Ventilation and Dry Fix FAQ's

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 Sitework Guide, or by contacting the technical department.

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.

If your decision is to mortar bed, then you should ensure that the correct 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.

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.

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 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.

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 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

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 other 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.

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.

Dry fix solutions have developed considerably over the last couple of decades as an alternative to mortar bedding. In fact Marley has been at the forefront being 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 mechanically secured alternative to mortar which is not only maintenance free, but in the case of ridge and hip systems, also provides ventilation to meet current Building Regulations, British Standards and National House Building Council (NHBC) guidelines.