NHBC: Raising the Standard of Pitched Roofs

NHBC: Raising the Standard of Pitched Roofs

Five years ago NHBC launched an industry-wide campaign to help raise standards of pitched roofs on new homes across the UK in a bid to reduce claims and re-occurring problems. NHBC Technical Projects Manager Chris Derzypilskyj examines how the situation has improved and what exactly has been done since 2011.

Back in 2011 NHBC claims figures made for unhappy reading in relation to pitched roofs. By analysing our claims we discovered that more than one in every 60 homes built in the year 2000 had a valid pitched roof claim.

This amounted to 60% of all claims that NHBC received in 2010. Critically, over half of all pitched roof claims related to mortar issues.

In terms of cost, NHBC spent more than £11m on pitched roof claims, but repairs undertaken directly by builders substantially increased this cost.

Conservative estimates at the time pointed to a wider industry cost of around £30m. Additionally, thousands of homeowners were affected by disruptive works to repair this damage.

It was clear that many of these claims could be avoided by improved design and workmanship. With the help of industry professionals including the National Federation of Roofing Contractors we completely reviewed the content of Standards Chapter 7.2 Pitched Roofs and identified a number key changes which, it was anticipated, would improve quality, and in due course help to prevent further expensive and disruptive remedial work.

After an initial period of raising awareness via a comprehensive engagement campaign which included free pitched roof training seminars, NHBC introduced the revised Chapter 7.2 in January 2012.

dry-hipThe right mortar mix

A key element of the initial campaign was a strong emphasis on achieving the correct mortar mix. The roofing mortar mix should be 1:3 cement:sand with plasticiser; with the mix based on sharp sand with soft sand added to achieve good workability.

Because sands will vary, roofers were urged to make slight adjustments to accommodate regional differences. However, the proportion of sharp sand must not be less than one-third of the total sand content.

Alternatively, proprietary roofing mortar mixes that are shown to have similar strength, durability and workmanship are acceptable. However, adapted mixes, such as silo mortar or factory produced retarded bricklaying mortars with additional cement added are not acceptable.

Mechanical fixing of ridge and hip tiles

For many years it was traditional to bed ridge and hip tiles in mortar to secure them to the roof and experience shows that pitched roofs will be subject to some movement during the life of the property.

Mortar is generally not tolerant of that movement and can easily crack or de-bond, making ridge and hip tiles vulnerable when subjected to high winds.

Therefore, since 2012, NHBC has required all bedded ridge and hip tiles to be mechanically fixed.

In addition to this and to help address failure to deep sections of mortar bedding, where ridges and hip tiles are bedded on mortar to rolled tiles, concealed or decorative dentil tiles should be fully bedded into all joints in excess of 25mm thick.

Another change to Chapter 7.2 addressed mortar placement to verges, which NHBC has seen to be susceptible to the effects of thermal movement, freeze/thaw action and failure due to incorrect mortar placement.

Where verge tiles and slates are to be wet bedded, they should be bedded on a 100mm wide bed of mortar on an undercloak with the mortar placed in one operation i.e. formed by either bedding and pointing immediately or bedding and then pointing following initial ‘stiffening’ of the mix.

There were other changes with regards tile suitability, fixing (nailing and clipping), acceptable overhang and tile batten length.

This guidance combined with the revised mortar mixes should greatly reduce the risk of claims where wet systems are used.

dry-verge-2Small cut tiles

Small cut tiles are difficult to fix and are vulnerable to slipping or being dislodged by wind. Therefore, the Chapter was revised again in 2013 with guidance aimed at avoiding the use of small cut tiles.

The revised guidance included;

  • Purpose-made plain tile-and-a-half should be used. Cut plain tiles are not acceptable.
  • Natural slate verges should be formed with full slates and either slate-and-a-half or half slates that are a minimum 150mm wide.
  • Where interlocking tiles are used to form verges, they should be at least ½ tile width and fixed in accordance with BS 5534 (it is good practice to locate cut tiles on the more secure right-hand verge where possible).

 Industry changes

The revisions in 2014 to British Standards BS 5534 ‘Slating and tiling for pitched roofs and vertical cladding – Code of practice’ generally brought it in line with NHBC Standards (in terms of mechanical fixings of bedded ridge and hip tiles, mortar mix ratios and small cut tiles).

One key element related to the specification and use of roof underlays. We encouraged industry to become familiar with the revisions and to incorporate them into their roof designs and construction as soon as is practicable: i.e. Ensure that underlays are suitable for intended location by using the ‘zonal classification’ system (suitable for most roof designs) or by calculation,  make sure that laps in underlays are adequately secured; and finally, the Zonal Method of fixing should no longer be used, with all fixing specifications now to be provided by the tile manufacturer in line with the revised British Standard.

The standard also requires all tiles to be twice fixed at verges to resist wind uplift. One of these mechanical fixings can be a tile clip or it can be a dry fix capping system where this can be shown to provide adequate resistance to wind loads.

A new BS is currently being drafted for dry fix systems for ridges, hips and verges and will include tests for resistance to wind loading.

slatesLatest information

In August 2015, a pitched roof covering survey was completed on around 3,400 sites in order to help NHBC understand how well the industry has adopted these changes.

We were also able to compare against the last two roofing surveys in 2010 and 2012 respectively, as well as gathering valuable information from new questions.

In summary we found that this was a ‘good news’ story, which will ultimately lead to reductions in claims.

Key points uncovered from the survey included;

  • an increase in the use of dry systems across the board
  • an increase in site specific fixing schedules from 31% of sites in 2010, to 41% two years later and finally 61% in 2015
  • an increase from 58% to 96% of small cut tiles that are either mechanically fixed or bonded with manufacturers recommended adhesive, and finally;
  • in 76% of all sites surveyed, perimeter tiles are mechanically fixed twice

The increasing proportion of sites moving to dry systems, reducing or removing some of the risks previously highlighted with wet work; follows an approach that has been adopted in Scotland for quite some time (which historically have lower pitched roof claims than England and Wales).

However, it should be noted that when using dry systems, it is vital to ensure that the correct system is specified for the pitch and that the tile and dry system is components are compatible. Also, don’t forget to keep your eye out for the eagerly awaited new British Standard briefly discussed above.

Despite the 2015 survey mainly being a positive story, inevitably there are some areas for further improvement, notably incorrect mortar mix and mechanical fixing of the last ridge or hip tile.

From a claims perspective, the percentage of roofing claims that relate to mortar has significantly reduced since NHBC first launched the campaign to reduce pitched roof claims and raise overall build standards. In 2011, this figure was at 79% but over the following years has fallen to 51% (2015).

For more information click here. 

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