What is a ridge beam?

Within a common pitched roof assembly, the ridge beam is a timber that runs down the centre of the span of the roof, supporting the rafters at the apex of the structure. This timber provides lateral stability to the rafters during the construction process. The ridge is typically wide enough to receive the full width of the rafter plumb cut, though the exact width is often specified on the drawings. For regular residential roofs, the ridge board is typically a single 8”/9” timber. In roofs with collars or joists, this ridge is supported within the assembly by the rafters. In roofs that feature a vaulted ceiling with no collars or joists, the roles are reversed, with the ridge beam supporting the rafters at their intersection. In order to achieve this, the ridge beam is built into a gable or bulkhead at each end of the vaulted ceiling, supported by a padstone or an engineered post. In these more heavy duty scenarios, the specific composition of the ridge beam is likely to change. A double or triple bolted timber beam, possibly with an integrated flitch beam, or even a full RSJ beam being commonly used for these more structural applications.  

What are the criteria of a ridge beam?

The ridge beam must be installed level within the roof assembly. The top of the rafters must meet the top edge of the ridge beam. If all of the rafters are cut to the same length, then the level of the ridge will follow the level of the plates. It is therefore imperative to establish perfectly level wall plates before the erection of the roof. As the rafters are installed in pairs against the ridge beam, it's important to ensure that the beam remains straight down its length, with no sideways deviations. This could cause a hump or hollow in the roof.   

How are ridge beams installed?

Traditionally, smaller 4” rafters received into thin 1” thick ridge board. In modern construction, the thickness of the ridge board has been increased to a common 2” thickness. By far the most common intersection in both modern and traditional roofing sees the top of the plumb cut of the rafter meeting the top edge of the ridge timber. This intersection allows for the roof battens to reach the very top of the ridge of the roof. Where a wider ridge beam is present for use in structural vaulted ceilings, a different type of plumb cut intersection is required. If the top edge of the rafters were to meet a wide ridge, a flat spot would then be present at the top of the roof. This creates difficulty for the roof tilers, presenting a lack of fixing surface for the top batten to receive the tiles. In this instance, a non structural birdsmouth is cut at the top of the rafter to receive over the wide beam and provide additional timber fixings above the ridge beam. The rafters then meet each other at the very apex of the roof, sitting on top of the ridge. This drops the ridge down a little in relation to the height of the rafters. 

For gable end roofs that feature an overhanging gable ladder, the ridge may extend past the external skin to provide stability to the gable ladder. The portion that projects outside of the building may have to be ripped down so that it does not interfere with the installation of the plastic capping after the fact. The ridge may be left long until the ladder is installed, with the excess material being cut off afterwards. 

In roofs that feature a hip end, the lateral position of the ridge terminates at a very specific geometric location. In order for the effective installation of the hip rafters to occur, in combination with the same desired pitch continuing around the corner of the hip, the crown rafter at the end of the roof must meet the ridge perfectly. For a standard hip end consisting of 2” timbers, the crown rafter is located directly in the centre of the span of the roof, as shown in the diagram. The two last common rafters are located down the plates at half the span less half the thickness of the crown rafter in from the end of the wall plates. In a modern 2” setting, this location is equidistant for the last common rafters and the crown. This position of the outside of the last common rafter is also the location of the end of the ridge in the hip end assembly. When the rafters are installed, the crown rafter will perfectly meet the end of the ridge at the correct angle, creating an ideal setup for the installation of the hip.  

In a scenario as can be seen in the drawing, we have multiple tiers of a hipped roof that all marry into one another. Visually, the outside hip of the larger portion of the roof comes down to meet the ridge of the lower portion of the roof. In reality, the end of the ridge is fixed into the side of the larger hip. During the construction process, the larger roof is typically pitched first in its entirety. With the larger hip in place, the small roof can begin to be pitched. As the rafters are pitched the ridge can be installed between them, receiving into the side of the hip on a 45-degree splay cut. This provides lateral stability to the ridge as it is fixed to a stable point within the roof structure. From this point, a couple of options are available. Once the ridge is fixed, the excess hip on the outside of the ridge can be cut away, and the common rafters can be filled in for both roofs. This is the easier option as less jack cuts are required, and a simple reinforcing post can be installed under the ridge/hip intersection. Depending on the scenario though and size of the roof, this larger hip may be a very important structural member. If this is the case, the hip may remain in place, with more jack and cripple cuts being made to fill in the portion of the lower roof. This requires more work.  

Take a 4 hipped roof for example. We’re constructing an extension on the back end of this building. The new roof ties into the existing hipped roof with 2 different height hip ends. The specific height of these roofs is predetermined by the pitch of the existing roof in combination with the span of the new wall plates. We can separate this new construction into 2 separate roofs, each coming into the larger portion. In the process of erecting this roof, we’re trying to leave the existing roof covered in for as long as possible to prevent the weather from driving into the existing furnished house. In this scenario, we can see the short ridge coming off of the existing hip and terminating in a new hip and a cropped hip. This ridge is relatively short, and exists almost exclusively over the profile of the existing roof. The question we’re looking to answer here is, how are we going to install this short ridge in the correct place with the existing roof still in place?  

The answer lies in the principles of a properly installed ridge board. Within the larger span of the new plates, there are no common rafters, as all of the components are that of a hip end and valley. As such, the short ridge does not extend over the new construction. The first step in the success of this installation is to do our best to ensure that the long wall plate is running straight with the existing wall plates. This can be hard to check with the existing roof still in place, but provided that the new masonry structure is running in line with the existing building then the wall plates that are bedded on top should be in line with the existing wall plates. If the long wall plate is in line with the existing plate, and the new plate opposite this one is parallel, then 2 identical pairs of rafters cut for this span will help us install the ridge correctly. Temporarily pitching these pairs of rafters, a long ridge can be installed between them.

The end of the ridge will either land on top of an existing jack rafter, or between the jacks and directly into the edge of the existing hip. An approximate test hole must be opened up in the felt of the existing roof to determine the desired cut on the end of the ridge, either a 45 splay, or an angled cut at the pitch of the roof. The ridge can then be installed between the pitched rafters and pushed into the existing roof. Pinched between these identical rafters, the long ridge is forced into place in the centre of the larger span, at the correct height for the span. Provided that the ridge timber itself is straight, the cut end of the ridge will intersect with the existing hip at the correct location. This intersection can then be fixed in place.  

A crown rafter can then be pitched off of the plate next to the long ridge to signal where the ridge will be cut off, and where the hip end intersection begins. If the centre of the larger span is off of the plates, and within the smaller span, then the crown rafter will not land on the wall plate. In its final location, it will be a cripple rafter, receiving into the valley that will be installed later. Ignoring this momentarily, we can set up a temporary timber to create a bearing for a full crown rafter to establish the length of the ridge. A block can be screwed to the underside of a joist. The top of this block is the same level as the top of the wall plate. The joist is placed in line with the back of the end plate and square up. The plumb cut of the birdsmouth of the crown is cut all the way up, to create a short rafter with no tail. This can then be placed in the emulated crown position to establish the length of the ridge.  

How are ridge boards joined for length?

When roof assemblies are longer than a single length of a timber ridge board, multiple lengths of timber must be joined to create a long ridge board. There are a handful of common joints that are used to join lengths of ridge boards. Here we will discuss the most common, highlighting their strengths and weaknesses, and how they are laid out.  

Simple scarf joint

This joint is among the simplest to cut and is suitable for use in most lightweight applications. The joint features two even parts that fit together one on top of the other. Whilst this joint type is easy to cut, it possesses no lateral force resistance, relying solely on mechanical fixings for its lateral restraint. Under extreme forces, the joint could pull apart. There are a handful of varieties of layouts for this joint.  

Typically, the length of the total joint from hard corner to corner is approximately 3 times the width of the boards in order to provide adequate strength to the timbers. It could also be an angle at a ratio of approximately 1:10. It really depends on where you go and who you are asking. To set this joint out, mark in 3 times the width of the board distance in from the end of the board. Mark up and down approximately 2 inches at each end respectively to form a decent thickness for the joints thinnest end. We don't want the splice to come to nothing, as there is no strength in it, so we need at least some timber thickness to provide stability to the thinnest part of the joint. From the marks, we can draw a connecting line to create the exact angle to cut to. The remaining marks can be left square, perpendicular to the diagonal line, or returning from these points at the same angle as the angled line. With one end cut, it can be laid over the next board to accurately mark a matching piece for the joint. Ensure that the joints are cut in the correct orientation to ensure that both lengths are crowned upwards when joined together.  

In traditional post and beam construction, this joint is used to connect beams directly above supporting posts. In these instances, it's common for the entire assembly to be connected through the use of dowels through the joint and into a tenon cut on the post. This provided lateral restraint to the beams as well as securely fastening them to the post.  

Wedged scarf joint

The wedged scarf joint is a variation of the simple scarf joint. This joint is more suitable for use in applications where mechanical lateral restraint is required. The two faces of the joint are staggered so that they lock into one another. A space in the middle between the two pieces is left so that a pair of timber wedges can be installed to lock the joint in place. With the joint assembled a large amount of lateral restraint is created. Similarly, this joint is also used in traditional post and beam construction for use in joining longer beams.  

Dovetail joint

The dovetail joint is less common in modern construction, though by all accounts was a common form of joining ridges during the later half of the 20th century. This type of joint also features lateral restraint in its design, with a dovetail shape receiving tightly into another board with a matching receptacle. The ratio of the dovetail itself is somewhere around a 1:7-1:8 pitch. The length of the dovetail itself is determined by the layout criteria in combination with the overall width of the boards. The thinnest part of the dovetail is centred on the end of the board, and is approximately half the width of the total boards width. The dovetail then grows outwards at an appropriate pitch until around 2 inches of material is left at the top and bottom of the board. Any thinner and the material may split out. Once the dovetail is cut, it can be laid over the other timber and traced to mark out an accurate receptacle for the dovetail.  

Half lap joint

The half lap is a very easy form of joining ridge timbers, and is perfectly suitable in most applications, though it possesses none of its own lateral restraint. The total joint is 1-1.5x the width of the board, and features a matching pair of half laps that sits atop one another. To keep the joint together, nails or structural screws are skewed through the timbers. Additionally, nail plates or other types of mechanical ironmongery can be installed over the joint. The advantage of this style of joint is that the secondary ridge can be rested on the first ridge piece that is pitched to aid in pitching the second ridge.  

Notes on wide ridges

In some instances, large structural beams are implemented as ridge boards to support vaulted ceilings and other related applications. These ridges may be wide steels, engineered timber beams, or site bolted timber beams. In any scenario, they're likely to be wider than a single timbers thickness. If the rafters are installed to meet the top edge as with a single timber ridge, a wide flat spot is created at the apex of the roof. For most roof covering solutions, especially battening and sheathing, this flat spot creates an area without adequate fixings. As such, special considerations must be made in these instances. If the flat spot is created by a non shapeable material such as a steel, an angled fillet piece can be installed on top of the steel to take the pitch of the roof all the way to the apex. Alternatively, the steel can be lowered in the structure slightly to allow for a non structural birdmouth to be cut at the top of the rafters and receive over the steel. This birdsmouth provides additional material above the steel that meet together at the apex of the roof. The exact proportion of the cutout is irrelevant, as the main stability of the rafter against the ridge is provided by the typical plumb cut, and so the birdsmouth purely provides additional fixing material.  

If the material can be shaped such as with timber beams, the appropriate dual pitch can be cut on the top of the beam to accommodate the apex meeting in the roof. Technically speaking, this is the case with any ridge, as geometrically the planes of the roof should be at a point in the centre. That being said, for single thickness timber ridges, the additional work to cut this backing angle is wasted due to the typical installation of the top battens lower on the roof.