EWI verge detail at flat roof

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for the junction of an external wall insulation system with an existing flat roof, where no additional insulation is applied to the existing roof buildup. To retain the existing parapet, the EWI system is carried up beyond the existing roof level to form a new parapet detail.

A new waterproof membrane is lapped under the existing mastic roof trim, and carried over the EWI to prevent water ingress into the woodfibre insulation system. New flashings are used to cap the top of the EWI parapet, and are glue fixed to the membrane beneath.

Technical description

Section detail. EWI to existing wall up to existing flat roof. Form verge.

Suggested Installation Sequence:

  1. Lift/cut back roof trim over fascia board. Tape additional waterproofing membrane to back edge of trim, over existing membrane.
  2. Fit insulation to external wall and over fascia board. Render.
  3. Fit cap flashings. Glue edge of membrane to inside face and fix flashing into insulation.

Reference Specification


New flashing and membrane to BS EN 10088-2 and BS EN ISO 9445-1 or -2. with sealant to flashing to BS EN ISO 11600.

For all works to existing roofs refer to Approved document C (resistance to contaminants and moisture), with reference to Section 2, and section 6.

Reference should be made to Document H (drainage and waste disposal). Document L1b should be referred when working on existing dwellings, with particular attention to sections 5 and 6 regarding material alterations. Document L2b should be referred to with existing buildings other than dwellings.


Membranes/Underlays: Butt jointed onto dry substrate, edges fixed with steel staples, or extra large head clout nails. Avoid tears and punctures when handling. Do not lay over eaves and drip/ step stainless steel underlaps. Keep dry and cover with stainless steel at the earliest opportunity.

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 existing roof and wall structure institute repairs and/ or additional works necessary to make the existing construction suitable to receive EWI system.

Parts list

  • External Wall Insulation (Woodfibre), 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/

  • Stainless Steel cap Flashings [RF09]

    Flashing at junction between flat roof verge and external wall insulation.

    • To BS EN 10088-2 and BS EN ISO 9445-1 or -2. with sealant to flashing to BS EN ISO 11600.
    • Manufacturer: Contractor's choice.
    • Type: Austentic alloy grade 1.4301 (304).
    • Thickness: 0.5mm.
    • Finish as delivered: Mill finished.
    • Minimum lap: as per drawing.
    • Fixing: Wedge fixed into joints, chases and damp proof course joints.

    Contractor's choice

  • Airtightness works to ceiling [DP03]

    Seal all ceilings and repair any cracks in plaster to ensure no air-leaks.

    • Product: Standard decorators caulk
    • Location: All ceilings

    Contractor's choice

  • Roof Underlay [RF06g]

    Roof underlay. Sealed to phenolic foam insulation and dressed to drainage point at the end of each run of guttering to allow water to exit construction.

    Green Building Store. Tel: 01484 461705.
    Web: http://www.greenbuildingstore.co.uk
    Product Reference: Solitex Plus


  1. Admin - 2015-04-29 11:24:36 [Displayed version]

    General formatting changes. DWG file updated.

  2. Admin - 2015-03-27 16:24:23

    Preview image and .pdf file updated.

  3. Admin - 2015-03-27 12:39:27

    General error removal. Codes amended so concurrent with drawing.

  4. Admin - 2015-03-23 16:16:20

    General re-formatting. Technical description expanded with installation sequence. Parts list expanded and parts added.

  5. Admin - 2015-03-23 16:15:37

    General re-formatting. Technical description expanded with installation sequence. Parts list expanded and parts added.

  6. Admin - 2015-03-13 15:48:10

    Technical description and parts list updated.

  7. Admin - 2015-03-11 15:52:05

    Parts list, introduction and technical description added/updated.

  8. Admin - 2015-02-27 12:56:18

    Version 1

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