OSB Board Decking For Flat Roofs 

Updated Info For 2021

Realistic Installation Cost Guides

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A Look at OSB Boards and Flat Roof Decking Options

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.

OSB boards can often be found under fibreglass, bitumen and also rubber flat roof coverings.

This page is all about your OSB decking options, which board is best suited to your project, how much they cost to buy and install and how many boards your roof will need.

OSB Board For Typical Flat Roof Replacement Projects

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.
  • 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.
OSB board

When You Need to Completely Replace Decking With OSB 18mm 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.

When an OSB Overlay Can be Considered

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.

How Much Does it Cost to Install OSB Decking Boards to a Flat Roof?

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

We’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 us 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 ours:

OSB 11mm overlay supply-only£23 each inc VAT
OSB 11mm overlay supply and install£38 each inc VAT
OSB 18mm replacement supply-only£32 each inc VAT
OSB 18mm replacement supply and install£50 - £55 each inc VAT

The prices above were sourced in early 2021 and are for 2.44m x 1.22m boards. The “supply and install” figures are an average of the prices given to us by a selection of flat roofing firms in the UK.

How Many OSB Boards are Needed to Cover a Typical Flat Roof?

A 2.44m x 1.22m 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.

OSB decking for rubber roofs
OSB board stack

Get a Custom Flat Roof Price

We hope you found our guide to flat roof decking insightful.

To get a custom price for your replacement roof project, tap the button below and fill in the contact form:

  • Get a flat roof installation price quickly online.
  • Read vetted ratings and reviews.
  • Compare the prices from three flat roofing firms.


Our OSB board and decking guide excludes prices for insulated decking, you can find more info about flat roofing insulation here.

All references to OSB boards on this page are for OSB grade 3 boards, the most common board found on flat roofs.

Here’s a description of the grades:

  • OSB/1 – General purpose boards, and boards for interior fitments (including furniture) for use in dry conditions.
  • OSB/2 – Load-bearing boards for use in dry conditions.
  • OSB/3 – Load-bearing boards for use in humid conditions.
  • OSB/4 – Heavy-duty load-bearing boards for use in humid conditions.

Flat Roof Replacement or Repairs

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Traditional Bitumen Flat Roofs

An alternative to rubber EPDM is traditional bitumen-based torch-on felts. They don’t last as long as rubber or fibreglass but are cheaper and in many cases still last for decades.

Fibreglass Flat Roofs

Fibreglass can be found on ships, kayaks, pond liners, cars and hundreds of other items that keep water out. It also makes for a very good flat roof covering. 

Rubber Roofs

Rubber is a durable, long-lasting and modern material that’s become very popular during the last two decades. It’s perfect for laying over a flat roof and is easy to install.

Their prices for roofline installations were correct; we were given quotes from several firms, all were similar to [Quotaton Check’s]

Quotation Check’s guides to flat roof prices are very accurate, they include all the extras many other price guides conveniently exclude.

Why only rely on three or four quotes for comparison? Quotation Check sourced prices from hundreds of firms for various home improvement tasks.

Roofing firms often charge more than they should because they think consumers have no idea about how much the parts and labour should cost. Quotation Check’s guides are easy to read and they explain in detail about the materials needed and how long the work will take.

If you want to know how much it costs to replace a flat roof with rubber felt, read the guide over at [Quotation Check].