Detecting and understanding roof leaks

Leaking roofs are a very common problem identified in home inspections. HouseCheck home inspections show that there are three main issues to consider when diagnosing and fixing leaking roofs:

  • The condition of the roof covering. This includes cracked, broken or dislodged tiles or slates, damaged or corroded roof sheeting, weathered thatch. Incorrect installation can render any type of roof coverings ineffective.
  • The pitch of the roof. Generally only “flat” roofs are water-proofed; most pitched roofs are “weather-proofed”. This means that the roof is designed to shed water quickly to prevent rain water from penetrating the roof covering. Generally speaking, the steeper the pitch of the roof, the less likely it is that the roof will leak.
  • Weak points as regards potential leaks are the areas where roof surfaces intersect with walls, chimneys or other roofs. These intersections are usually weather-proofed with “flashing” of one kind or another.

Tiles
The home inspector will check for cracked, broken or dislodged tiles or slates. Poor installation techniques which can result in leaks include: Cracked mortar on ridge or barge capping tiles; lack of mechanical fixing of tiles in vulnerable areas along eaves and ridges and a roof pitch which is too low.
Slates
Slates are usually installed over a water-resistant underlay – often bituminous felt (malthoid). Leaks arise from broken or dislodged slates and from a perished underlay. If mild steel, rather than copper or aluminium fixing nails, have been used then corroded nails may also lead to slipping slates. Ridges on slate roofs are normally finished with facing ridge slates over a “double soaker” – this is an overlapping layer of underlay extending over the ridge. If this double soaker becomes damaged or perished then leaks can occur along the ridges.
Roof sheeting
Metal roof sheeting often corrodes along the overlaps and around the fixing screws. Too-short end-laps and inadequate side-laps which face the prevailing weather are common installation mistakes.
Cracked and weathered fibre-cement or plastic roof sheeting can also be a problem.
Thatch
The top layer of grass in a thatched roof, which is exposed to the elements, slowly rots and needs to be periodically combed out and replaced to preserve the weatherproof qualities of a thatched roof.
Semi-flat roofs
These can be either concrete slabs or board – often surrounded by parapet walls.  These roofs must have adequate drainage. The top surface of the semi-flat roof needs to be effectively waterproofed – usually with heat-applied torch-on bituminous felt topped with UV ray resistant silver aluminium paint. If the waterproofing is old or has been badly installed with inadequate overlaps or poor bonding to the substrate, then leaks may occur. The solution is to either patch or remove and re-install the torched-on waterproofing.

Roof pitch
The South African National Building Regulations prescribe minimum pitch levels for roofs – depending on the roof covering.

Flashing
Flashing, which is usually either metal flashing or acrylic membrane waterproofing, is  installed where roof surfaces intersect abutments.  The flashing is designed to waterproof these weak points in the roof.
Properly installing metal flashing and counter-flashing is a tradesman’s art which is rapidly becoming scarce in South Africa. More and more modern roofers resort to flashing roofs with acrylic membrane. Acrylic membrane flashing is often untidy, but is reasonably effective assuming that the acrylic is of good quality and assuming that the flashing is recoated every few years. If the acrylic flashing is not properly maintained then the harsh South African sun soon causes deterioration and debonding and leaks are likely to occur.
Finding the leak
Roof leaks are mostly noticed from inside the house – damp patches on the ceiling or on walls.
Because water often tends to run down the roof timbers before manifesting itself in a visible damp spot, detective work inside the roof cavity is often useful in determining exactly where the roof is leaking. Damp staining on the top chords of the roof trusses and sometimes spots of daylight visible through the roof covering are often the best indication as to where the leak originates.

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