6.2 External timber framed walls
6.2 - Appendix 6.2A | |
Certification | |
6.2 - Appendix 6.2B | |
Packing under timber sole plates | |
6.2 - Appendix 6.2C | |
Differential movement |
Appendix 6.2-A
Certification
Certifiers
The timber frame certifier is required to complete and sign a certificate confirming that he has assessed the structural adequacy of the timber frame superstructure for a specific project.
The completed and signed certificate must be given to the registered builder.
Certifiers must be suitably qualified civil or structural engineers with at least three years' experience in timber frame construction and Chapter 6.2 'External timber framed walls'. The certifier must not be the designer of the timber frame.
Where the design is four storeys or more and the floor joists are solid timber the design is to be submitted to NHBC.
Applications to become a timber frame certifier should be made to NHBC Standards and Technical.
Registered builders
Registered builders should ensure that the completed timber frame certificate is available on site for inspection by NHBC.
Appendix 6.2-B
Packing under sole plates
The finished surface of the substructure supporting the timber frame should be reasonably level. However, where packing is required to ensure the timber frame or sole plate is level, the following three alternative options are acceptable.
The options given are for packing up to 20mm. Packing exceeding 20mm should be agreed between the timber frame manufacturer's engineer and NHBC.
1 Permanent structural packing under sole plate
The sole plate is levelled on temporary spacers. Once the first lift construction - wall panel and first floor floor structure (or roof structure in a single storey building) - has been erected, permanent packing is placed under the sole plate.
This permanent packing can be:
- free flowing non-shrinkable grout for the full length and width of the sole plate, or
- individual packers placed under each load point (e.g. stud or post).
2 Bedding of sole plate
The sole plate is laid and levelled on a continuous bed of mortar, prior to the erection of the wall panels. The bedding should extend the full width of the sole plate. Care is needed to ensure that the bedding is not disturbed during the fixing of the sole plate.
3 Doube sole plate 'sandwich'
The lower sole plate is fixed to follow the contours of the supporting structure. The upper sole plate is fixed on top and levelled with temporary spacers inserted between the sole plates. Once the first lift construction has been erected permanent packing is inserted under each load point (e.g. stud or post).
Because this method introduces an additional sole plate the extra shrinkage should be taken into account. See Appendix 6.2-C.
Notes
- Permanent packing should be designed and approved by the timber frame designer to suit the horizontal and vertical loads on the sole plate and should be at least the same plan area as the load points (e.g. studs or posts). Hollow plastic packing with reduced bearing surfaces is not acceptable.
- Temporary spacers can remain in place providing they are durable and non-degradable.
- Grout and mortar are both unsuitable for gaps less than 5mm and in such cases permanent packing should be used.
Appendix 6.2-C
DIFFERENTIAL MOVEMENT
INTRODUCTION
During the first two years after erection the timber frame will reduce in overall height as it dries out. Movement will occur between the timber frame and other parts of the structure.
This Appendix contains:
- guidance on differential movement of the timber frame, and
- common details for timber frame construction where differential movement will take place.
The sketches show examples of typical interfaces and illustrate general design principles.
In the absence of project specific calculations the gaps shown in the following table, are required:
Table of differential movement
Gap location | Opening and Closing gaps (mm) | |
Floor joists | ||
Solid timber (mm) | Engineered I-joist (mm) | |
Eaves/verge | Add 5mm to gap dimension at level below | |
Sixth storey | Specialist calculations to be submitted to NHBC | 61 |
Fifth storey | 53 | |
Fourth storey | 45 | |
Third storey | 45 | 35 |
Second storey | 35 | 25 |
First storey | 20 | 15 |
Ground storey* | 5 | 5 |
*Ground storey or lowest level of timber frame.
Notes
The gap sizes are based on the following :
- timber joist and rim beam/header joist maximum depth 240mm
- timber frame floor cross section is as shown below with maximum 45mm deep timber plates/binders
- single head binder at the eaves. Maximum double sole plates
- calculations, where required, to be based on BS EN 1995-1-1
- timber components are not saturated and normal moisture contents at the time of construction (e.g. less than 20%) and tight jointed construction
- movement gaps in excess of 35mm should be protected by cover strips
- the table allows for a 2mm thickness of compressible material in closing gaps. Check the manufacturer's product details
- outer leaf brickwork with expansion rates no greater than 2.5mm per storey
- brickwork up to 5 storeys with lightweight cladding above 5 storeys
- lightweight cladding - floor level joints to be 15mm for solid timber and 10mm for engineered I- joists.
- the ground floor is concrete. For ground floors of timber joists add 15mm for solid timber and 10mm for engineered I-joists
Timber frame construction on which the table of differential movement above is based.
Location of opening and closing gaps at windows.
COMMON DETAILS
The following sketches show examples of typical construction details and illustrate general principles relating to differential movement.
Further information is available from the UKTFA document 'Guidance on detailing to accommodate differential movement in timber frame buildings'. This UKTFA publication includes guidance on:
- the interface between a lift shaft and the timber frame
- the interface between lightweight cladding to the timber frame and balconies
- the support of roofs above lift shafts etc. constructed from masonry or concrete
- staircases within the timber frame
- services within the timber frame
- additional chimney details.
In the following sketches:-
= downward movement of the timber frame
= upward brick expansion (taken as 2.5mm per storey of clay masonry).
The design should ensure that the material used in movement joints will safely accommodate the amount of expansion or compression expected and, where required, provide a weather resistant and durable joint. See Clauses 6.2-D6 and S2.
Normal reveal
Check reveal
As built
After movement
WINDOW HEADS WITH MASONRY CLADDING
(movement gaps in excess of 35mm should be protected by cover strips) (cavity trays omitted for clarity)
As Built
After movement
As Built
After movement
As Built
After movement
WINDOW HEAD AND SILL WITH LIGHTWEIGHT CLADDING
(movement gaps in excess of 35mm should be protected by cover strips)
Head with lightweight cladding
Sill with lightweight cladding
ROOF TO CHIMNEY ABUTMENT
As Built
After movement
TIMBER FRAME INTERFACE WITH CONCRETE OR MASONRY STAIRS AND COMMON AREAS
As built
After movement (transition piece removed)
EAVES (movement gaps in excess of 35mm should be protected by cover strips)
As Built
After movement
VERGE
As Built
After movement
SERVICES
As built
After movement
DRIVE THROUGH
As built
After movement
Section A - A
Section B - B
BALCONY ABUTMENTS (movement gaps in excess of 35mm should be protected by cover strips)
Balcony - lightweight cladding
As built
After movement
Balcony - masonry cladding
As built
After movement
Juliet balcony
LIGHTWEIGHT WALL CLADDING - JOINT AT EACH FLOOR LEVEL (with and without insulation in cavity)
As built
After movement
LIGHTWEIGHT CLADDING AND MASONRY PLINTH
As built
After movement
WALLS TO FLAT ROOF ABUTMENT
As built
After movement