Ventilation and Dry Fix

What is dry fix?
Should I use mortar or dry fix for my ridge and hip tiles?
Is dry verge easy to install?
What is breathable underlay and can I use it?
How do I ventilate a cold roof?
How do I ventilate a warm roof?

What is dry fix? 

Dry fix solutions have developed considerably over the last couple of decades as an alternative to mortar bedding. In fact Marley Eternit has been at the forefront of innovation in this area and were the first manufacturer to launch a dry ridge and verge system.

Although mortar has historically been the popular choice to bed hips, ridge and verges, there a number of factors that cause mortar to fail. Dry fix systems provide a superior and mechanically fixed alternative to mortar which is noted for it's poor performance and failure. In addition to being maintenance free, Marley Eternit dry ridge and hip systems, also provide ventilation to meet the requirements of BS5250 the code of practice for the control of condensation.

Should I use mortar or dry fix for my ridge and hip tiles? 

The question of whether to use mortar or dry fix really comes down to the issue of future maintenance and cost. It is a fact that mortar only has a limited life span and is also affected by building movement and environmental conditions. Although the cost of having ridge and hip tiles secured by mortar may be cheaper in the short term, there may be long term maintenance costs.

Recent changes to BS 5534 - the code of practice for slating and tiling - have introduced some important changes to the way that roofs are fixed. In particular, mortar is no longer recognised as being suitable for providing a sole means of fixing. To comply with the new standard, any mortar bedding, such as traditionally found on the ridge, hip and verge, must now be accompanied by a mechanical fix. Marley Eternit provides a range of mechanical fix solutions for any mortar bedding requirement. In addition to providing a mechanical fix, you'll need to ensure that a suitable mortar mix is used. the NHBC has guidelines which state that an acceptable ratio for roofing mortar is 3:1 sharp sand/cement. There are also pre-mixed roof mortars that are available and are accepted by the NHBC.

Is dry verge easy to install? 

Although dry verge is easiest to install on a new roof by your roofing contractor, it can also be fitted retrospectively by anyone, presuming the tiles are laid to the correct gauge. It is recommended that before working at height, adequate measures are taken to ensure safety.

Once the verge has been stripped back of any mortar it may be necessary to extend the battens using a batten extension piece (as part of the dry verge refurbishment kit) to provide a 50mm overhang for a suitable fix . It is also important to ensure that the plastic eaves guard is in place before the first verge is fitted. This provides protection against birds and large insects getting into the void.

Further information can be found either in the Marley Eternit Sitework Guide, or by contacting technical department.

What is breathable underlay and can I use it? 

There has been much confusion and misunderstanding on the subject of breathable underlays. Many people group these products together as one and don’t distinguish the different types including their differing capabilities. Breathable felts come in two types:

- Vapour Permeable

- Vapour and Air Permeable

Vapour permeable underlays are often the cheaper of the two types. The fibrous structure of vapour permeable underlays is sufficiently dense to prevent liquid water from penetrating; while allowing water vapour to diffuse. Although water vapour can diffuse, there is still an argument for having additional ventilation to carry this vapour out of the roof space. In fact the NHBC has recently implemented guidelines that state when using a vapour permeable underlay, there should also be high level ventilation to provide sufficient air flow to draw this vapour out of the building.

Air open underlays are generally the most expensive form of underlay. Air open underlays (according to their manufacturers) have the lowest vapour resistance and negate the requirement for any additional roof ventilation. Whilst this is a claim that is supported by these manufacturers, there are still some questions over the long term performance and its suitability when specified with a close fitting roof covering. Where an external covering (such as fibre cement slates) is relatively airtight, there is a risk of interstitial condensation forming on the underside of the underlay and the external covering; to avoid that risk the batten space should be vented (See BS 5250:2011). There is also often a requirement for additional components such as sealant tapes. Due to the increased airflow properties of this underlay, there is also some question over its impact on the thermal performance of the roof space.  

In all cases the use of eaves vent system in conjunction with a ventilated dry ridge is not only the cheapest way to ventilate a roof, but also the most effective and assured in supplying over-and-above the minimum free air flow required to meet building regulations.

How do I ventilate a cold roof? 

A cold roof is the most common roof construction often in the form of an uninhabited storage space. Insulation is laid at ceiling joist level, leaving the roof space relatively colder than the accommodation below. If moisture laden air from the living area is allowed to condense in the roof space, it can eventually cause structural damage, or damage to any contents stored within.

BS5250 (Control of Condensation in Buildings) sets out the minimum ventilation requirements of a cold roof taking into consideration span, pitch and roof area. The wider the span, the more free airflow required.

Where insulation is at ceiling level and the void is therefore uninhabited and ‘cold’ BS 5250 specifies sufficient ventilation as being:

  • 25mm along the length of the eaves for pitches of 15° or less
  • 10mm along the length of the eaves for pitches of more than 15°

Additional continuous 5mm ventilation at high level for roofs where pitch exceeds 35°, or for roofs of any pitch with a span of more than 10m for lean-to or mono-pitch roofs.

How do I ventilate a warm roof? 

A warm roof, for example a ‘room in the  roof’ or a loft conversion will require ventilation at high and low level to remove moisture laden air from the batten cavity.  For a simple duo-pitch roof BS 5250 specifies sufficient ventilation as being:

  • 25mm at eaves on each slope and 5mm at ridge.
  • If ridge or eaves ventilation is not possible, ventilation tiles can be used at high or low level to achieve the required airflow.

A minimum 50mm clear air path must be maintained between the insulation and the underlay to ensure a clear airflow from the eaves to the ridge and can be achieved through the use of counter battens.

Marley Eternit provides a range of Universal ventilation products and accessories that can be used to achieve the required airflow for a healthy roof space and can be used to comply with the British Standards BS 5250. 

Here at Marley Eternit, we are always on hand to offer practical and experienced support. Whether you have a query, are unable to find what you are looking for or would like to report an issue, drop your details below and we will be in touch.
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