Substructure thatched roofs
Some people claim that the substructure of a thatched roof has little or no impact on the roof's lifespan. At Verhoek Riet, we think differently. Of course, the quality of the thatch is vital for the roof's lifespan, yet we cannot ignore the fact that faulty substructure often has catastrophic consequences.
An example to illustrate:
Suppose you grab a bottle of soft drink from your fridge. This one still feels nice and dry at first. But what happens if you put the bottle in a warm house for half a minute? Right, then condensation appears on the bottle. The moisture from the hot air settles on the bottle.
This is similar to when hot moist air comes through the roof structure and thatch package. Depending on the thickness of the insulation package, somewhere in the thatch layer, the hot air will encounter the cold outside air and start condensing. The result? Thatch will rot faster and is more susceptible to moss or mould. A good substructure can prevent such problems.
Guidelines for substructures
As a company, we are very committed to the industry. Therefore, we would like to give you more information on the different types of substructures and the steps you need to take to make these substructures condensation-free.
Closed substructure (screw fixing roof)
Screwing on underlayment board without insulationNo vapour barrier is needed; the condensation point is outside the roof. If you do want to vapour seal the hood, always do so on the inside of the board
Screwing on an insulated substructureAlways apply a vapour barrier to the inside of the insulation
Screws on battens on an insulated substructure with space between battens and insulationAlways apply a vapour barrier on the inside of the insulation
Open construction (on battens)
The ideal construction is the open substructure. Unfortunately, we often see homeowners start
insulating and panelling the inside anyway. We, therefore, recommend taking this into account to
prevent condensation early on.
Sewing on battens (open construction) with an insulated and panelled hood on the insideAlways apply a vapour barrier on the inside of the insulation
Influence of condensation on the insulation
Installing insulation, depending on the thickness of the insulation, draws the condensation point inwards. Do you want to install insulation on the inside? Then it is crucial to apply a vapour-proof layer that prevents condensation, which makes sense because if no moisture can penetrate the hood, it also has no chance to condense.
Another important thing to keep in mind: the thicker the insulation is, the colder the roof becomes. As a result, the roof dries less quickly, and moisture (rain, mist) from outside has more time to affect the thatch. Therefore, try to limit the thickness of the insulation as much as possible.
With an insulation layer over 10cm, creating an air cavity between the board and the thatch layer is always advisable. This compensates for the delay in the drying process of the roof. If this is not possible, we recommend using a coarser type of thatch. This allows for more air and ventilation in the thatch package, creating better drying.
Moisture from the cavity wall
Moisture from the cavity wall is often underestimated but is a serious problem. Therefore, ensure the moisture rising through the cavity walls cannot condense in your thatch package. You avoid this by sealing the cavity wall 100% vapour-proof at the top. This applies to all cavity walls, whether internal, external or intermediate.
Always check that the contractor uses a suitable vapour sealant. For example, have the owner check what PU foam the contractor uses during the work or check it yourself. This also applies to the masking of roof panels. Not all PU is vapour-tight, and contractors often choose cheap PU foam. A shame, as this can cause unnecessary condensation problems.
The above points of interest and advice have come about through real-life experience. We heartily recommend that you take this advice seriously, as it will help you avoid many problems.
Do you have any other questions about the construction of your thatched roof or the substructure? Contact us; we will be happy to help!
Verhoek Riethandel B.V.
8281 PN Genemuiden
+31 (0)6 53 39 86 00