What flat roof substrates are suitable for mechanical fixing?

It can be used with most types of roof deck substrates, but there are some exceptions where mechanical fixing is not recommended or where you will need to undertake testing before starting installation. It's also important to be aware that some substrates you may encounter on refurbishment projects are no longer considered suitable for flat roofing applications.

Here we look at the suitability of mechanical fixing for a wide variety of different roofing substrates.

• Steel

  Steel substrates on flat roofs must have a minimum thickness of 0.7mm for mechanical fixing, unless specialist fasteners are used (such as peel rivets)

• Aluminium

  There are different grades and thicknesses of aluminium decks. If the aluminium is at least 1.2mm thick, it may be possible to fix into it using a threaded stainless steel fastener, but we would recommend a fastener pull-out test is carried out. If the result achieved is unacceptable or the deck is less than 1.2mm thick, it may be possible to use a peel rivet, and again, if peel rivets are to be used, a pull-out test is recommended

• Composite panel

  There are many different types of composite panels with different top skins. A site investigation to establish the type and thickness of the top skin is recommended. It is also important to check the top skin is held securely in place. If mechanically fixing is considered to be a viable option, a fastener pull-out test is recommended

• Standing seam

  Two examples of standing seam roofs include the traditional standing seam roof, and the modern self-supporting standing seam roof. A site investigation to establish the type and thickness of the top skin is recommended, and it's important to check the top skin is securely held in place. If mechanically fixing is considered to be a viable option, a fastener pull-out test is recommended

• Plywood

  To conform with BS EN 1995-1-1, plywood used for flat roof substrates must be a minimum of 18mm thick. In some cases, it might be possible to fix to a thinner deck using specialist fasteners such as peel rivets - however, a fastener pull-out test is recommended. Alternatively, it may also be possible to fix through the plywood to the timber joists

• Oriented Strand Board (OSB) 3

  OSB 3 must be at least 18mm thick to comply with BS EN 300. As with plywood, it may be possible to fix to a thinner deck using specialist fasteners such as peel rivets - however, you should consult your fastener manufacturer before starting installation, and a fastener pull-out test is recommended. Alternatively, it may also be possible to fix through the plywood to the timber joists

• Closeboard timber

  For mechanical fixing, Closeboard timber must have a minimum thickness of 25mm. It can be possible to fix to a thinner deck using specialist fasteners, such as peel rivets - however, you should speak to your fastener manufacturer for advice before starting installation, as a fastener pull-out test is recommended. Alternatively, it may also be possible to fix through the timber deck to the timber joists

• Chipboard

  Chipboard is not a suitable substrate. If this has been used, it should be removed. If it's not possible to remove, it may be possible to overlay with a new structural deck. The new deck will need to be secured through the chipboard to the timber joists, and the extra weight of an overlay and new roof might need to be approved by a structural engineer

• Structural Insulated Panel Systems (SIPS)

  SIPS panels are generally manufactured using plywood or OSB sheets. Firstly, the thickness of the plywood or OSB needs to be established: if it's 18mm thick or more, then it should be possible to mechanically fix the roof. If it's less, please refer to the guidance for Plywood and OSB (see above)

• Concrete substrates

  There are different types of concrete decks. These can be broken down into two main groups: cast-in-situ & pre-cast. Cast-in-situ is where the concrete is poured on-site, and pre-cast is where the product is manufactured in a factory

  • Cast-in-situ concrete: Traditional formwork is erected, the concrete poured, and the formwork removed, or the concrete is poured on a structural metal deck

  • Pre-cast concrete: Examples include bison-type hollow beam, concrete planks, and beam & block. Another example is Reinforced Autoclaved Aerated Concrete (RAAC). RAAC must be checked by a Chartered Structural Engineer or Chartered Building Surveyor before work commences to ensure the substrate is structurally sound. Refer to SPRA technical guidance S16/22, and check out our article here: Can I fix into a Siporex RAAC deck?

  Some concrete decks are covered with a screed to provide a smooth surface. Screed can also be used to create a fall to direct water to a gutter or rainwater outlets - this is sometimes known as a laid to falls screed. Screeds are not normally structural so cannot be fixed into - you must fix through the screed to the structural concrete below. This will require different length fasteners depending on the thickness of the screed.

  When fixing to concrete, a fastener pull-out test is required.

• Woodwool slab

  HSE classifies woodwool slabs as fragile, so NFRC Guidance Note 43 must be followed. Some woodwool slabs have steel channel reinforcing bars along the edge - normally 1.2-1.6mm thick, at 600mm centres. Depending on the roof system being installed, it may be possible to fix into these - however, if the woodwool does not have these bars, it will be resting on timber joists that are normally at 400- 600mm centres. Again, depending on the roof system being installed, it might be possible to fix through the woodwool to the joists. It may also be possible to fix through the woodwool using a peel rivet, however, a site pull-out test is recommended. If you discover the substrate is made from woodwool, always consult a suitably qualified professional before starting the work

• Stramit

  Stramit is considered to be a fragile deck and can no longer be used as a substrate. If you find stramit on a refurbishment project, this will need to be removed and replaced with a suitable substrate such as plywood, OSB, or metal deck. If it is not possible to remove the stramit you may be able to overlay it, although the extra weight will need to be considered by a structural engineer