Internal Finishing

Blockwork Finishes – Internal

There are 2 methods that can be used for internal wall finishing, namely wet plastering techniques and dry-lining with plasterboard. With wet plastering techniques, a certain amount of drying time must be allocated before decorating. This is not the case with dry-lining. Usually, the type of insulation being used will have a bearing on the method used, e.g. in hollow block construction where the insulation must be placed on the inside, dry-lining must be used.

The sound requirements for party walls in terraced houses or semi-detached houses should also be taken into account. Technical Guidance Document E of the Building Regulations defines the minimum density of a wall as 415 kg/m^2^ and the minimum thickness of plaster on both sides as 12.5 mm. The document states that concrete blocks of 1900kg/m^3^ combined with 13 mm of lightweight plaster each side will meet this requirement.

Alternatives

Please see detailed alternatives in the Technical Guidance Document E included in this app.

Finishing Walls & Ceilings

Using Plasterboard-based Finishes

Commonly internal wall finishes are provided by plastering methods incorporating plasterboard. Along with wall finishes, plasterboard also forms the basis of most ceiling finishes in domestic construction. Thereare 2 types of plasterboard-based methods, namely “dry” systems and “wet” systems.

“Dry” System

In this system, large plasterboard sheets that are 12.5 mm thick are fixed to the walls and joint-filling methods are used to leave the boards ready for painting/decorating.

Finishing Of Ceiling

Scratch coat of bonding plaster to a thickness of at least 8 mm; scratch to form a key and finish ceiling with a 2 mm nominal thickness of finish coat. Finish smoothly using a steel trowel.

How To Avoid Cracking Of Plaster In Ceilings

Moisture content should not exceed 20%; ensure timber is properly dried.
For the type of board being used, all joints should be caulked, taped, or scrimmed appropriately.
Provide adequate support, noggings etc.
In cut roof construction, ensure struts that prop purlins are supported adequately.
Ensure sizing of joists is appropriate to their spans.

Alternatives

1) Tape and fill all joints and internal angles and leave ready for painting/decoration using a proprietary priming agent or a slurry of joint finish.

2) Tape and fill all joints and internal angles and finish with a 2 mm coat of skimcoat plaster finish suitable for dry-lining boards.

Alternative Procedures For Wet Plastering To Blockwork

1)

Scud blockwork using a thick slurry of 1:2 sand cement ratio to a depth of 3-5 mm.
Apply a scratch coat of 1:1:6 (cement:lime:sand) or 1:6 (cement:sand) with plasticiser to a depth of 10-16 mm. Scratch the surface thoroughly to form a key, then, once dry, finish with a 2 mm coat of gypsum plaster.

Failure of the gypsum plaster finish may occur if the scratch coat is not allowed sufficient time to dry.

2)

Apply a coat of proprietary gypsum base coat plaster to a depth of 9 mm; scratch well to form a key. Finish with a 2 mm coat of gypsum final coat plaster once the base coat has set adequately. Ensure base coat has dried fully before plaster finish is applied. Correct application of the base coat is essential to ensure finishing coat does not fail.

Cracking in sand/cement base coat due to shrinkage causes delamination/boasting of the finishing coat. Poor scratching; no key provided for finishing coat. Good scratching; good key formed for finishing coat.

Fixing

Fixing Plasterboard To Walls

Fix the boards with the bound edges vertical to vertical battens located at less than 600 mm centres. A continuous horizontal treated batten fixed at no more than 450 mm centres should be provided at the top and bottom of each sheet to act as a fire stop for the wall to ceiling junction and for proper fixing. Window or door reveals should be reinforced with angle bead or with flex corner tape.

Diagram E1 - Fixing of wall linings to treated battens

Dabs of drywall adhesive can alternatively be used to fix plasterboard. A continuous fillet of drywall adhesive or timber battens as described in the previous paragraph can be used for firestopping at floor and ceiling level.

Illustrated are forms of firestopping in cases where the insulation material is laminated to the back of the plasterboard slabs that are fixed by dabs of drywall adhesive.

The requirements of agrement certificates and the recommendations of the plasterboard with insulant backing manufacturers should be followed.

Secondary Fixings

Plasterboard incorporating insulation, typical detail. Once the boards are in position and the drywall adhesive has set, secondary fixings should be provided. The minimum secondary fixing incorporates 2 no. suitably approved mechanical fixings per board.

Locate fixings 15 mm from the vertical edges along the horizontal centreline of the boards.

Diagram E2 - Plasterboard incorporating insulation - typical fixing detail with a continuous drywall perimeter seal

Continuous treated timber battens can be used as an alternative to continuous drywall adhesive perimeter seal. These battens should be fixed mechanically to the blockwork with the plasterboard and insulation mechanically fixed to the battens at 300 mm centres. Notching of insulation material around the battens can help ensure that plasterboard is fixed appropriately.

Diagram E3 - Plasterboard incorporating insulation - typical fixing detail with a continuous treated fire stop batten

Fixing Plasterboard To Ceilings

Plasterboard should be fixed to the ceiling such that the bound edges run perpendicular to the joists and are lightly butted and staggered in order to prevent a continuous end joint. For 12.5 mm plasterboard, 450 mm joist spacing is recommended; if noggings are incorporated between joists, this value can be increased to 600 mm. The edges of the plasterboard should be supported around the perimeter approximately every 150 mm centres.

When driving nails, ensure nails are driven home firmly without piercing the paper surface. Nails should be 40 mm long, galvanised, with a 7 mm diameter head and a 2.5 mm diameter shank. Screws may be used as an alternative; they should be of similar size and at 200 mm centres. Take care to form all opes for services neatly and to fix slabs level and flat.

Fixing Plasterboard To Ceiling Joists

When fixing plasterboard to ceiling joists, ceiling joists must be at least 38 mm wide. Boards should never be fixed less than 10 mm from the edge of the tapered board edges, less than 13 mm from the cut edges or less than 6 mm from the edge of the timber. In the case of trussed rafters with a span of less than 11m, BS 8212: 1995 Code of practice for dry-lining and partitioning using gypsum plasterboard, Section 4, Para. 4.3.3 allows the thickness of the joists to be 35 mm.

Treatment Of Joints

A joint filler compound in conjunction with a paper-jointing tape should be used to fill all joints where joints between boards are taped and filled. To complete filling, a joint finisher should be used after. Once completed, the plasterboard is then ready for painting/decorating.

Fixing Of Plaster Lath – Ceiling

The bound edge of the plaster lath should be staggered to avoid a continuous end joint and run perpendicular to the ceiling joists. Support should be provided around the perimeter of the board as well as at every joist at approximately 150 mm centres. Take care to drive nails fully, without piercing the paper covering of the plasterboard.

With a 90 mm jute scrim, reinforce all internal corners, i.e. the wall-to-ceiling junction. Board finish plaster should be used to caulk all joints. Downstand beams with galvanised metal angle bead should be used to reinforce external angles. Butt ends of boards and lightly butt cut edges. For boards 9.5mm thick, use 30 mm nails and for 12.5 mm boards, use 40 mm nails. Nails should be galvanised with 7 mm diameter heads and 2.5 mm diameter shanks.

Wet Systems

Wet System – 2 Coat

Round edge plaster lath must be used for this system. The standard sizes for these boards are 1200x400 and 1219x400 in thicknesses of 9.5 mm and 12.5 mm. The recommended maximum spacings of joists for each thickness are outlined below.

Table E1 - Maximum joist spacing for ceiling plasterboard

Wet System – 1 Coat

Finishing Of Ceiling

Install tapered edge wall board, then tape and fill all joints as outlined. Finish the ceiling with a 2mm finish coat; smooth with a steel trowel.

Finishing Of Walls

Apply a coat of gypsum board finish to a thickness of 3 mm. Finish smooth using a steel trowel and ensure that all reveals are accurately formed. Never install slabs until house is closed to avoid damage resulting from dampness; i.e. ensure the roof is completed and doors and windows are installed and sealed.

Plasterwork