Movement in Masonry: Factors affecting movement joints

What to consider when specifying movement joints.

Poorly detailed masonry can cause problems to a building’s appearance, function and longevity.  In this blog we take a look at what you need to consider to get the specification of movement joints right.

There are lots of factors that cause masonry to move including moisture, changes in temperature, structural loading, subsidence, environmental and chemical influences. Clay brick and aerated and aggregate blocks are particularly influenced by moisture and thermal movement, which could become more of an issue as our weather gets wetter with more extreme temperatures.

In addition, some materials move more than others and some, such as clay bricks, tend to expand over time, while others, such as blocks, tend to contract.  To avoid future problems, you need to account for this and use both horizontal and vertical movement joints.

Here’s what you need to consider …

How many do I need?

The first thing to consider on a building is the maximum length of uninterrupted brick or blockwork around the perimeter for vertical movement joints and the height and/or number of storeys for horizontal ones.

For vertical movement joints PD9967 recommends that for clay brickwork you need one every 10-12 metres and also install them not more than half this spacing from a corner on a building.

Looking upwards on a building, the spacing for horizontal movement joints in the outer leaf clay brickwork should be at every third storey or 9 metres, whichever is less. But if your building is no taller than 12 metres and only 4 storeys tall then you might not need one. For timber or light weight steel framed buildings you will need to design the brickwork as an off the frame single skin up to 9 metres. Above this height you will need to talk to the brick manufacturer as specification becomes more crucial.

The vertical movement joint spacing for blockwork is different. Aerated blocks have them installed every 4.5 to 6 metres, medium dense blocks every 6 metres and dense aggregate blocks every 9 metres.

So far so good, but when you are allowing for movement, there are other factors that you need to be aware of.

Height difference

The shape and proportions of masonry will affect how it moves, which means that if you have two attached structures with different heights then you will need to introduce a movement joint between them to reduce potential differential movement.

What about doors and windows?

And you may think that having doors and windows means that you need fewer movement joints.  In fact, it’s even more important that you include them because uninterrupted lengths of brickwork will move more than the short tall piers between the openings, which means there will be more stress points.

For windows that are spaced far apart you should maintain the spacings for the movement joints and position them between the openings. Where this is not possible you may be able to introduce a vertical movement joint at the window jamb, but I would strongly recommend that you talk to your brick manufacturer about this.

North vs south

Believe it or not even the direction that an elevation is facing can affect your specification. Those facing south will have greater thermal movement and may need more joints.  In contrast north facing elevations will generally be wetter and have lower temperatures so will take longer to dry and therefore move in response to moisture.

Restrained or unrestrained?

You also need to think about areas of rigidity in masonry, such as where there is an internal or external return. If these returns are less than 675mm then you may need extra movement joints located at this detail to reduce stress and avoid cracking.

Freestanding walls and material differences

Lightly or unrestrained masonry, such as parapets or freestanding walls, are freer to move, tend to have stronger mortar mixes and are more exposed to the weather. You may also use different materials above and below the damp proof course and for the coping at the top, all of which move differently over time.

This means that you need to use more movement joints.  You should space the joints no more than 6 metres apart and not more than 3 metres from a corner in the main wall.  You should also space them at no more than 3 metre centres in the masonry coping.

Even in a building you may construct the ground floor from brick and then the second floor in rendered blockwork.  One expands over time, while the other contracts, so you need to design a solution for this differential movement at this junction point; I would recommend consulting a structural engineer about this.

Supporting the joint

A movement joint should be continuous for the full length of the masonry, which can reduce the load that masonry will withstand.  To overcome this, you should position wall ties either side of the joint. The addition of de-bonded wall ties across the joint will allow movement and increase flexural strength, as long as they are installed correctly.

Appearances matter

While movement joints are necessary for the long-term performance of masonry and to avoid issues later, a long continuous line down brick or blockwork can be unsightly. To counter this, you can hide them behind internal returns or perhaps behind rainwater pipes. For the latter you need to make sure that the fixing brackets for the pipes allow the joint to move.

Equally you may want to apply a render or other finish and movement in the masonry or blockwork could lead to cracks. To overcome this, you should continue the joint from the masonry into the finish.

Final thoughts

As you can see there is a lot to consider with movement joints and we haven’t even touched upon things like differential movement between the external leaf and internal structure or frame, or how to install a movement joint.

For more guidance on masonry construction, you can turn to PD6697.  You can also attend a free CPD seminar that we host called “Movement in Masonry” find out more here.