EWI to lower flat roof [Version ID #183]

Author details

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Detail summary

Detail of EWI junction with an existing flat or sloping roof projecting form an external wall below the main roof of a building. Shown is junction between woodfibre insulation layer and Insulation suitable for splash zone, cut to slope of lower/projecting roof. For use where existing flashing to be retained on roof. EWI to roof with new flashing installed at joint of insulation to new roof. Appropriate air tightness work to be carried out on ceiling below existing roof.

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.


1) Fix membrane to existing solid wall and lap in to existing roof membrane. 2) Fix external wall insulation. 3) Fix supporting bracket. 4) Friction fit water resistant insulation in splash zone between external wall insulation and existing roof. 5) Fix Stainless Steel flashing. 6) Render.


Perimeter insulation suitable for below ground use and drainage layer. To BS EN 13164. Rainwater drainage: please refer to approved document H, section H3, rainwater drainage for guidance. 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 substrate, to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the structural substrate; institute repairs and/ or additional works necessary to make the substrate suitable to receive EWI system.

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

  • External Wall Insulation (Woodfibre), rendered finish [EW01]

    For use on solid and hard to treat cavity walls. Composite woodfibre board. UdiTherm SK back board, Diffutherm NF (T&G, renderable) top board. \ Fixed mechanically to substrate in accordance with manufacturers recommendations. \ 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. \ Thermal conductivity: 0.045W/m2K. GWP 0.

    Unger \ www.unger-diffutherm.de \ Reference: UdiFRONT. \ Render: UDI Perl

  • External Insulation at roof abutment level [EW01]

    Extruded Polystyrene (XPS) to BS EN 13164. Suitable for below ground conditions and for application of render. \ Thickness as external wall insulation. Depth: 600mm below internal floor level or to footings. Tight butt joints. \ Thermal conductivity: 0.033W/m2K. GWP<5.

    BASF \ http://www.plasticsportal.net/wa/plasticsEU~en_GB/portal/show/content/literature/styrodur \ Reference: Styrodur 2800C.

  • Woodfibre EWI adhesive [EW01]

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

    Unger \ www.unger-diffutherm.de \ Reference: Udi Dispersion Adhesive

  • Stainless Steel Apron Flashings to Lower Roof Abutments

    H75 stainless steel strip or sheet coverings and flashings to BS EN 10088-2 and BS EN ISO 9445-1 or 9445-2. \ Austenitic alloy grade 1.4301 (304). \ Mill finished \ Thickness 0.4mm \ Minimum lap to roof covering: 150mm. \ Fixing to contractors choice, using masonry joint sealant: High modulus elastic sealant, colour to match render.

    Contractor’s choice.


  1. Admin - 2015-04-29 12:06:55

    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 [Displayed version]

    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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