How to Replace and Insulate a Damaged Flat Roof Deck

30 June 2020 EcoTherm Insulation
With occasional care and maintenance, a flat roof should provide lasting protection against the weather. However, if issues such as slow leaks or water ponding aren’t dealt with, they can eventually damage the roof structure, either rotting the deck and supports, or causing them to bow or crack under the weight. In this blog we look at how roofing contractors can tackle a typical re-roofing project and provide tips on how to speed the process up with time saving solutions such as pre-insulated flat roof decking.
How to assess and strip flat roof

First inspect the ceiling below for signs of damp or bowing – making a careful note of where these are located. Next, use  a ladder or scaffold to look for obvious issues such as tears, loose felt, water ponding or bulges in the membrane (these are caused by water getting beneath the membrane and then evaporating).
Once you have identified any likely problem areas, move on to the roof surface, taking care to keep your weight above the rafters. It is usually possible to remove the existing waterproofing with standard hand tools such as a spade or pry-bar. As you strip the surface away, keep an eye out for areas of rotten or damp decking. Once the water proofing membrane layer is removed you can pull up the affected decking boards and assess the condition of the rafters beneath to determine whether these will need to be replaced or strengthened.

How much insulation should be installed when replacing a flat roof?

In many cases, older flat roofs will have been fitted without insulation – meaning lots of heat loss and higher energy bills. If you are replacing the roof, it will need to be properly insulated to comply with the Building Regulations/Standards. Typically, this means it will need to achieve a U-value (thermal performance) of 0.18 W/m2K (you can find out more about U-values and other common insulation terminology here).
By fitting insulation with a lower thermal conductivity (therefore insulated more effectively), you can reach this target with a thinner construction. This is particularly important as re-roofing projects will typically be warm roof constructions (where the insulation is fitted above the rafters) and this can lead to fairly sizeable constructions if lower performing insulation materials are used. Our polyisocyanurate (PIR) flat and tapered roof insulation boards have a low thermal conductivity, making them a great choice for these applications.
One of the easiest ways to check how much insulation you’ll need is with our U-value calculator. Simply enter the construction you’re using and the target U-value and it’ll give you a recommended thickness and build-up. For added assurance you can even your output to be checked by our specialist technical team.

How can you save time with pre-insulated flat roof decking?

We offer a range of flat and tapered roof insulation solutions to suit your project requirements. For re-roofing projects where speed is essential, our pre-insulated flat roof decking board, EcoTherm Eco-Deck, offers a great solution.
With Eco-Deck, the insulation comes pre-bonded to a 6 mm layer of exterior grade plywood on one side and can be installed directly above the rafters. This means you can skip the step of having to lift and fix deck boards separately – saving time and helping to get the roof fully weathertight more quickly.
The installation process is also straightforward – first check the rafters carefully to ensure they’re clean, free of nails and angled to the correct fall. You’ll also want to measure out the roof for the Eco-Deck boards, fixing 50 x 50 mm (2” x 2”) noggins in any areas where the board edge won’t be supported by a rafter or will be cut to fit around features such as rooflights.
Next, apply a continuous bead of mastic across the top of all these supporting timbers then lay the Eco-Deck boards over the top with the plywood side facing up. Take care to ensure that all edges are supported by at least 20 mm of supporting timber and that long-edges run over the centre of joists. You can then screw them into the rafters/noggins at 300 mm centres, ensuring the fixing points are at least 10 mm from the edge and 50 mm from the corner, and that the screws are long enough to penetrate the rafters by 35 mm.
The deck is now ready for the waterproofing system. Eco-Deck supports a range of options including mechanically fixed single-ply waterproofing membranes, mastic asphalt, roll & pour and adhered systems. Whichever option you choose, make sure you fit a 25 mm thickness of insulation on any upstands.

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EcoTherm Insulation Ltd

Harvey Road

Burnt Mills Industrial Estate



SS13 1QJ

01268 591155