The pitched roof on our garage extension left the wall leaking

We live in a 35-year-old three-bed semi in Co Wicklow onto which we built an extension last year.

The extension included a new pitch roof on the garage at the gable end, but water continues to appear down the inside wall of the garage despite triple layers of flashing.

Our builder has been very helpful in trying to resolve this, but we all think the external plastering is so old it is now porous and allowing rainwater seep through and down the concrete block.

Before I go re-plastering the whole house, have you ever come across this problem before and are we missing something?

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The alterations you describe are typical where an old flat roof is replaced by a pitched roof during refurbishment or the conversion of an attached garage. The problem here relates to the continuing ingress of water.

The question is whether this is as a result solely of poor external render or are other factors at play.

Your house is 35 years old, and therefore the render should not be at a point where it is failing. It is possible however that the render could have been of poor quality at the outset. It may not be performing correctly as a result.

However, the replacement of the render may not solve the problem. Therefore, I would not proceed to condemn or replace the render without a correct diagnosis of the defect at hand.

There are a number of additional issues to consider when dealing with this type of water ingress. The gable wall to your home is an external wall once it projects through the new pitched roof over the garage.

However, this wall also forms and internal wall within the accommodation below, typically separating the original house from the garage and side extension.

It follows, therefore, that water that enters the wall at high level may percolate down within the wall thickness and either evaporate or escape into the room below.

If you imagine a twin leaf wall, with an inner and outer blockwork leaf and a cavity between, you can picture the outer leaf of blockwork becoming wet or saturated during heavy or wind driven rain. This water can travel down the internal cavity and into the room below.

Damp proof

It is for this reason that a damp proof course (DPC) called a cavity tray will typically be introduced at the roofline where the penetration of the wall takes place.

The insertion of a tray across the cavity is necessary to prevent the percolation of water into the internal space.

If you have not installed a DPC into the cavity at the new pitched roof line and also made provision for the escape of water from the wall structure this may be the source of the problem.

Houses built 35 years ago may have been constructed using hollow blocks. These are semi-solid.

This means that a DPC cannot be inserted and the eradication of ingress can be more difficult.

A good quality well-finished render coat on the gable is required in these cases.

Although I’m slow to recommend brush applied sealers these can be useful in these situations, particularly if the existing render appears to be in good condition.

The sealer, if it is successful, would need to be reapplied every few years.

Cold

A third issue that can arise is cold bridging. Again, as the gable wall travels through the roof structure, the wall can transfer cold temperatures from the outside to the inside.

The cold surface internally can then be prone to condensation and dampness. This issue is difficult to resolve in older buildings.

When diagnosing any building defect, it is imperative to firstly identify the actual form of construction used.

If cavity or twin leaf construction is used has a tray been provided? Is the wall of hollow blockwork and is there evidence of direct ingress through the semi solid structure?

Is the render of sufficient quality to accept a brush applied sealer? What insulation is used and are thermal breaks inserted to prevent cold bridging or is improvement required?

An inspection by your local chartered building surveyor should help to quickly identify the forms of construction used and the potential route of entry of water.

With the assistance of your builder and perhaps some minor opening up your surveyor should quickly provide a correct diagnosis and a practical solution.

With all building defects, as with any ailment, correct diagnosis is the key to successful treatment.

Noel Larkin, is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie