Rafter insulation

Rafter insulation exerts shear and wind forces and the fasteners need to be able to absorb them. With its cylindrical head for easy countersinking, fixing thread, CUT point and WIROX surface for corrosion protection, SPAX Iso not only facilitates work but also ensures top safety.

The main facts at a glance:

  • Boost the energy efficiency of your roof with rafter insulation. However, since this involves completely re-covering the roof, such a project is also extremely complex. 
  • Because rafter insulation exerts high shear and wind forces on the roof, you will need screws that can reliably withstand them
  • If you are using soft insulation material for your project, make sure you go for SPAX ISO – otherwise, the insulation layer will be squashed and will lose its effect. 

Are you looking for particularly efficient, clean insulation for your roof? If so, rafter insulation is the answer. You simply place hard or soft insulation boards directly on the rafters and fasten them from above using a batten construction. This has the advantage that the insulation boards create a continuous layer, thus preventing thermal bridges. It also means that no room height is lost inside, since the insulation lies on top. This offers particularly efficient energy saving. However, the outlays for this project are by no means cheap – because the entire roof has to be completely re-covered, it is a good idea to opt for rafter insulation, especially if major renovation work is planned anyway.

What else should be kept in mind?


Rafter insulation causes two particular types of forces to be exerted on pitched roofs, which fasteners must absorb. On the one hand, shear force, which occurs because the insulation naturally wants to slide down the rafters. On the other hand, the suction force of the wind, which tries to lift the insulation complete with covering off the roof in the lee. In order to withstand both forces, screws are required that reliably stop the shear forces and hold the rafters and battens together even in the strongest of winds.


Another thing that needs to be considered with soft insulation material is that normal screws would cause soft insulation to be squashed between the rafters and battens, rendering much of its effect useless. In order to prevent this, you need a screw with a shank geometry that holds the lower screw thread securely in the rafter, has a smooth shank inside the insulation and is equipped with a fixing thread below the screw head to ensure spacing between the battens and rafters. SPAX with partial thread are suitable for hard insulation.

What solutions does SPAX offer?


The SPAX range offers craftspeople and DIY enthusiasts the perfect solution for both soft and hard insulation materials. For hard insulation material: SPAX with partial thread, whether washer head or flat countersunk head with WIROX surface and slide coating. For soft insulation material: SPAX ISO. In contrast to the washer head screw, this has a fixing thread below the head, which holds the counter batten firmly in place and transfers the tensile force. This also creates space between the counter batten and the insulation, thus ensuring it can maintain its shape. With its cylindrical head for easy countersinking, 4CUT point and WIROX surface, it also offers maximum corrosion protection.

Discover now our screws for rafter insulation


2 Products

Facade screw, Fixing thread, raised countersunk head, T-STAR plus, 4CUT, stainless steel A2 antique
Façade screw, stainless steel A2 antique
  • Head Shape: Raised countersunk head
  • Drive: T-STAR plus
  • Material: Stainless steel A2
  • Point: 4CUT
  • Thread Shape: Fixing thread
Façade screw, partial thread, raised countersunk head extra small, T-STAR plus, CUT point, stainless steel A2
Facade screw stainless steel A2, extra small head
  • Head Shape: Raised countersunk head
  • Drive: T-STAR plus
  • Material: Stainless steel A2
  • Point: CUT
  • Thread Shape: Partial thread

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