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A warm roof is where the insulation is laid above the decking and the vapour control layer. The vapour control layer is to stop moisture moving up into the insulation. The process of installing a warm roof offers a greater level of thermal insulation, for the whole property. And it is easy to install if the existing roof deck is in good condition.
• The insulation is in a continuous quilt which means there is less thermal bridging.
• The cost of installation is lower, as all the work can be carried out from above the roof.
• It is a simple installation meaning less can go wrong.
• A warm roof construction leads to a higher roof depth as the insulation is above the roof joists. This means a warm roof may not be suitable if features of the building prevent the roof being more than a certain height.
• Due to the insulation the final finish of a warm roof construction is soft. This means a warm roof is not suitable for foot traffic. However, there are options to make a warm roof suitable for foot traffic. This includes decking laid above the insulation and a harder roof covering.
On a new flat roof construction the first step is to lay the vapour control layer on to the decking. On an existing roof the decision needs to be made on what needs to be done for the installation to commence. Sometimes the insulation can be laid directly on top of the flat roof and other times the roof covering will need to be removed and prepared before a vapour control layer is applied.
This vapour control layer controls how much vapour can pass through it into the insulation. If the decking is not particularly good, it is important to invest in a high quality vapour control layer. The vapour control layer should go up vertically around any of the flat roof's features. Features include walls and skylights. The vapour control layer should go to the height of the insulation.
Next, the insulation can be laid, it is important to keep the insulation tight and butted up against all the of the walls and roof features. It is best to fix this insulation using thermally broken fixings. However, the insulation can also be glued to the vapour control layer. Any gaps in the insulation must be back-filled using foam. Any skylights are fitted over the insulation to eliminate any possibility of thermal bridging. It is also important to fill any exposed gaps in the roof joists with foam. The end of the joists should be treated with primer. If any timber fascia boards are to be installed these should also be primed.
Many contractors use glue to fix the insulation of a warm roof to the vapour control layer. However, the problem with this is the glue is only as good as what it is adhered too. Often, the foil attached to the insulation board will come away from the insulation. It glue is to be used it is important to ensure it is binding quality insulation to a quality vapour control layer.
Contractors also often use nails to secure the insulation to the vapour control layer. There are two problems with this, the first one is the nails are not galvanised. If the nails are not galvanised they will deteriorate quickly. This means within just a few years the nails may lose their function and cause weaknesses in the warm roof. Another problem is if there is movement in the roof from foot traffic, cracks in the roof covering can occur. This is because the nail is solid and does not allow for movement. What often happens here is the nail breaks through the roof covering leaving a hole. Although above we described the problem using nails as an example the same can apply for screws.
The way around these problems is to use thermally broken fixings. This is where a screw is inserted into a tubular fixing. The screw goes into the fixing and once fully inserted allow for a small amount of movement. This eliminates the problem of foot traffic causing cracks in the roof covering. Instead, the screw just moves up and down inside the tubular fixing. Another problem this tubular fixing is they are thermally broken. This is because the metal screw is inserted inside the tubular fixing meaning the metal screw does not run straight through from the inside of the roof to the outside of the roof. This disconnection of the metal screw from the inside to the outside because of the plastic tubular fixing is why they are referred to as thermally broken fixings.
If you would like RJ Evans to provide any information relating to warm roof construction, please contact us or call us now on 01277 375 511. One of our friendly flat roofing specialists will be delighted to help.