Timber Frame Construction 3A9 Construction Technology Dr S Pavía Dept of Civil Engineering Trinity College Dublin

Lecture Notes tures/

Load bearing wall structure vs frame structure. Timber members and components forming a structural frame which transmits all loads to the foundations. Timber as a structural material. –Favourable strength/weight/cost ratios –Easily joint and prefabricated –Low self-weight facilitates handling and erection and reduces dead weight of the structure –Sustainable material –Combustible but fire resistance is high if members not less than mm section (smaller sections protected by non combustible material) –Pre-cutting and assembly reduces site work –Timber is very stiff in relation to its weight-E value fairly constant

External cladding non-loadbearing, protective and aesthetic roles. Site operation or pre-fabricated. –Variation in the extent of factory construction from simple sheathed stud panels to others including components such as joinery, insulation, cladding, external linings. –Work on site usually limited to erection and fitting out- few companies making and erecting frames on site.

Examples of Prefabricated Panels Kingspan Products Standard Timber Frame Structural Insulated Panels

A frame wall consists of vertical timber members (studs-spaced at mm centres), framed between horizontal members at top (head plate) and bottom (sole plate or piece). Joints are simple butt and nailed, the frame is therefore non rigid and requires bracing to provide adequate stiffness. –Diagonal braces –Board or plywood external sheathing to stiffen the structure.

Main terminology of of wall Butt and nailed joint

Systems of frame construction Platform frame Balloon frame Modified frame Independent frame

Foundations for frame walls In all types, timber structure not in contact with ground moisture. –base wall or foundation beam rising to dpc level –edge of a concrete raft floor A wall or cill plate (base for the structure) to be set, levelled on the dpc and anchored to the foundation by 13 mm holding down bolts at no more than 2.40m centres. A damp-proof course (DPC) is a horizontal impervious barrier in a wall designed to resist moisture rising through the structure from the ground by capillary action. J S Foster 1994

Platform frame –Walls and partitions bear on the platforms formed by the floor structure. –Frames are single storey in height. Balloon frame –Studs are continuous through 2 floors. –First floor joists fixed individually to the studs. Chudley & Greeno 1988

Platform frame I Seeley 1995

Platform frame Wall frames consist of studs at mm centres, framed between a bottom sole plate and double head plates (head binder and lwr head plate- nailed to form a continuous member). Wall frames bear on the floor structures. They are erected by nailing them through the sole plate on to the floor platforms. Ground floor structure: joists nailed to sole plate & header plate. –Closes floor cavities –Completes the platform on which wall frames are erected. Foster1994

Platform frame Upr floor structure: joists built on the ground floor wall frame nailed to head binder and header plate. In platform frame: option of laying the floor before building is closed A provisional sub-floor can be laid on the joists providing the floor structure on which the frame bears- avoiding damage to floor finishings. –Useful working platform –Enhances thermal insulation of ground floor and sound insulation of upper floor. Foster1994

Advantages of platform frame Relatively short members are required for the studs. Storey height panels facilitate transport and site handling. Studs and floor joists align (helpful when working with grid layouts). Cross-grain moisture movement of the floor is the same throughout the building as both external wall frames and partitions bear on floor platforms. Disadvantage: as floor construction must precede erection of walls covering cannot occur as early as with other methods-temporary protection.

Balloon frame Foster1994 Wall frames independent from floor structures. Studs are continuous through 2 floors. Studs extend from sole plate to double head plates at roof level unbroken by floor structures. Sole plate nailed directly to cill plate on the foundation-no headers Studs and ground floor joists bear on the sole plate. Joists face-nailed to studs- stiffening the frame on the other direction.

Balloon frame Foster1994 Upr f. joists bear on a continuous member let into the studs ( ribbon or ledger )- provides lateral stiffening to the long studs.. Continuous cavities from ground to roof between studs linking to similar floor cavities. Cavity barriers (50 mm members cut-in between the studs and joists) must be built at 1 st floor level and between the floor and the wall against fire and smoke spread.

Advantages of balloon frame Wall frames can be erected independent of the floor structure and up to roof level in one lift - covering of the building at an early stage. The continuity of the studs and the face-nailing of joists to studs provides rigidity to the structure. Absence of cross-grain (no breaks by floors) minimises moisture movement in the frame - no differential movement between the cladding and the frame. Disadvantages: –joists and studs do not align; –cross grain movement still occurs (in internal walls)

Prefabrication and joints Traditionally, joints were covered by external sheathing and internal linings-after erection. In fully pre-fabricated systems ‘through the wall’ joints are produced. –Need to exclude rain and wind but ensure easy assembly. Simple joint techniques incorporate: –A seal to act as a vapour barrier and wind excluder near the inner face. –A joint near the outer face that prevents rain penetration and allows condensed warm vapour from the inside to drain away.

Angles and junctions Good tie between adjoining walls provide nailing support for the interior finish and exterior sheathing. Foster1994

Junctions Layout of the structure based on a co-ordinated system of gridlines: incorporate filler pieces at junctions, of the same thickness as that of linings or floor finishes, to maintain the grid. No grid system is followed: set double studs blocked apart at junction. Nailing boards fixed to noggins set between the studs at junction. Foster1994

Elements Internal lining-gypsum plasterboard Vapour barrier. Structural timber frame of vertical studs at 400 or 600 mm centres, nailed with simple butt joints to top and bottom rails, noggings, sole plate and head-factory produced with sheathing board and breather membrane attached. Incombustible insulating quilt. Sheathing board-plywood or other wood material nailed to studwork. Breather membrane to keep rain/draught out allowing wall to breathe. Cavity External cladding I Seeley 1995

Internal lining: –13 mm lining-gypsum plasterboard- incombustible (class 0 spread of flame rating), with taped and filled joints or a skim coat of plaster. It provides fire protection to the timber and added rigidity to the wall frames. Thermal insulation: –Mineral or fibre glass in the form of batts or paper- faced quilt –Rigid polystyrene –Incombustible I Seeley 1995

Timber Frame Stress graded timber required. Joints are simple butt and nailed, the frame is therefore non rigid and requires bracing to provide adequate stiffness. Bracing provided by: –Wood-based sheet material nailed to external frame face –Diagonal bracing (optional) –Internal plasterboard Durability: Any timber used as cladding and rated lower than Class 3 (moderately durable- EN 350-2) should be treated with preservative.

Sheathing/Boarding applied to external frame face: –9 mm resin-bonded plywood sheathing or 25 mm T&G boarding –Functions: Provides rigidity to the structure by bracing the panels. Stiffens the studs in the direction of their smallest dimension. Produces stiffer individual panels for handling & transport Forms a nailing surface for external claddings Improves thermal insulation of the wall. Foster1994

Moisture and vapour barriers -prevent entry of water into frame cavities. Vapour barriers: –Placed on inner, warm side of wall frame –Exclude any warm water vapour passing through inner lining which may condense inside the colder cavity. –Non-ferrous metal foil, polyethylene sheathing, Al- faced plasterboard. –Must be continuous and imperforated-all joints sealed. Moisture barrier: –Placed on the external face of wall frame –Exclude wind-driven rain and moisture from external cladding. –Building paper ‘breather type’-excludes draught and external moisture but permits passage of water vapour. Foster1994

External cladding Wide range (brick, tiles, slates, timber boarding, ply sheathing, cement rendering). When cladding is brick, a 40 mm cavity between cladding and timber frame is required- water penetration & insulation. Galvanised light steel ties are nailed to the frame and built into the brickwork-flexible. A dpc must be inserted into the brickwork extending over the bottom of the cavity and up the face of the frame behind the moisture barrier. Open perpends (‘weep joints’) needed at the bottom course. The cavity must be closed at the top and around all openings by cavity barriers. Foster1994

Types of cladding: self-supporting and non- self-supporting. Horizontal timber boarding. Tile /slate hanging. Render on metal lath. Typical brick clad timber frame wall. TRADA Technology Library, 2010.

Timber structure connected to brickwork by flexible wall ties. Cavity-50mm wide, ventilated. I Seeley 1995

Openings in wall frames Studs on each side are doubled: –To provide support for the lintel and large nailing faces for finishes around the opening. Lintels are: Double solid timbers of depth appropriate to the span. Plywood boxes built using the wall frame and plywood. J S Foster 1994

Internal walls-partitions Vertical dividers to separate internal space. Giving support to floors /roofs. Timber stud partitions: a stud frame (factory produced) lined (on site) on each side with plasterboard or other sheet material. Nogging pieces between studs to stiffen the partition. See their junctions with frame walls. Sheathing board can also be applied for extra stiffness. Stud partitions of upper floors usually supported by floor joists.

Separating walls Two independent wall frames spaced mm apart. Preferable combined width of 250 mm for acoustic performance. Lining of 2 or 3 layers of dense plasterboard on each side to provide fire and acoustic protection. Insulating quilt in one leaf or the cavity to provide thermal insulation. Light metal restrain straps at approx. 1.2 m centres to tie the 2 leaves at ceiling level-to stiffen the frames during construction. I Seeley 1995

Cavity barriers required to prevent smoke and flame penetration and restrict their spread. At different positions: –Around all openings in external walls –Junctions of external with separating walls –In all cavities at a maximum of 8 m centres –At each floor level in external and separating walls and at eaves level. Fire stops (mineral fibre or wool, cement material or plasterboard) fixed in the cavity –between brickwork and frame) to prevent spread of fire. I Seeley 1995 Types of cavity barriers- preservative treated timber battens; mineral fibre board; wire- reinforced mineral wool blanket; polyethylene sleeved mineral wool.

Advantages of timber frame Shell constructed fast (1 day)-trades working under cover. Can be pre-fabricated-lower cost-fewer site operatives. Full insulation values achieved immediately after construction. Choice of external claddings. High levels of insulation are achieved. Problems of drying shrinkage eliminated. Security on site (faster) Modifications to internal walls are simpler and cheaper- internal layout assumes greater flexibility. The lightweight and well insulated walls warm up faster than brickwork or blockwork.

Problems Associated with poor practice- found in all forms of building construction. –Lack of provision for vertical relative movement between the frame and the cladding. –Inadequate provision for the exclusion of rainwater. –Inability to provide effective barriers against fire. –Torn or missing breather papers. –Poor construction of flashings, gutters and windows leading to water penetration. Performance of timber framed housing not less satisfactory than that of traditional construction.