Everything you need to know inc installation prices

Written by Daniel Woodley. Fact checked by Hannah Miller. Published to Prices. Updated: 6th March 2023.

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My name is Daniel, and I trained as a flat roof installer back in 1998, since then I’ve completed hundreds of installations.

This is my guide to flat roof decking, what it is, why you may need to replace it and how much it costs to install.

I updated this page in 2023 with information about insulated decking – Part L of the Building Regulations states that when over 50% of the flat covering is replaced, the insulation must meet current standards.

For many of you, that will mean installing new insulation and possibly lifting the roof to make space for that insulation.

Note: Insulation and Part L of the regulations do not apply to porch canopies, garages and sheds.

OSB Decking Boards

OSB stands for oriented strand board and is the number one choice for roofers who need to install a strong decking board to support a flat roof covering.

Unlike plywood or chipboard, OSB boards are created by layering strips of wood in particular patterns and layers to achieve strength. The resin that binds the wood strands together is both tough and water-resistant.

I’ve installed OSB boards under fibreglass, bitumen and also rubber flat roof coverings hundreds of times.

When You Should Use OSB Decking Boards

If you’re replacing an old flat roof with a new covering, you’ll need OSB boards if:

  • The existing decking is made of chipboard – this is a weak type of board that often sags and breaks very easily, especially when wet. It should be replaced with modern 18mm OSB decking boards otherwise the roof will be unsafe.
  • The existing decking, of any type, is rotten, flaking or has failed.
  • You want to install a fibreglass roof – fibreglass must be laid onto new, clean boards that won’t flex or sag, OSB is perfect for this task.
  • You’re planning on replacing a tar or bitumen flat roof with a rubber covering. Rubber membranes shouldn’t be laid onto tar or bitumen, including residues, as it will stop the rubber from sticking to the decking.
  • You need to raise one end of the flat roof to stop water pooling on the surface.
Closeup of OSB decking board

When to Use 18mm Decking Boards

If your existing flat roof decking has sagged it should be obvious just by looking at the roof from above – look for dips between the roof joists.

If this is the case, the existing boards should be removed a replaced with OSB 18mm boards.

Also, if any of the existing boards are wet or showing signs of rot, they’ll need to be replaced before the new roof covering is installed.

18mm OSB decking is load bearing and is the number 1 choice for flat roofers.

When to Use 11mm OSB Flat Roof Boards

Rather than fully removing and replacing decking boards, a process that takes time, creates lots of waste and adds to the cost of the project, one can consider an overlay.

If the existing flat roof decking is in good condition, with no signs of sagging or rot etc but isn’t suitable for fibreglass or rubber, perhaps due to contamination with bitumen or oils etc, then an overlay would be the best option.

An overlay is simply an OSB board, often thinner than a regular board, laid over the top of the original boards.

Thinner OSB boards are cheaper and overlays are quicker to install, so you save money on the cost of the project.

A good option for flat roof overlays is 11mm OSB boards.

11mm OSB board are not suitable as a load-bearer on a flat roof and must be placed over another board.

How Much Do OSB Decking Boards Cost to Buy and Install?

Below you’ll find my cost guide for flat roof OSB decking.

I’ve displayed prices on a “supply only” basis and also separately, a “supply and install” basis.

The prices below were sourced from roofing stores and also flat roofing installers that gave me prices for a new flat roof.

If you’ve been given quotes from flat roofing companies and they include the cost of new OSB decking, you can compare their figures to these:

Item:Price inc VAT:
11mm 2440mm 1220 mm OSB Decking (Supply Only)£22 each
11mm 2440mm 1220 mm OSB Decking (Supply & Install)£40 - £50 each
18mm 2440mm 1220 mm OSB Decking (Supply Only)£33 each
18mm 2440mm 1220 mm OSB Decking (Supply & Install)£55 - £65 each

The prices above were sourced in early 2023 and are for 2.44m x 1.22m boards. The “supply and install” figures are an average of the prices given to me by a selection of flat roofing firms in the UK.

How Many Decking Boards Are Needed to Cover a Flat Roof?

A 2.44m x 1.22m OSB decking board covers around 2.9 square metres so measure your roof area to determine how many boards you need. You should add 10% for wastage.

A typical single garage roof is 18 – 20 square metres so will require between 7 and 8 boards.

A double garage flat roof often requires 13 – 18 boards.

A porch may only require 1 or 2 boards.

Overhead view of OSB decking board
Stack of OSB boards

How Decking Boards Are Installed

OSB decking boards are nailed to the roof rafters and a small 5mm expansion gap is left at the joins.

Insulation and Part L of the Building Regulations

The Building Regulations are not optional, they must be adhered to.

The current regulations state that:

If you replace over 50% of the roof covering, the insulation present must be up to the standards set in the document

For many (in my opinion; most) homes, that will mean the flat roos must be lifted and extra space created for new insulation.

Here is a photo of an insulated decking board with an OSB finish on the top:

Insulated decking

The thickness and insulating value of the boards will depend on the specifics of the project.

As a general guide to pricing, the extra cost of insulated decking boards is around £50-60 per square metre.

This is a significant amount that the buyer should budget for.

Don’t forget; Part L only applies to habitable parts of the home, not porch canopies, sheds or garages which are uninsulated.

Examples of Obsolete and Acceptable Flat Roof Designs

This page contains photos and examples of flat roof designs, inc obsolete and those that meet the building code with regard to Part L.

Get Your Custom Flat Roof Price Today

These price guides are based on real quotes from firms and individuals in the UK, but every project is different.

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Why You Can Trust This Guide

I know there’s a lot of nonsense published online about roofing projects, usually by those that have never worked in the industry.

Here’s my experience:

1997-1999: I trained as an installer and fitted fascia and soffit boards and guttering as a roofline installer.

1999-2003: I trained as a flat roofer (built-up systems and fibreglass).

2003-2008: I went self-employed and worked in flat roofing, roofline and general roofing (mostly in domestic settings).

2008-2015: I worked first as a project cost estimator and then as a project manager for a housebuilder, a property renovating firm and later a landscaping firm.

2015 onwards: I continued working as a project manager, but mostly on larger landscaping projects.

As accuracy is so important, I gathered prices from flat roof installers so I could get a picture of what the going rate is in the UK.

I also asked Paul Farley to fact-check the important statements I’ve made in this guide.

Author: Daniel Woodley

Daniel has over 18 years of experience in the construction, home improvement, and landscape garden industries, including a spell working as a project manager.

In his spare time, Daniel enjoys gardening, hiking and scrambling. He frequently posts videos to his popular YouTube Channel and is well known in the hiking community.

More About Daniel Woodley.

Daniel Woodley

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This guide to installing flat roof decking was created by the team here at DIY Gardening

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