In today’s article, we have a close look at some defects to roof frames that we found conducting building inspections in Adelaide suburbs recently. Roof frames, be they timber or steel are a major structural component of any building. They provide the support to the “lid” (roof covering) on a dwelling that protects the rest of the structure from the damage that rainfall and falling damp can cause. If you want to do your home a favour, make sure you maintain the general condition of the roof frame and its covering.
Inspectors should always crawl the roof space during their inspections where possible
Whenever safe and reasonable access is possible, a good building inspector should endeavour to crawl the roof void to inspect the roof frame and the underside of the tiled or metal roof covering details. It just isn’t good enough to simply poke a head through a manhole and call that an inspection. However, it is expected, and accepted, that roof voids have many visual inspection limitations and access issues. Components such as insulation, air conditioning ducting, sarking and the general low method of construction over eave areas can prevent a complete examination of all roof framing elements. Some second level roof cavities often do not even have a manhole fitted to allow access. All these potential limitation issues should be pointed out to the client in an agreement prior to the inspection, and within the report itself if they actually occur.
Roof construction types are many and varied…
Out of all the different roof construction methods available to builders, two seem to be predominately used in residential homes throughout Adelaide, these being the pitched roof, and the skillion or flat roof. Due to a lack of height, access and inspection within a skillion roof is not possible, but most pitched roofs have enough height to enable bodily access to take place. Pitched roofs are given their structure with either conventional raftered or pitched roof framing, or, as is commonly used today, roof trusses.
Our first property located in Aberfoyle Park had two noticeable issues to its pitch gabled timber roof truss system. Firstly, the top cord of the far right hand truss had been modified with over notching to accommodate some nogging members between trusses. As a result, the top cord had split and visible sagging could be seen at the eaves to the rear right corner, and visible undulation to the tiled roofline surface.
Secondly, some of the nail plates to the majority of the roof frame trusses were beginning to separate from the timbers joints, on average by at least 1-2mm. This is known as delamination and was mainly noted to the top apex joints, and to a lesser extent to some web member joints. This can be caused by the truss timbers absorbing atmospheric humidity lifting the moisture content of the timber. Expansion of the timber results, taking the plates with it, and when the timbers dry out they shrink, but the plates stay where they were pushed to.
Fortunately, no significant visible deflection was noted on the day to the roofline surface. However, roof trusses are an engineered product and they rely on the nail plates to connect the members to each other to meet their design requirements. When these nail plates start to separate, failure can occur to the truss components at the joints and potential roof frame collapse is always a possibility in the future. Delamination is a condition that doesn’t improve by itself, so a further assessment by a structural engineer was recommended to ascertain the extent of its seriousness as remedial work may be required as a prevention against more serious and expensive roof failure repairs at a later date.
Another inspection and more defects…
Our next example is a Bellevue Heights home with a similar pitched gabled roof structure, again with timber roof trusses, but with much larger spans. The external roof surface inspection revealed some undulation, albeit minor, to the front left area of the external roofline. When an inspector observes this, a mental note is made to particularly check the internal roof frame within this area, as undulation can in most cases be symptomatic of framing issues. True to this theory, sideways buckling could be seen to the top cords of a number of trusses in this area which seemed to be the result of poor diagonal steel bracing that was not under tension. Steel brace only works in tension and must be fixed to every truss member it crosses to help keep them upright. The timber tile battens also assist with this, but inspection of these members was not possible due to the installation of sarking.
Also, settlement of the bottom cord of one truss to the left of the dwelling over the living room may have been existing as evidenced by visible cornice distress within the living room cornice in one area. When situations like this are occurring with roof frame trusses, it is prudent to err on the side of caution. A good inspector with will always recommend a further investigation by a competent professional such as a structural engineer to prevent against any deterioration, or potential roof frame collapse in the worst case scenario.
To find out whether the roof frame over your head is going to be structurally sound, why not contact us today to check it out before buying that new property.