Decking Construction: Joist Spacing, Bearers, Fixings & Building Regs

Summary

Garden decking has been one of the most popular outdoor improvements in UK residential properties for twenty years. Done well, it creates a durable, attractive surface that extends usable outdoor space. Done badly — with untreated timber, inadequate drainage, or poor fixings — it rots in five to eight years and becomes a liability.

The regulatory position trips up many contractors. Permitted development allows most domestic decking without planning permission or Building Regulations notification, but there are significant exceptions: proximity to boundaries, height above ground, attachments to the building, and properties in conservation areas all affect what requires consent. Getting this wrong leaves the customer and the contractor exposed.

Structural adequacy is non-negotiable. Decking must carry live loads safely — a deck full of garden furniture and people is a significant load. Joist sizing, span, bearing, and fixing must be calculated correctly. The structural hierarchy is: foundation (posts or pads) → bearers → joists → decking boards.

Key Facts

• Planning permission: Not normally required for decking at single dwelling if it does not exceed 300mm above ground AND covers no more than 50% of the garden area AND is not in the curtilage of a listed building or designated area
• Building Regulations: Not required if deck is under 300mm above ground, but may be required if attached to house or if over 300mm [verify with local authority for project specifics]
• Conservation areas and AONB: Permitted development is restricted — decking may require planning permission
• Standard decking joist size: 47×100mm C16 regularised treated softwood
• Joist spacing: 400mm centres maximum for 28mm decking boards; 600mm centres for 32mm+ thick boards
• Bearer size: 47×150mm or 47×200mm depending on span between posts
• Post size: 75×75mm minimum for low decking; 100×100mm for elevated or loaded decking
• Post centres: Typically 1.8m–2.4m maximum depending on bearer span
• Concrete pad footing: 300mm × 300mm × 300mm minimum for low-level deck; deeper for elevated structures
• Timber treatment class: UC3 (above ground, exposed to weather) minimum for joists and bearers; UC4 (ground contact) for posts in or near soil
• Post base connectors: Steel post base connectors (e.g. ATA post bases) allow UC3 timber if post is raised above the concrete — preserves timber at the base
• Decking board thickness: 28mm (most common), 32mm, or 38mm for heavy-use commercial
• Decking board width: 90mm and 120mm most common; wider boards are more prone to cupping
• Board gap: 5–8mm between boards to allow drainage and expansion
• Fixing type: Stainless steel decking screws (A4 grade) or hidden decking clips — do not use zinc-plated screws (will rust and stain)
• Screw length: Minimum 2.5× board thickness — 65mm screws for 28mm boards
• Handrail requirement: Required where deck is 600mm or more above ground — 900mm height minimum
• Balustrade sphere test: 100mm maximum gap (same as internal stair balustrade — see staircase regs)

Quick Reference Table

Detailed Guidance

Planning and Regulatory Check

Before pricing or starting:

1. Measure deck height above ground — if any part will exceed 300mm, Building Control notification is likely required
2. Check property type — listed building, conservation area, AONB, or new-build (50% garden rule may apply)
3. Check attachment to house — decking bolted to the wall requires structural consideration; if the ledger board is bolted to the house joist, it becomes a structural connection and Building Regulations apply
4. Boundary check — decking close to a boundary in a conservation area may require planning permission

When in doubt, contact the Local Planning Authority — a free pre-application enquiry is better than enforcement action.

Foundation Design

Low-level deck (under 300mm):

• Concrete pads at each post position: 300×300×300mm minimum
• Allow concrete to cure for minimum 7 days before loading
• Set a post base connector (Twistfix or similar) in wet concrete, aligned with a string line

Elevated deck (300mm–1.2m):

• Larger pads required — 400×400×400mm or 450mm diameter tubular form
• Post base connectors allow UC3 posts; if posts are to be set in concrete, use UC4 timber only
• Depth of foundation: minimum below frost line (300mm in most UK locations) and into stable ground — deeper on soft or made ground

Ledger board (deck attached to house):

• Bolt the ledger through the house wall into the floor joist using M12 structural bolts at 400mm centres maximum
• Install a flashing above the ledger to direct rainwater over the top of the board and away from the wall
• A DPC behind the ledger is essential to prevent moisture bridging

Framing

1. Set and fix all posts plumb, using post base connectors or post sets in concrete
2. Fix bearers to post tops using post cap connectors or notch and bolt
3. Fix joists to bearers using joist hangers (galvanised or stainless steel — never unprotected steel)
4. Check deck frame is square: measure diagonals before fixing off
5. Add blocking at joist ends to prevent rollover and at any mid-span support points

Joist hangers: Use the correct specification for the joist size (manufacturer-specific) — undersized hangers have lower capacity than the joist. Nail with the correct galvanised joist hanger nails (typically 30×3.75mm), not standard wire nails.

Decking Boards

Board direction: Typically run away from the house for visual effect. Boards running parallel to the house look wider; perpendicular emphasises depth.

Pre-drilling: Always pre-drill near board ends (within 150mm of end grain) to prevent splitting — even with self-drilling screws.

Hidden fixings: Clip systems (e.g. Camo Edge Fastener, Ipe Clip, DecTie) give a clean surface appearance. Most require a specific gap dimension — check manufacturer's guidance. Hidden fixings reduce maintenance access if a board needs replacing later.

First and last boards: The last board at each edge should overhang the outer joist by 25–40mm for a clean finish. Cut the overhang using a chalk line and circular saw after all boards are fixed — do not try to pre-cut every board to a perfect length.

End grain sealing: Treat all cut ends with a compatible end-grain sealer immediately after cutting — end grain absorbs water far faster than the face and is the primary entry point for moisture that causes checking and splitting.

Timber Species and Durability

All structural timber (posts, bearers, joists) must be UC3 minimum, UC4 if in ground contact. Decking boards require UC3 treatment or a naturally durable species.

Handrails and Balustrades

Where deck level is 600mm or more above adjacent ground level:

• Handrails/balustrade required — 900mm minimum height for private domestic use
• 100mm sphere test applies — no gap larger than 100mm
• Balustrade must resist 0.36 kN/m horizontal load at the top
• Post spacing: maximum 1.8m for standard residential balustrade
• Connection to deck frame: through-bolted or structural post base — never just screwed to the face of the joist

Frequently Asked Questions

Do I need planning permission for garden decking?

In most cases, no — domestic garden decking is permitted development. Exceptions are: decking exceeding 300mm above ground level, decking covering more than 50% of the garden area, properties in conservation areas, listed buildings, national parks, or Areas of Outstanding Natural Beauty. If the deck is attached to the house and might affect the structure, Building Regulations may also apply.

What is the best timber treatment for decking?

For structural components (posts, bearers, joists), use timber factory-treated to UC4 standard (for ground contact) or UC3 (if clear of the ground on metal connectors). For decking boards, UC3 pressure-treated softwood is adequate; finish with a quality decking oil or stain annually. Avoid creosote (prohibited for residential use), and avoid water-based stains on timber that sits wet frequently — they peel.

How do I stop decking boards from cupping (bowing upward at the edges)?

Cup occurs when one face of a board takes up more moisture than the other. Fit boards bark side up (the curved grain lines point downward when viewed from the end). Ensure air can circulate beneath the deck — a minimum 150mm ground clearance is ideal. Oil the underside of boards before fitting. Avoid very wide boards (over 120mm) — they cup more than narrow boards.

How long should decking last?

Properly constructed decking with treated softwood and stainless fixings should last 15–20 years with regular annual maintenance (cleaning and re-oiling). Untreated or poorly treated decking may rot within 5–8 years. Hardwood or composite decking lasts longer — 25+ years with minimal maintenance.

Can I attach decking directly to the wall of my house?

Yes, using a ledger board bolted into the structural wall. This approach requires a proper flashing above the ledger to prevent water ingress, a DPC behind the ledger, and structural fixings into the wall framing or masonry. The connection is a structural junction and must be adequate to carry the deck loads — this is the point where Building Regulations may apply.

Regulations & Standards

• Building Regulations Approved Document A (Structure) — applies if deck is elevated or attached to house

• Building Regulations Approved Document K (Protection from falling) — handrail and balustrade requirements

• Town and Country Planning (General Permitted Development) (England) Order 2015 — permitted development limits for decking

• BS 8417 (Preservation of wood) — timber treatment classification (UC ratings)

• Eurocode 5 (BS EN 1995) — structural design of timber; joinery span design

• Planning Portal — Decking and Outbuildings — Permitted development guidance

• Wood for Good — Decking Guide — UK timber decking guidance

• Timber Decking and Cladding Association (TDCA) — Industry best-practice guidance

• staircase regs — Balustrade and handrail requirements

• timber spans — Joist and beam span tables

• working at height — Safe working when constructing elevated decking

• building control — When building control sign-off is needed