Roofs and parapets are a major area of problems with art deco houses, largely because of water ingress.
Flat or very low-pitched roofs with parapet walls are prone to water ingress from wind-blown driving rain.
The roofs on art deco houses are therefore likely to have been re-roofed, often with a different roofing material from the original. For example, metal trough section roofing may have replaced corrugated iron or built-up roofing; or modern flat roofing materials such as a torch-on membrane, fibreglass or rubber-based products may have replaced built-up roofing. Any new membrane roofing is likely to have been installed over a new ply substrate – most modern membranes cannot be laid over the original boarding.
Parapet and other flashings may have been replaced.
Today, manufacturers recommend a minimum roof pitch of 8° for installating corrugated sheet roofing, but in many situations art deco houses had corrugated roofing installed with roof pitches of 7° or less. This may have resulted in water getting under the laps between sheets or at flashing locations and being unable to readily drain out. There may also be ponding of water at any lower points or indentations in the roofing.
If the roofing material is the original corrugated galvanised steel, it may require replacement. If this is the case, unless the roof pitch is 8° or greater, a different metal roof profile such as a trough section roofing must be installed.
Trapezoidal symmetrical and unsymmetrical rib profiles and standing seam, fully supported flat sheet metal roofing materials may be used with roof pitches as low as 3°. Confirm manufacturers’ recommendations for minimum roof pitch for specific materials.
It is important, particularly when replacing membrane with profiled metal on a low pitch, to ensure that flashings or internal gutters are extended sufficiently under the roofing material to prevent water tracking or being blown under the roofing and leaking beyond the flashing. Generally, to avoid this risk, it is advisable to replace low-pitched roof cladding with a membrane system with gutters integrated into the membrane.
The early built-up roofing had a limited serviceable life as bituminous materials were prone to a number of problems including:
- hardening and cracking of the membrane
- rotting of organic fibres in early the felts as the bituminous coating deteriorated allowing moisture to penetrate
- differential movement between the roofing material and the substrate and insufficient elasticity in the roofing
- loss of adhesion of stone chips and subsequent loss of UV-protective coating to the membrane
- breakdown of the paint finish and subsequent loss of UV-protective coating to the membrane.
Roofs with low pitches that were slow to drain, were more likely to suffer some water penetration through any minor breaches in the roofing membrane and this may result in eventual failure of the roofing.
If roofing is still original it is likely to be in poor condition and should be replaced.
A new membrane roofing can be applied, but a new ply substrate is likely to be needed first. It may be worthwhile considering increasing the roof fall before re-roofing. The internal gutter should also be checked against current building requirements for depth, width and fall.
Common options for membrane roofing systems to replace an existing roof:
- Synthetic rubber (butyl and EPDM), which is laid across the slope, fully adhered to the substrate and installed with lapped joints that are bonded using adhesive or lap tape (lap tape is recommended where there is any risk of water being held at the joint)
- PVC, which is laid across the slope, fully adhered to the substrate and installed with lapped joints that are heat-welded
- Polymer modified bitumen, typically torch-on material, which is laid across the slope using heat to activate the under surface to provide full adhesion to the substrate. Joints are lapped and adhered. It is recommended that modified bitumen be installed as a two-layer system, with the upper surface having a cap layer or protective covering to protect the bitumen from UV light.
- Liquid applied membranes, which are typically an acrylic emulsion or polymer gel that is roller or brush applied in two or three applications directly to the substrate and often reinforced.
For membrane reroofing applications, always obtain the product-specific installation requirements from the membrane supplier/manufacturer, especially the requirements for the detailing of joints in the plywood substrate. More information on membrane roofing systems and their installation is given in the BRANZ Good Practice Guide Membrane roofing.
Moisture, whether from external sources or from internal condensation (see below) may lead to deterioration of timber roof framing. Roofs should be checked for evidence of sagging, which may be due to inadequate timber sizes, or rot, which may be due to moisture penetration.
Because art deco houses were hard lined and uninsulated, there is a risk that condensation will occur on walls and ceilings, particularly where the house is shut up during the day, and particularly where the original timber windows have been replaced with aluminium. Ensuring that the house is well ventilated, insulated if possible, and heated, will reduce the amount of condensation forming.
An issue that can arise with low-slope skillion roofs is the accumulaion of moisture within the roof space, which can form as droplets on the underside of the timber sarking or metal roofing. Research has shown that moisture within roof spaces is carried there by air movement from moist internal spaces into the roof space. A number of art deco houses incorporated vents into parapet walls to provide ventilation of the roof space.
Where there is a membrane roofing system, some of this moisture is transferred through the permeable substrate and then condenses (on cold nights) on the underside of the membrane. As the sun warms the roofing in the morning, the moisture cannot escape back through the substrate, and bubbles form as the water tries to dry to the outside. Unless the source of moisture is removed, the bubbles will continue to expand, and because the substrate remains wet, there is an increased risk of timber damage.
For metal roofing, droplets that form on the underside of the metal may drip back down onto the ceiling, which may result in damage to the ceiling and/or ceiling framing.
Reduce the possibility of moisture getting into the concealed skillion roof:
- Ensure ceilings are airtight – a flush-stopped plasterboard sheet is considered sufficiently airtight. Where the ceiling is not airtight, BRANZ recommends that it be installed over an air barrier (such as flexible wall underlay complying with air barrier requirements of Table 23 of Acceptable Solution E2/AS1). For existing air-leaky ceilings, consideration should be given to replacing the ceiling where a moisture problem has been identified.
- Do not install recessed downlights in the ceiling, and remove any that have been installed.
Finish ceilings in wet areas such as bathrooms, laundries and kitchens with an oil-based sealer or a full oil-based coating system to provide a vapour check so that the materials will be less likely to absorb moisture from within the space.
Corrosion of metal components such as the roofing, fixings, flashings or accessories, may appear as the familiar red rust, or as a white discolouration on the surface of materials containing zinc, which is called white rust. It may be caused by:
- moisture combined with salt spray, dirt, or hydrogen sulphide from coal-burning fires or geothermal activity
- contact between dissimilar metals resulting in electro-galvanic action
- run-off from copper-based timber treatments.
Houses built close to the sea are likely to be affected by salt or chloride contaminants. The parts of the house most likely to be affected are the subfloor, roof cladding and flashings, window and door flashings and cladding nails.
Flashings, fixings and accessories that are significantly affected by corrosion (for example, pitted or pinholed surfaces) should be replaced.
Where corrosion on mild steel is on the surface only and is able to be removed by sanding, the steps that should be carried out are:
- Sand with grit paper to expose shiny metal.
- Prime with a zinc-rich primer.
- Apply a solvent-borne metal primer.
- Apply two finish coats.
The risk of future corrosion can be reduced by painting steel, specifying factory-coated steel and using hot-dip galvanised steel or stainless steel fixings and brackets.
It was not uncommon for walls and roofs of art deco houses to be built without any form of building paper or underlay. If they have previously been re-roofed or reclad, underlay may or may not have been installed.
Options for retrofitting underlay include to install underlay:
- when replacing roofing
- by folding into the framing cavities for walls when the interior lining is being replaced.