Are you thinking about replacing a flat roof on your property? If so, we have a helpful and impartial guide where you’ll discover the different options available and their pros and cons.

This guide to the best flat roof is brought to you by Dan at Job Prices.

Decking  – The Most Expensive Part of a Flat Roof?

The large sheets of decking underneath most flat roofs may need to be replaced if you’re installing a new roof covering. You might be able to leave the original boards in place if they’re not made from chipboard and they’re in good condition.

Our research in 2022 has shown that roofing companies charge between £60 and £75 to replace each sheet with a new OSB plyboard.

A typical single garage has up to 6 sheets. 6x £60 = £360 and that is just the decking.

But do you need to replace the existing boards or can they be reused?

Here is our guide:

If you want a fibreglass roof then yes, in most cases, you will also need to replace the decking boards.

If you have chipboard decking under your existing roof then yes, in all cases it should be replaced as chipboard is unsafe and when wet, will not support the weight of someone walking on the roof.

If you have plyboard under your existing felt, then each board will need to inspected for rot or distortion and replaced as required. Plyboards are usually long-lasting and durable so shouldn’t need replacing unless subject to water for a period of time.

If you have close-boarded decking (a particularly strong type of board) under the felt, then it shouldn’t need replacing unless you want a fibreglass roof covering.

Here is a pro tip from an experienced roofing contractor with over 20 years of experience:

Check the roof for any dips, bowing or pools of water. This is usually the result of a failed decking board that will need to be replaced. If it isn’t then rainwater will pool on the new roof and potentially shorten its lifespan.

What is the Best Flat Roof Covering?

Below is a table exploring our personal opinion of the best types of flat roof covering (bitumen, asphalt, rubber and fibreglass).

We suggest you have a think about what is most important to you and how this fits in with your budget.

The visual appeal of a flat roof that is overlooked by several prominent windows in the house is probably more important than a dormer flat roof located so high up that you can’t see it from any window.

For those on a low budget, a bitumen torched-on felt system is usually advised. These products have been around for decades and there’s plenty of competition that keeps the prices down.

Fibreglass and rubber roof systems last the longest but are usually more expensive.

Pros and Cons of Each Flat Roof

Below is a guide to each type of flat roof covering:

Fibreglass

This material has been used for decades and is often seen on boats and kayaks. It was first used on flat roofs several decades ago but got an undeserved bad reputation due to poor workmanship and techniques that have now been improved with updated standards.

Positives:

• It’s visually appealing
• Tough and unlikely to crack from impacts such as dropped tools
• Easy to patch repair
• Easy to clean with a mop
• Insects, moss and mould won’t damage it
• No naked flame used during the installation

Negatives:

• Should be installed by a specialist with experience of this material
• Not as reliable on large flat roofs that experience flexing of the roof timbers
• Price – at the top end of the price range
• Standard fibreglass is slippery, especially when wet

Torch-on Bitumen Felts

This type of felt, also known as a built-up or layered felt system, is manufactured in rolls and has a tar-like material called bitumen on the back which is heated with a gas torch as the material is unrolled over the roof.

The felt is built up into layers. A 3 layer system is typically used over a habitable part of the home, while on garages and sheds it’s not uncommon to see a 2-layer application.

Positives:

• Affordable and is in the middle of the price range
• Plenty of roofing businesses do this type of work so there’s lots of competition
• Can be patch repaired if damaged by falling tree branches or similar objects
• Popular for large roof areas
• Can be joined to most materials if you share a flat roof with a neighbour

Negatives:

• Looks patchy as each roll is joined to the next with a visible line of bitumen (tar)
• Doesn’t last as long as modern alternatives as it’s affected by sunlight
• Can’t walk on it during hot weather as the material is much softer and may stick to the underside of shoes
• Requires extensive use of gas blow torch during installation which means increased fire risk and health and safety considerations

Rubber Membranes

These first hit the markets about 20 years ago and have become very popular due to their longevity. There are essentially two types of rubber membranes in use today and they are slightly different.

The one-piece system has no joints and is made to measure. This is typically used on smaller roofs such as single garages and dormers.

The most common type of rubber flat roof comes in rolls and each joint is sealed with a chemical glue that bonds the material together.

Both systems come with edging trims and accessories to finish off the roof neatly.

Positives:

• A long-lasting product that isn’t easily affected by sunlight or insects
• Impact resistant up to a point
• No gas torch used in the installation
• Were very expensive but prices have come down slightly due to competition
• The one-piece rubber membrane is attractive as it doesn’t have any joints
• It flexes with the roof and shouldn’t crack or split
• Easy to clean with a mop
• Lightweight

Negatives:

• The price – still at the top end of the range
• The one-piece version has been known to pull away from any vertical walls due to thermal movement, this is worse if it hasn’t been correctly fitted. Extra care and attention should be made to large areas as they will experience more thermal movement
• The jointed version isn’t visually appealing and can look plasticky and patchy
• There are plenty of cheap, inferior and low-quality one-piece variants on the market that target DIY installations. These typically have poor quality trims, adhesives and have a tendency to form ripples and creases.

Mastic Asphalt

This material was once one of the most popular flat roof materials in the UK and has been in use for hundreds of years. However, poor workmanship by a minority and the emergence of new products that are quicker and easier to work with has reduced its popularity.

Asphalt is often used in waterproof tanking and damp proofing. It is one of the best waterproof materials in use but does have some limitations.

Positives:

• Looks appealing with a consistent smooth grey finish
• Has a very tough and durable surface that won’t break when objects are dropped onto it, can be walked on all year round
• Should never need a complete replacement, just an extra layer added to the top when it comes to the end of its life

Negatives:

• Asphalt is very heavy and some flimsy roof structures are not suitable for it
• Considered by some to be an old and outdated way to waterproof a flat roof. Fewer and fewer companies now offer this service as it’s more difficult to lay than a roll of felt.

Insulation – A Quick Guide

Since 2006 all flat roof replacement projects around the home must now meet minimum insulation thresholds.

So, if your flat roof is over a habitable part of the house, you’ll need to ensure that adequate insulation is also included.

That could mean extra expense, although it depends on how much (or little) insulation you currently have in this part of the roof.

The good news is that insulation is not too expensive and there are tons of options available. Your house will also be more efficient so you’ll get some of your money back over time due to reduced energy bills.

Insulation is not required for garage flat roofs or sheds.

Neither do you need to meet this requirement if you are laying a new roof covering over the top of an existing one.

The insulation rules only apply to flat roof replacements, not refurbishments/repairs.

Below is a decking product that contains a sturdy ply deck, a thermal insulating layer and a waterproof layer to keep condensation out:

Celotex insulated flat roof decking sheet

These boards are 1200mm wide and 2400mm long and cost around £100 inc vat (exc fitting fee).

See how much they currently cost by visiting this price list here.

They are used instead of traditional decking sheets, so if you intend to replace your old sheets, just go for this product instead, they are more expensive but they’ll meet current building regulations.

A typical small dormer will require one sheet.

A small extension will need 3-4 sheets and larger extensions 5+ sheets.

There’s no need to use these on separate garages and sheds as they aren’t usually insulated or heated anyway.

What About Prices?

We have created a helpful guide to flat roof prices that you can see below.

This guide includes example prices for torch-on felt, rubber membranes and fibreglass:

See our flat roof price guide here

A Warning About Asbestos:

A recent survey by the Guardian Newspaper revealed that over 50% of homes in the UK contained asbestos at some point since their initial construction.

One of the most common places to find asbestos is on flat roofs.

If you think you have an asbestos roof, it will need to be removed or encapsulated by a professional before a new roof covering can be installed.

Below is our guide to asbestos removal prices in the UK:

Asbestos removal cost guide.

Can You Answer This One Question?

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