Social Housing Group Event 21 st June 2012

Transcription

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2 Social Housing Group Event 21 st June 2012 NHBC Standards Chapter 7.2 Pitched roofs 2012 Paul Cribbens

3 Aims of this presentation Reminder of why change was needed What are the common causes of failure for roof coverings What changes have been made in Chapter Be clear where to focus attention Questions

4 Claims performance headline figures More than 50% of all section 3 claims (by number) relate to pitched roofs Over 50% of all pitched roof claims involve mortar issues We ve seen that some builders are experiencing up to 70% of all claims being related to pitched roofs

5 Cost ( m) Pitched Roofs Claims performance Headline figures More than 25% of section 3 claims costs relate to pitched roofs In 2009/10 annual claims costs on roofs passed 10M 2010/11 costs in excess of 12m (and this only represents cost of repairs to claims undertaken by NHBC) Is this the peak? Pitched roof claims costs

6 Review Chapter 7.2

7 Mortar Previous guidance asked for the mortar mix to be 3:1 sharp sand/cement. Modified pre-mixed mortar was also acceptable. 3:1 sharp sand/cement is difficult to work and achieve a reasonable finish. Modified mix or soft sand used due to workability and availability.

8 Pitched Roofs Mortar Several winters have been considerably colder than average

9 Pitched Roofs Mortar- Material failure

10 Mortar

11 Mortar, why does it fail Mortar is asked to perform differently on roofs. Gap filler and adhesive. Roofs are generally the most exposed part of the building. Roof mortar connects materials with different thermal expansion rates. Modified pre-mix mortar, and soft sand mix has poor tensile strength.

12 Mortar- New Guidance Roofing mortar should be 1:3 cement:sand with plasticiser. The mix should be based on sharp sand with soft sand added to achieve workability. The proportion of sharp sand should not be less than 1/3 of the total sand content.

13 Mortar- New Guidance Mechanically Fixed ridge and hip tiles Dentil tiles used where the joint is in excess of 25mm

14 Design out problems

15 Designing out future problems?!

16 Dry systems Around 40% of sites surveyed in England and Wales currently adopt dry systems, ridges were higher at about 60% On sites surveyed in Scotland this was higher, at over 90% Is this relevant? Our claims figures suggest a lower ratio of pitched roof claims in Scotland compared to England & Wales Dry systems don t remove all risk but might be worth considering?

17 Ensure that components are installed correctly.

18 ACTION Ensure correct mortar mix is specified and used Bed and point verges in a single operation Consider whether dry systems may provide a better alternative If dry systems are used, check they re suitable and correctly installed

19 Pitched roofs Graded battens Battens should be marked in accordance with BS 5534

20 Pitched roofs Small sections of cut tile Guidance introduced to prevent the use of small sections of cut tile that are difficult to fix Typically found at abutments and valleys

21 Pitched Roofs Detailing of verges

22 Detailing of verges Battens should be set a minimum 25mm-50mm back Verge tiles should be placed on a minimum solid 100mm bed of mortar Recommended overhang for a verge is 38-50mm (plain tiles) 30-60mm (interlocking tiles)

23 Pitched roofs 2013 Guidance for verge tiles At verges: cut plain tiles are not acceptable and purpose made plain tile-anda-half should be used small sections (less than a half width) of cut single lapped interlocking tiles should not be used natural slate verges should be formed with full slates and either slate-and- a-half or half slates that are a minimum 150mm wide all tiles and slates should be mechanically fixed at the verge in accordance with Appendix 7.2-A

24 Questions?