EWI to lower pitched, or flat roof

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for external wall insulation to the top of an existing flat or gently sloping, lower roof, projecting from an external wall to be insulated. For use where the existing flashing to the projecting roof is to be retained, but with new flashing over the junction with EWI and existing roof.

Detail also indicates junction between woodfibre insulation system and insulation suitable for splash zone. EWI system is cut to slope of lower/projecting roof and flashed over to prevent any gaps in insulation.

Technical description

Section detail. EWI to external wall at junction with projecting/lower roof. Junction between woodfibre insulation layer and XPS insulation, cut to slope of lower/projecting roof.

Suggested Installation Sequence:

  1. Fix membrane to existing solid wall and lap in to existing roof membrane.Tape and seal joint.
  2. Fix external wall insulation. Adhesive to side in contact with XPS insulation.
  3. Friction fit water resistant insulation in splash zone between external wall insulation and existing roof.
  4. Fix Stainless Steel flashing. Slot cut into XPS to take lip of flashing. Ensure watertightness.
  5. Render insulation up to flashing.

Reference Specification:

Guidance should be sought from Approved documents C, section 5 and 6; and Document H, sections 2 and 3 for rainwater drainage.

Insulation suitable for use in a splash zone and/or drainage layer. To BS EN 13164. Stainless steel strip flashing to BS EN 10088-2 and BS EN ISO 9445-1 or -2.


Care should be taken to ensure air-tightness. Seal all service penetrations through EWI and existing wall. Lap and tape all membranes around edges of EWI system.

All insulation to be continuous to avoid thermal bridging.

Preparation of external wall: Remove any existing loose material and prepare in accordance with manufacturer's requirements.

Any alterations to drainage and other fixtures, or work to prepare the existing roof, to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the roof structure; institute repairs and/ or additional works necessary to make the existing construction suitable to receive EWI system.

Existing Services:

Rainwater pipes and waste water pipes to be moved to the outer face of insulation as standard, and swan-necked back into gullies where possible. Any work additional to this should be agreed in writing with the contract administrator.

Soil Pipes: An assessment of the condition of soil pipes should be undertaken prior to external wall insulation being undertaken. Drawing assumes soil pipes will remain in-situ. However in some circumstances in order to do this they will need to be replaced, and in others they will need to be moved. This is to be agreed in writing before start of work.

Parts list

  • EWI (Woodfibre), with rendered finish [EW01]

    Composite woodfibre board for use on solid and hard to treat cavity walls.

    • Insulation back board: UdiTherm SK.
    • Insulation top board: Diffutherm NF (T&G, renderable).
    • Thermal conductivity: 0.045W/m2K. GWP 0.
    • Base Coat: levelling on substrate: 90% contact required around edge of boards between board and substrate to prevent thermal bypass. If substrate is very uneven, use base-coat material around edge of board to achieve contact.
    • Fixing: Fixed mechanically to substrate in accordance with manufacturers recommendations.
    • Render/carrier: to manufacturers standard.
    • Render: Udi Perl
    • Decorative finish: tbc
    • System guarantee: 25 years - ECO compliant insurances and guarantees.
    • System Certification : ETAG

    Unger-Diffutherm. Product Reference: UdiFRONT.
    Supplied by: Back To Earth.
    Tel: 01392 861763
    Web: http://www.backtoearth.co.uk/

  • External Insulation at roof abutment [EW01]

    Extruded Polystyrene (XPS) suitable for splash zone and for application of render.

    • Thickness: as EWI.
    • Depth: Minimum 300mm above roof abutment level.
    • Thermal conductivity: 0.033W/m2k. GWP: <5.
    • Joints: Tight butt joints

    BASF. Product Reference: Styrodur 2800C insulation.

  • Woodfibre EWI adhesive [EW01]

    Adhesive for use at joint between perimeter/ plinth insulation and external wall insulation.

    Back to Earth. Tel: 01392 861763
    Web: http://www.backtoearth.co.uk/
    Product Reference: UdiSpecial Adhesive.

  • Stainless Steel Apron Flashings to Lower Roof Abutments [EW01]

    H75 stainless steel strip or sheet coverings and flashings to BS EN 10088-2 and BS EN ISO 9445-1 or 9445-2.

    • Manufacturer: Contractor's Choice.
    • Type: Austenitic alloy grade 1.4301 (304).
    • Mill finished.
    • Thickness 0.4mm.
    • Minimum lap to roof covering: 150mm.
    • Fixing into insulation to splash zone: Contractor's choice.
    • Masonry joint sealant: High modulus elastic sealant, colour to match render.

    Contractor’s choice.

  • Damp Proof Membrane. [FL06]

    • Manufacturer: Contractor's choice.
    • Product Reference: Agrément certified.
    • Recycled Content: Contractor's choice.
    • Thickness : 250 micrometres (1000 gauge)
    • Joints: Bond and seal as manufacturer's recommendations.

    Contractor's choice.


  1. Admin - 2015-04-29 12:06:55 [Displayed version]

    EW01 parts list contact details updated. DWG uploded. Technical description amended to remove references to "perimeter insulation"

  2. Admin - 2015-04-29 12:04:28

    EW01 parts list contact details updated. DWG uploded.

  3. Admin - 2015-03-27 16:33:38

    Preview image and .pdf file updated.

  4. Admin - 2015-03-24 12:29:28

    General re-formatting. Introduction revised and expanded. Technical description expanded with Approved document links and installation sequence. Parts link suppliers links added. Damp proof membrane added to parts list.

  5. Admin - 2015-03-13 11:38:38

    Suggested sequence updated. Introduction updated.

  6. Admin - 2015-03-13 11:37:22

    Suggested sequence updated.

  7. Admin - 2015-03-13 11:36:15

    Introduction and technical spec. updated and revised. Parts list added.

  8. Admin - 2015-02-27 16:30:10

    Version 1

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