Decking

It is the property owner’s responsibility to check whether planning or building consent is required before work starts. It is advisable to contact the local Planning Department if:
• the deck platform is more than 300mm (1’) from the ground
• the deck is visible within 20 metres of a highway
• the deck is likely to exceed more than 50% of the property’s garden
• the deck is in a conservation area, national park or attached to a listed building.

It is becoming increasingly popular for people to extend their living areas outdoors, into the garden. The commercial use of decking and walkways is also growing, with restaurants extending their dining areas and pubs providing smoking areas etc. One of the best ways to create ‘a room in the garden’ is by building a timber deck. Simple and quick to build, it requires less groundwork than masonry construction, particularly on sloping or rough sites, and can blend in well with existing landscape features. It is also relatively easy to provide for simple changes of level with ramps or steps, and to allow for future extensions.

Timber decks can be attached to a property or freestanding. The complexity of build depends on the site and height from the ground. There are four basic decks: Ground level - built directly onto the ground; Floating - raised less than 60cms (24”); High level - raised more than 60cms (24”); Roof top - an area on an existing flat roof.

Timber

Only use timber capable of giving a service life of at least 15 years. This means using wood that is naturally durable, like some hardwoods, or a softwood that has been treated to the right level for the job.

Timber strength class

All decks should be built with strength-graded timber. Strength class C16 is considered the minimum standard, with C24 recommended for heavy domestic or commercial structures. For hardwoods, D30 is the minimum strength class.

Metal fixings

Corrosion is the biggest threat to fixings used out of doors. Stainless steel, hot dipped galvanised, or high quality coated carbon steel fixings are best. Electroplated, brass, or uncoated steel fixings should not be used. Do not use aluminium either. Always use the same type of metal for fixings and connectors.

Wall plate

Site preparation

Clear all vegetation from under low level decks. If the under deck area is visible, lay weed-suppressing sheeting, held in place with clips or a layer of gravel.

Construction principles

• Deck boards are fixed to joists which are supported by beams attached to posts to create a raised deck

• Wall plates (sometimes called ledger boards) are used to attach decks to a property, leaving a gap of at least 10mm to allow rainwater to drain away freely. Take care not to damage the property’s dpc

• Post, beam and joist spacing varies depending on a number of factors. These include the size of the deck board being used, and the size and strength class of the framing materials.

When cross cutting or notching treated wood on site always brush the cut ends with an end grain preserver available from your timber supplier. This will ensure the wood continues to be fit for purpose and avoids invalidating the manufacturer’s guarantee.

Laying deck boards

• Always build a slight fall into a decked surface. Lay grooved boards in the direction of fall. Plain deck boards may be laid in any direction

• The space between boards should ideally be between 6mm and 8mm wide to allow the water to drain away quickly and stop dirt accumulating. If the boards are installed at a moisture content higher than they are likely to reach in use, the gap can be 5mm on the assumption that it will increase as the wood dries

Abutting boards

• Always locate abutting boards over a joist to which a 47mm batten has been attached for support. Fixing points need to be no closer than 15mm from board ends. Pre-drill fixing points to prevent splitting

Fixing

• Every time the board crosses a joist it should be attached with two fixing points positioned at the quarter points of the board 

• Always fix grooved boards at the bottom of a groove, ensuring all fixings are flush with the surface, not depressed below it, to avoid water retention

• When installing hardwood boards always use screws, and pre-drill every fixing point 2mm oversize. This allows for any seasonal movement in the wood that could cause screws to break under the tension

Deck parapets or balustrades

For decks raised less than 600mm from the ground, Building Regulations require parapets or balustrades to be at least 900mm high. For decks raised more than 600mm, balustrades must be 1100mm high. Spaces between individual components, like a baluster or rail, should never exceed 100mm wide. 
All newel posts should be capped to avoid water being absorbed into the grain.

Resource from Wood Campus

Treated wood - fitness for purpose

Based on BS EN 335:1 which defines the treatment ‘Use Class’ for different applications

Treatment level

Construction application

Risk of failure

Component examples

Use Class 1

Internal

Low

First floor joists, rafters

Use Class 2

Internal

Low / Medium

All roofing timbers, rafters and ground floor joists

Use Class 3

External: out of ground

Medium / High

Cladding, fence rails, deck beams, joists and deck boards

Use Class 4

External: in permanent ground or water contact

High

Posts and ground level deck joists

Maximum softwood joist spans

(Metres) - Strength Class C16

Nominal Joist size (mm)

Dead load <0.25KN/mDomestic decks Joist centres (mm)

<1.25KN/mCommercial decks Joist centres (mm)

400

500

600

400

500

600

50 x 100

306

2.94

2.67

2.52

2.42

2.15

50 x 150

1.95

1.84

1.60

1.57

1.49

1.31

Ground level decking: John Brash & Co Ltd.
Ground level decking: John Brash & Co Ltd.
Floating decking: Timber Decking Association
Floating decking: Timber Decking Association
High level decking: Peter Brook Design Ltd.
High level decking: Peter Brook Design Ltd.
Walkways: Timber Decking Association
Walkways: Timber Decking Association

 
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