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Volume 4
Chapter 4 Structural Stability

4.1. A house meets the tolerable standard if it is structurally stable.

Introduction

4.2. This element of the tolerable standard requires every house to be structurally stable. Instability in the structure of a house is an obvious threat to the occupants' safety.

4.3. This chapter provides guidance for assessors on how to judge if a house is structurally stable. Assessors should use this advice alongside their own experience and the support of specialists (where necessary) to make decisions.

4.4. As noted in chapter one of this guidance, staff from a wide range of professional backgrounds assess houses against the tolerable standard. Structural stability is a complex technical area. It is not our intention to provide a comprehensive guide to structural stability here. Instead, our guidance aims to help housing staff and other professionals who are not structural experts to recognise whether a house is potentially unstable and to identify when to call in a competent professional.

Legislation

4.5. The tolerable standard has always included the requirement for houses to be structurally stable. The Housing (Scotland) Act 2006 did not change this element of the standard.

Definitions

4.6. Structurally stable: The structural elements of a house should exhibit no signs of recent or fresh movement, evidence of which may indicate to the assessor that the house may be at risk from either partial or total collapse. The main structural elements of a house are:

• roof structures, including chimneys;
• chimneys;
• load-bearing walls including external walls;
• lintels, sills and mullions;
• floors and stairs;
• load-bearing beams and columns; and
• foundations.

4.7. There is more information on these elements later in this chapter.

Making the Assessment

4.8. The flow-chart below will help guide an assessor through the process:

4.9. When assessing a house against this element of the tolerable standard, an assessor should look for visible signs of potential instability in any structural element of the building. This will allow him / her to consider the stability of the building as a whole.

4.10. We explained in chapter three that housing staff and other professionals assess houses against the tolerable standard for two distinct purposes: local / national house condition survey work; and possible enforcement action. This difference is particularly important for structural stability as it influences the factors an assessor will be able to consider:

• Where a local authority is examining whether an individual house is structurally stable for the purpose of possible enforcement action, it is able to monitor the house over time. Staff will be able to track any pattern of change that might indicate that the house is structurally unstable. Non-specialist staff should use our guidance to help identify indicators of possible structural stability problems. Where staff have concerns, they should ask a competent professional to carry out a detailed assessment. A competent professional will be responsible for judging whether a house is structurally unstable using his/her professional knowledge. A house which is not structurally stable does not meet the tolerable standard.
• Where an assessor is examining a house as part of a house condition survey, he/she will need to base his/her judgement on a one-off walk-through survey. Assessors may be able to form a clear opinion during a single visit in some cases. But there are inevitably limitations because the assessor cannot track the building over time.

Key indicators

4.11. This chapter gives information to assessors on what they should look for when considering if the house is structurally stable. A house that is structurally unstable will normally show signs that it is moving, or that a structural element is likely to fail. The tables later in this chapter describe some of the most common indicators of instability.

4.12. The first two tables cover fresh cracking and movement or displacement of structural elements. These contain information on indicators that may show a house is unstable because it has moved and may still be moving. The third table covers damage, deterioration, rot and timber infestations, and describes indicators which may suggest a house is unstable because a structural element has been weakened.

4.13. The tables show indicators of potential instability and do not necessarily mean on their own that a structural element, or the house as a whole, is structurally unstable.

Information on cracking

4.14. An important indicator of instability and movement in houses is cracking. There are two types of cracking that can happen because of movement, and an assessor should try to tell them apart. Assessors may need to take advice from a competent professional to do this.

4.15. Historic cracking is cracking which has occurred in the past. It shows that the house has previously moved. But this is not always evidence of current structural instability. Historic cracking may be the result of building 'settlement' after construction, thermal expansion and contraction of the building fabric, or vibration from passing traffic. This does not in itself indicate that the house is still moving.

4.16. The assessor should look for evidence that the cracks are historic. For example, old cobwebs or dirt in the cracks may suggest that the house has not moved for some considerable time. A house with only historic cracking will not normally be below tolerable standard on the grounds of structural instability.

4.17. Fresh cracking is an indicator of possible structural instability. This is cracking that has happened recently and shows that a house has moved and may be continuing to move. The assessor should look for evidence that the cracks are fresh. Possible indicators are the absence of old cobwebs or dirt inside the crack or the presence of masonry dust or flaking paint in, around or on the ground below the cracking. In some cases, a local authority may have monitored the progression of cracks over time using glass "tell tales" or other similar devices. Glass "tell tales" are similar to glass laboratory sample plates which are placed across the crack and cemented in place so that any movement will cause them to crack. This information will be very helpful in assessing the house against the tolerable standard. A house showing fresh cracking may be below tolerable standard.

4.18. Cracking is an indicator of potential structural instability. But it can have other causes. These include vibration caused by traffic on a nearby road, settlement cracks that appear soon after construction, and thermal cracking caused by expansion and contraction of the house due to temperature changes. These types of cracking do not normally indicate a defect in the structure of the building.

Information on movement or displacement

4.19. When a house has moved, it may result in a distortion to the normal shape or profile of the structure. Assessors should look at the structural elements of the house, included in the table below, for any abnormal qualities indicating potential instability. This includes walls that are bulging or sloping, roofs that slope to one side or dip in the middle, and internal floors that slope or are uneven.

4.20. Assessors should use their experience to help them determine visually when structural elements of a house have moved or have been displaced. If there is evidence of movement, an assessor needs to decide if this means the house is at risk of being structurally unstable and may be below tolerable standard. If this is the case, the assessor should ask a competent professional to inspect the house.

Information on damage, deterioration, rot and timber infestations

4.21. The structural elements of a house may be weakened because of factors associated with the house's age, location, maintenance and build quality. The table below includes examples of some of the most common problems resulting from damage, deterioration, rot, and timber infestations. Each of these has the potential to reduce the load-bearing capacity of specific structural elements. If the structure of a house is significantly weakened, it is at risk of being unstable and may be below tolerable standard. Again, an assessor may wish to consult a competent professional if he/she suspects the house is unstable.

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