboard ceiling, using the interaction between the two to set clarity between key rooms.

Case Study - Lewisham Rear Extension

Let's look at the Lewisham rear house extension in a little more detail to see how an expose timber beam ceiling was chosen and use to fulfill the brief.

Brief - One of the key items for the client's brief was to retain and use a tall ceiling within the existing house, the client was also very keen to create a bright ceiling space with a clear distinction between the kitchen, dining area and living area inside an open plan room.

This being the case - a mix of the existing more traditional ceiling was used in areas where the existing building continued upwards onto the first floor. The ceiling then transitioned to an exposed timber ceiling as it crossed into the new extension.

This created a clear distinction between old and new - even in the interior. This also lined about to provide:
- Traditional ceiling over the kitchen.
- Fully exposed timber beams over the dining area.
- partially exposed timber beams over the living area.

The ceiling in it's entirety shows the construction logic of the building, providing a clear level of clarity throughout the interior. Finally, the rooflights provide natural light that plays with a feeling of infinity above the timber joists and being framed in other areas.

This all comes together to take advantage of the pros of exposed timber ceilings, namely:
- it provides a consistent material palette on the interior.
- It celebrates and displays the construction of the extension in a positive aesthetic way.
- It emphasizes a clear visual distinction between the different rooms in an open plan space.

Steel vs Timber Beams

Often on projects - whether it's internally or with clients - there will be a discussion about whether:

- Timber beams should be exposed
- Steel beams should be exposed (if there's steel beams)
- Both or neither should be exposed.

When deciding if timber beams should be exposed comes down to taste, the design theme and whether the feature marries with the site and existing building (if applicable).

Steel beams can often be exposed as a design feature - normally as more of a 'feature beam' rather than anything else. However - much like with standard timber beams - This will normally mean fire resistance is a key factor to solve, normally we find that steel beams require more thought in order to get a good design finish when compared to exposed timber as well, given that they often need to do more structurally on a project.

You can see an example of exposed steel beams as a design feature on our Seaside House Project.

Exposing both timber beams and steel beams normally provides a level of consistency on an interior space, but this does force the interior down a very specific aesthetic theme, normally something more industrial / modern industrial.

Concealing everything is definitely the more standard route and often that can be for a good reason, while sometimes this might be an ease of construction of budget constraint, some spaces benefit from having a more toned down ceiling space so other items within the interior and pop out - as mentioned throughout the guide - it's all about finding the right solution for each specific project and design as a whole.

Exposed Timber Beam Ceilings on Flat Roofs Vs. Pitched Roofs

Having designed projects with exposed timber beam ceilings on flat roof and on pitched roofs, it's important to note some of the differences and potential challenges you may face with either type.

Flat roofs - much like with the Lewisham house extension and another extension in Bexleyheath (drawings immediately above) the key is ensuring that a slight roof fall can still be accommodated on the roof while having the exposed beams still appear flat on the interior.

This can typically be achieved with furring pieces in the roof construction buildup, or by accommodating the fall in a way that compliments the interior and it's layout.

For Pitched roofs - much like the Kent countryside house extension and a new stone bungalow we're designing in Bradford (drawings below) the key is ensuring items such as cross battens for roof tiles at the top can be attached securely.

In both cases - the finish in between the exposed timber beams on the ceiling and the beams into the wall is equally crucial. Sheathing board can be painted or treated in order to provide a clean finish or plasterboard can be fitted between the beams with a skim and white paint to finish. For the point where the beams actually meet the wall typically there will be a few options.

1. A high quality joist hanger (such as those provided by Rothblaas) can provide a high quality modern or traditional industrial look or a minimal look with their hidden slot hanger range.

2. The joists can rest on the wall itself - in these cases the finish of the wall (for example plasterboard) need to be considered carefully to ensure a consistent high quality finish.

How to Design Exposed Timber Beams in Modern Extensions (UK Guide)

Why Choose Exposed Timber Beams?

Types of Timber Beams Used

Building Regulations For Exposed Timber Beams

Common Design Approaches

Case Study - Lewisham Rear Extension

Steel vs Timber Beams

Exposed Timber Beam Ceilings on Flat Roofs Vs. Pitched Roofs

Costs & Considerations

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