Floors

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Plywood

Plywood suitable for flooring in domestic dwellings:

American Plywood

Grades: C-D, C-C, exterior, unsanded.

Typical Markings:

  • C-D. Exposure 1, PS1-95.

  • C-C. Exterior, PS1-83.

Canadian Plywood

Grades: B-C, exterior, unsanded.

Typical Markings:

  • Exterior.

  • DFP or CSP. (Douglas Fir Plywood/Canadian Softwood Plywood)

Finnish Plywood

Conifer and Birch plywood, Grades III and IV, sanded.

Typical Markings:

  • Finnish Birch-Faced.

  • Exterior.

  • Bond Class.

  • Conifer.

  • WBP.

Swedish Plywood

Pine or Spruce, Grade P30, unsanded.

Typical Markings:

  • S15.

  • Trident symbol.

Please Note this list is not complete; other types of plywood are available if approved for use.

Moisture Content

Conditioning of plywood to the expected moisture content when in service is recommended. The maximum moisture content when being placed is 16%.

Laying and fixing of plywood

  • Lay plywood with grain perpendicular to the floor joists.

  • Ensure all end joints of plywood occur on joists evenly spread across the floor.

  • Keep line of longitudinal joints straight.

  • Support edges with noggings where boards are square edged.

Expansion Gaps

At edges where sheets meet the walls in a room, leave a minimum gap of 10 mm or, alternatively, 2 mm for each meter the floor spans in the particular direction. In some cases, intermediate gaps may be required.

Plywood Fixing

Typically, fix at 150 mm centres at perimeter and at 300 mm for intermediate supports using flat head, 3 mm diameter, 50 mm long annular ring shank nails. Depending on thickness of plywood, nail size may change; ensure the nail is at least 2.5 times the plywood thickness and penetrates the joist to a depth of at least 32 mm. Depending on manufacturer’s guidance, other fixings are permitted.

Table C14 - Typical fixing centres for different types of plywood

Applicable Areas

  • Use Type C-D Exposure 1 for timber frame prefabricated floor panels.

  • Type C-C Exterior or type C-D Exposure 1 are suitable for all locations in both masonry construction and timber frame.

Chipboard

For use as floor decking, chipboards must be flooring grade. Acceptable grades are as follows:

Types

  • C4 – Moisture resistant

  • C5

Typical Markings:

  • C4 – green and red stripes

  • C5- green and yellow stripes

Types:

  • II/III – C4 equivalent

Typical Markings:

  • II/III (C4) – on underside green code mark

Moisture Content

When chipboard is being laid, the moisture content should be less than 15%. Before being placed, the chipboard should be given sufficient time to adjust to the in-service moisture conditions.

Laying & Fixing Chipboard

  • In the case of square edged boards, the long edges should be tightly jointed on joists and kept parallel.

  • End joints should be supported by noggings and staggered appropriately across the floor.

  • Boards with tongues and grooves are to be laid at right angles to joists with the short edges tightly jointed at joists.

  • For boards with tongues and grooves, there is no need for support when joists are spaced correctly. Stagger the joints between the shorter edges appropriately across the floor.

  • Continuous support of edges from joists and noggings is vital around the perimeter.

Gaps For Expansion

Expansion gaps are required between edges of the boards and walls. The gap should at least 10 mm or, alternatively, provide 2 mm for each metre the boards span across the floor. An expansion gap is manufactured into the underside of tongued and grooved boards to allow for intermediate movement.

Chipboard Fixing

Typically, fix at 300 mm centres at perimeter and at 300 mm for intermediate supports using flat head, 3 mm diameter, 50 mm long annular ring shank nails. Depending on thickness of plywood, nail size may change; ensure the nail is at least 2.5 times the plywood thickness and penetrates the joist to a depth of at least 32 mm. Depending on manufacturer’s guidance, other fixings are permitted.

Table C15 - Typical fixing centres for chipboard

Gluing Joints

To prevent creaking in tongued and grooved joints, joints are glued using Poly Vinyl Acetate (PVA) type adhesives.

Areas Applicable:

  • Type P5 to BS EN 312: 2010 Particleboards (BS ref. C4) must be used for timber frame prefabricated floor panels.

  • For timber frame flooring, use type P4 to BS EN 312:2003 Particleboards (BS ref. C2) fixed on site in a dwelling fully sealed from outside weather conditions. In kitchens, bathrooms and wet areas, use type P4.

  • Use type P4 (C2) for general areas and type P5 for kitchens, bathrooms, and wet areas in masonry construction.

Even with moisture resistant chipboard, care should be taken to avoid exposure to wet weather to avoid damage.

Oriented Strand Board

Softwood flakes are bonded with phenolic resin adhesives, chemical binders, and wares to form a structural panel called Oriented Strand Board (OSB). This type of board is produced in a range of grades and thicknesses.

For OSB, a voluntary colour coding system exists that consists of 2 colours; the first colour is either white or yellow. White shows that the board is for general purpose and yellow that the board is for load bearing. The second colour is either blue or green. Blue shows that the board is fit for use in dry conditions and green that the board is fit for use in humid conditions.

Table C16 - Colour coding system for OSB

Table C17 - Voluntary colour coding system for OSB

The following markings are printed on OSB decking: OSB 2 EN 300 or OSB 3 EN 300.When used alongside colour coding, these markings make identification easy on site. Colour coding and markings vary with manufacturers; it is important that guidance is sought.

Moisture Content

OSB 3 boards are factory conditioned to between 5% and 13%; panels should be allowed time to adjust to the conditions that will be encountered during service prior to being placed.

Laying & Fixing:

  • Fix boards with short edges parallel to the floor joists and staggered appropriately.

  • Cross joints need to be staggered.

  • Use noggings to support all cut edges that are not supported by joists and all edges of square-edged boards.

  • Support needed for board edges around the perimeter of the floor.

Gaps For Expansion

Gaps of 3 mm are required between each floor panel as well as around the perimeter of the floor. Around the perimeter, the minimum gap required is 10 mm or, alternatively, 2 mm for every metre the floor spans. It may be necessary to include intermediate gaps for large floors.

OSB Fixing

Typically, fix at 150 mm centres at perimeter and at 150 mm for intermediate supports using flat head, 3 mm diameter, 50 mm long annular ring shank nails. Depending on thickness of plywood, nail size may change; ensure the nail is at least 2.5 times the plywood thickness and penetrates the joist to a depth of at least 32 mm. Depending on manufacturer’s guidance, other fixings are allowable.

Table C18 - Typical fixing centres for OSB board

Applicable Areas

  • Use OSB 3 for timber frame flooring that includes prefabricated floor panels.

  • Use OSB 3 or OSB 2 when panels are fixed on-site in a dwelling completely enclosed from the elements, or use OSB 3 for kitchens, bathrooms, wet areas, and where there is a risk of wetting on-site for timber frame flooring.

  • Use OSB 2 or OSB 3 in general areas and OSB 3 for kitchens, bathrooms, and wet areas for general masonry construction.

Even with moisture resistant OSB, care should be taken to avoid exposure to wet weather to avoid damage.