In his latest consultant case study, pitched roofing consultant John Mercer focuses on efflorescence which can form on the surface of concrete roof tiles, but also looks at the emerging trend for efflorescence forming on the underside of the tile…
I have often been called to roofs where efflorescence is forming on concrete roof tiles.
Although efflorescence has always been a problem associated with many concrete building products, including bricks, roof tiles and mortar, in the last two or three years a new trend is emerging in which the efflorescence is appearing on the underside of roof tiles and then running down the top surface of the tiles below.
Firstly, a quick recap on what efflorescence is:
Efflorescence occurs naturally when water in the form of rain, condensation or dew penetrates the pores of concrete and then carries calcium hydroxide, which is formed in concrete during the hydration process of the cement, to the outer surface of the concrete. The water then evaporates, leaving a white film, or bloom, on the surface.
Once deposited on the surface of the product, the calcium hydroxide then reacts with carbon dioxide, which is present in the atmosphere, and becomes an insoluble calcium carbonate. The process soon ceases as the outer pores of the concrete are closed by the insoluble carbonate. Because the efflorescence is now insoluble it will not be quickly washed from the roof tiles by subsequent rain downpours. Rainwater is slightly acidic, therefore long-term weathering will eventually remove the efflorescence, but it is impossible to predict how long this will take.
Admittedly, the problem does not appear on most roofs; it may be partly connected to the amount of salts in the tiles, which can vary during their manufacture. The building’s internal climate, the amount and even timing of the vapour passing through the underlay may be significant – see in the image below how the efflorescence has only formed over the semi-detached house on the left and not on the house on the right, even though the roof tiles came from the same batch and were installed at the same time.
Concrete tile manufacturers coat the top surface of their tiles during manufacture to prevent visible efflorescence. Due to the method of manufacture, it is not possible to coat the underside of tiles, nor has it ever been considered necessary to do so.
So why are we now experiencing this new phenomenon? One theory which seems to be gaining some traction, is that the large volume of moisture passing through the permeable underlay, particularly during a new building’s drying out period, is having a detrimental effect on the roof tiles.
We now have some extremely efficient vapour permeable underlays that can be used without traditional roof space ventilation and which prevent condensation from forming within the roof space. However, all the water vapour that passes out of the structure through the underlay ends up in the batten cavity for a period before it dissipates out through the roof tiling. I am seeing increasing evidence of this causing excessive amounts of condensation on the undersides of the roof tiles, thus causing the natural salts in the roof tiles to come to the surface.
BS 5250: British Standard Code of practice for control of condensation in buildings recognises that permeable underlays will lower the risk of water vapour in the loft, but may increase the risk of condensation forming in the batten space unless there is enough air movement through the external covering.
There is a test method in BS 5534 for determining the permeability of external roof coverings such as tiles and slates. For close fitting roof coverings, BS 5250 recommends that either counter-battens are installed with low-level and high-level batten space ventilation, or the loft be ventilated.
Clay and concrete tiles are generally considered to be sufficiently air-open to allow vapour to dissipate out into the atmosphere. Though increasing cases of efflorescence on the backs of concrete tiles and even some cases of delamination on the backs of clay tiles perhaps indicates that relying on the tiles or slates alone to dissipate water vapour may no longer be enough.
An unventilated roof space, where vapour is safely dissipated through the underlay, provides a much more thermal efficient roof system. Therefore, the answer lies in removing the water vapour from the batten cavity before it can cause any harm.
In mainland Europe, the risk of excessive condensation forming on the undersides of tiles is well understood. The batten cavity is routinely ventilated at low and high level to encourage air flow to remove water vapour and thus prevent condensation from forming.
In traditional ‘English’ cold roof construction, we generally drape the underlay, unsupported, directly over the rafters, then fix the tile battens over the underlay. However, if counter-battens are fixed between the underlay and tile battens, we not only overcome any difficulties in achieving a sufficient drape in the underlay, we also provide an efficient path for air movement.
The addition of low and high-level ventilation into the batten space will help to reduce moisture levels by encouraging a free flow of air. Many roofs are already installed with ventilating dry ridge and hip systems, so all that is needed is eaves ventilation to drive airflow through the batten cavity and thus remove excessive moisture, all without impacting upon the thermal efficiency of the roof structure – see diagram above.
- Closed roof structures, where vapour can pass through the underlay without traditional ventilation, are more thermally efficient, though increase the amount of vapour passing into the batten cavity.
- The use of counter-battens overcomes any problems in achieving a drape in the underlay, as well as providing a path for air movement.
- Eaves ventilation over the underlay, fitted in conjunction with a ventilated dry fix system at ridge, will encourage air movement through the batten cavity to remove water vapour.