Flat roof decking in old roofs used to be like floor boards and was called a close boarded roof deck.
Chipboard was used extensively in the 1970s and flat roofs were laid using this. It is not a very efficient type of material to use as when the flat roofs leaked the chipboard would become water logged and soggy, in-fact the roof would often become unstable and collapsed.
- Builders have gradually avoided using this substandard type of material and started using a product called stramit board or strawboard. This material consists of straw mixed with cement and formed into sheets which offer strength and insulation all in one.
- Realistically there is absolutely nothing wrong with stramit board or straw boards. We still come across flat roofs installed with it now and then and providing it has been installed with plenty of support it will still perform quite well as a flat roof decking.
The problem is that flat roofers in the past bonded the existing felt and when it is removed the original installed felt rips the board apart.
Ply came on the market and the vast majority of flat roof deckings were laid using shuttering ply. Unfortunately shuttered ply has a tendency to rot when it is penetrated by rain water.
This type of ply has recently come down in price and unfortunately so to has the quality. Some of these types of plywoods laminate with the smallest amount of damp getting to it!
OSB has now come onto the market and is regularly specified by architects and flat roofing material suppliers as the substrate of choice when replacing the decking to new flat roofs.
Personally we like using OSB and we tend to go for the higher quality T+G sheets, these are easier to lay and support the flat roof better when butting boards together.
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