Ventilation - Taking care with re-roofs

Klober share some hints for builders and roofers on ventilation pitfalls with re-roofs.

The proportion of pitched roofing work accounted for by re-roofs continues to grow due, in the main, to public sector housing upgrades as we near the Decent Homes Standard initial 2010 target date.

Already, however, common problems related to provision of adequate roof space ventilation are beginning to arise.

Keeping abreast of the swathe of building performance legislation, not least that relating to ventilation isn't easy, while product claims from manufacturers in an endless quest for market share are often confusing.

The effects of condensation can, however, be devastating, and risk of its presence in building structures continues to grow as building standards improve.

Condensation, however, is also a symptom of modern living, use of domestic appliances producing a greater amount of water vapour in the home than ever before.

Much of the older publicly owned properties previously had little or no loft insulation for decades after they were built, so condensation in the roof void was rarely a risk.

Now, with insulation being installed at thicknesses of 250-270mm, today's pre-occupation with avoiding heat loss, and fewer open fireplaces, the loft is colder and more prone to formation of water vapour.

As a consequence of improved insulation standards, getting roof ventilation right has never been as important.

How this is done, however, comes down to a straightforward choice of whether to ventilate in the traditional way or use a vapour permeable underlay to allow the structure to breathe.

A few basic and inexpensive precautions, however, will ensure the ventilation performs properly and avoid the vast majority of problems that can arise.

Fortunately, most publicly owned stock is of broadly conventional roof design, so any recommendations made apply to the greater proportion of it.

One of the commoner forms of roof refurbishment now employed uses a non-breathable HR underlay with existing eaves ventilation.

Problems often arise if improved insulation, often installed some time after the roof refurbishment, is pushed into the eaves.

The ventilation path becomes blocked, allowing stagnant air pockets to form and creating an ideal environment for condensation to form.

Ironically, a re-roof provides the ideal opportunity to install accessories such as a rafter tray or eaves vent system, which are fitted more easily from outside than from within an enclosed and restricted loft space.

Products such as the Roll Out Rafter Tray and Loft Vent Tray, supplied by Klober, and fitted with fascia or soffit vents, provide the equivalent of a continuous 10mm or 25mm opening, and ensure correct positioning of insulation at the eaves.

The Loft Vent Tray provides added convenience for the installer by adjusting itself to the pitch of the roof.

Fascia vents are both cost-effective and desirable as they offer a discreet and unobtrusive means of eaves ventilation.

Their use, however, does present a problem on mixed ownership terraced or semi-detached properties.

An over-fascia vent will increase the board height and so has to be continuous, given the effect on lower tile courses.

Vent tiles, often criticized for being unsightly, are the only option if a continuous roof line cannot be maintained or is impractical to install.

This problem has now been overcome, as popular tile profiles such as the large format concrete Double Roman, Double Pantile and Flat interlocking tiles such as the Sandtoft Calderdale now have matching Profile Line vent tiles.

These enable ventilation to be provided at high or low level and are all but invisible from the ground.

With the option of a similar vent for the smaller 15 x 9 tile, so common in the South East, the greater proportion of tiled and slated properties can now benefit from unobtrusive roof ventilation In areas of planning sensitivity in particular, this is a major advance.

Mixed ownership also presents the common problem of varied tiles and slates within a roof line, creating the need for a bonding gutter.

These typically have a sanded strip for mortar bedding so that tiles or slates can be butt joined.

Frequently, however, the strip is wrongly used as a valley with consequent potential for water to seep in.

Use of vapour permeable membranes such as Klober's Permo Forte has also proved effective over the past 6-8 years, and was one of the main reasons why the recently updated BS5250 was so eagerly anticipated.

Ordinarily, Permo Forte requires no additional ventilation, but although most vapour permeable underlays now have BBA Certification, problems can occur where comparison of performance between similar products is not carried out.

With 50 or so such products currently covered by such accreditation, a decision to switch source of supply can prove costly if what is supplied proves not to be like-for-like.

This is particularly true when considering product strength and risk of fall through.

Provision of ventilation at the ridge may however be necessary and is an added benefit that comes with dry fixing.

This offers far greater control over and consistency of workmanship, while reducing need for remedial work due to mortar failure.

Failed mortar joints were highlighted by Granada Reports following the recent spate of high winds.

Not surprisingly, with the benefits dry fix brings to those with long-term maintenance commitments for publicly owned properties, complaints tended towards privately owned dwellings.

Where used, the venting area follows the full ridge length but again, it is visually discrete.

To make life easy for installers, products such as Klober's Uni-Dry Ventilating Ridge and Roll-Fix are suitable for most half round and angled ridges in clay, concrete and slate.

Installation is pretty much unaffected by weather and without the mortar there is far less weight on the roof.

In practical terms too, jobs are more likely to be completed on time.

Improved visual effect and neatness of finish are an added bonus.

Provision of adequate roof space ventilation need not be a complicated or expensive process.

Even so, evidence suggests that there is still widespread inconsistency in the understanding of condensation's effects in a well insulated building.

There is already evidence of buildings having to be demolished where uneconomic to repair as a consequence of vapour getting into the structure.

So, a little care with assessment of ventilation needs while upgrading insulation could just prove to offer 'best value' in every sense.

Back to top