Roof Insulation for Increased depth Rafters

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for the internal installation of woodfibre insulation to an existing roof; where existing rafters are not deep enough to allow 200-250mm insulation to be installed. In this detail the existing rafters are extended with additional timber to fit insulation to the required depth.

To be used only where a suitable vapour-permeable roofing membrane is present above. (check brand and sd value upon inspection of roof).

Target U-value : 0.18W/m2K

Technical description

Section detail. Roof insulation to existing room-in-roof with no existing sarking or vapour permeable membrane present.

Suggested installation sequence:

Preparation of roof and loft space: Remove any existing loose material and prepare in accordance with manufacturer's requirements.

Any alterations to cabling, existing ventilation, and other fixtures, affected by the work to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the roof internally and externally, especially any structural timbers; institute repairs and/ or additional works necessary to make the substrate suitable to receive insulation.


  1. Removal of any existing light fittings, set aside for re-use. Remove any existing timber mouldings.
  2. remove existing ceiling to expose rafters.
  3. Check condition of existing breathable roof membrane.
  4. Fit new timbers to existing rafters.
  5. Fit flexible insulation between rafers. Fit vapour barrier
  6. Ensure air and vapour barrier airtightness.
  7. Fix new plasterboard to rafters. Plaster to face of board.
  8. Re-instate/replace and fixtures/ fittings/ mouldings as required.

Care should be taken to ensure air-tightness. Seal all service penetrations through insulation and roof. Lap and tape all membranes around edges to existing membranes or as shown on detail.

All insulation to be continuous to avoid thermal bridging. Phenolic foam eaves insulation: To BS EN 13166 or Agrément certified.

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

Approved Document F (ventilation) should be referred to with regards to ventilation to existing roofs and over eaves ventilation.

Any work should be checked that it does not affect existing routes of escape, access and facilities, internal and/or external fire spread to linings and/or structure in event of fire. Refer to: Approved document B (fire safety) for guidance for both dwellings and non-dwellings for existing buildings.

Approved Document A (structural safety) should be referenced when working with existing roof timbers or other roof structural members.

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.

Parts list

  • C16 Strength Timbers [RF02c]

    To increase depth of existing rafters. Fixed to existing rafters

    • Strength class: C16.
    • Depth: to increase existing rafters to 250mm.
    • Species: contractor's choice.
    • Centres: as existing.
    • Treatment: Not required.
    • Certification: All timber to be FSC certified (or re-used from existing on-site).

    Contractors choice.

  • Insulation Boards/Batts Fitted Between Rafters [RF02c]

    Woodfibre batts, friction fitted between rafters.

    • Thickness 250mm.
    • Width to suit rafter spacing.
    • Thermal conductivity: 0.40W/m2K. GWP 0.

    Unger-Diffutherm. Product Reference:Reference: UdiFLEX
    Supplied by: Back To Earth.
    Tel: 01392 861763

  • M20a Single layer plasterboard backings [RF02c]

    Vapour check plasterboard.

    • Thickness: 12.5mm thickness
    • Edge profile: Tapered. Joints taped for air-tightness.
    • Fixing: nail fixed as manufacturers recommendations.

    Contractors choice

  • M20a Single Coat Board Finish Plaster [RF02c]

    Two coat (undercoat+finish) plaster system.

    • Final Coat: Gypsum finish plaster 2-3mm applied in single coat.
    • Surface preparation: Tape joints.
    • Finish: Smooth.

    Contractors choice

  • Intelligent air and vapour control layer [RF05b]

    Intelligent breather membrane.
    Fixing: Laps And Edges: Sealed with vapour-tight adhesive tape. At floor and ceiling junctions ensure is appropriately sealed.

    Options for intelligent membrane


  1. Admin - 2015-04-30 10:55:29 [Displayed version]

    DWG uploaded. General formatting work to parts list.

  2. Admin - 2015-03-30 11:40:44

    Preview image and detail pdf updated. Title amended to match drawing.

  3. Admin - 2015-03-28 18:04:39

    Intelligent membrane amended in parts list.

  4. Admin - 2015-03-19 13:28:25

    Version 2 -Title updated. Errors is introduction removed.

  5. Admin - 2015-03-19 13:27:48

    Title updated. Errors is introduction removed.

  6. Admin - 2015-03-18 10:36:36

    Version 2- Introduction expanded with additional information. Technical description expanded with BS codeas an approved doc links. Parts list codes and manufacturers links added.

  7. Admin - 2015-03-04 14:26:13

    Version 2- preview image revised.

  8. Admin - 2015-03-04 14:20:27

    First version

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