Flat Roof Repair Guide

Roof Care & Maintenance
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Flat Roof Repair Guide

Whilst they keep rain out of our living rooms and prevent us from freezing in the winter, flat roofs are not infallible. Over time your flat roofing material, no matter how sturdy, will come to need repairs. In this guide we’ll go through the different types of problems you may encounter on different types of roofs as well as how the intrepid DIYer may go about fixing and/or preventing them.

Table of Contents

When You SHOULDN’T Repair Your Flat Roof

When looking to tackle a patch up job it is important to first consider if it is worth the time and money. Many problems that you might encounter, especially on older roofs, simply cannot be solved by just a repair. Of course, this can depend on the type of roof covering you have but generally you should be on the lookout for three key indictors:

  • Multiple leaks on an aged roof
  • Bowing, dipping, bending or buckling between the joists
  • Deterioration of the entire surface caused by age or UV damage

Not only does this imply a much larger issue that may need multiple sections, if not the whole roof, replaced, it also makes any repairs that you do attempt far more dangerous. This is because flat roofs, unlike pitched roofs, tend to hold water much longer and thus even small rips or tears can lead to significant damage, harming the structural integrity of the whole roof and making it difficult to work on safely. Therefore, if your roof does have one of these issues, or even if you think it might, the best thing to do would be to consult a professional.

If you are confident that your roof doesn’t have any significant problems and is safe to work on, then we can take a look at the different repair jobs that may need doing.

What Type of Flat Roof Do You Have?

Here are the most common types of flat roofs:

  • Felt Roof
  • Asphalt/ Bitumen Roof
  • Concrete Roof
  • Fibreglass Roof
  • EPDM (Rubber) Roof

How to Repair a Felt Roof

Felt roofs have been around for a while and thus could mean you have the older ‘roll out’ type of the more contemporary ‘torch-on’ version. Roofing felt is advantageous for many reasons, it’s lightweight, affordable, and flexible but most of all felt is one of the easiest materials to work with when it comes to repairs. Here are some of the problems you may encounter:

Splits, Punctures, Tears or Small Holes

One of the most common causes of leaks, tears or splits is typically caused by falling debris or temperamental weather. Fortunately, as a common problem, they have a common solution and that is to simply seal them up using a suitable roofing sealant. First, ensure that the area is clean and dry and then apply two to three layers and wait for them to cure. Note that this is more of a temporary fix and, at best, will last a few years.

For a longer-term repair, you could opt for a three-layer bitumen approach. This involves apply one layer of cold bitumen over the affected areas toped with a layer of scrim (webbing or mesh generally in tape or roll form) followed by a final layer of cold bitumen. Depending on the size, it is worthwhile considering this approach for the whole roof to ensure a clean-looking and even finish.

If these two approaches are not suitable and you want the assurance of a permanent solution without having to replace the whole roof, then you’ll be looking at torching on a new layer. This means that the material will be thermally bonded to your roof for a stronger seal and greater protection. However, as this work involved a gas-powered torch, you may need to be insured to work with naked flames. As such it may be best to outsource to a professional tradesperson.

Lifting Joints

This can occur if the original materials used were of poor quality or if there were issues during the installation. A lift in a joint will break up the roofline and allow an easy access point for rainwater to enter your roof structure. Again, a great temporary fix if this issue is limited to a small area can be sealant or paint on waterproofer. This can be applied in the same way as mentioned above. This is also the case for the permanent solution – a new torched on layer of felt.

Leaking Flashings

Flashings and mortar can be damaged in any number of ways, but mostly by time, debris or animals. Whichever may be the case, if water can get behind them it would spell trouble for the structure of your roof. Most of the time this can be fixed simply by removing the mortar from behind them and repointing with a new and correct mixture.

If the actual surface of the lead or lead alternative is damaged, you will be looking at a slightly bigger job which will involve removing and replacing any and all affected flashing. For guidance, take a look at our article on how to fit apron flashing. An appropriate lead sealant can also be used if the damage is not to extensive, but this will only delay the problem until the flashing can be replaced.

Dips and Bowing

As stated previously, any signs of felt dipping or bowing between your roof joists indicates a much more significant structural issue. This will result in tears and splits continuing to form, unfortunately making any repairs a pointless exercise.

As such, the best approach here is to remove the roofing felt so that you can inspect and replace any damaged boards which may be the leading cause of the problem. Depending on your DIY skill and level of confidence it might be best to leave this work to a qualified roofer.

How to Repair an Asphalt Roof

A higher-end option for flat roof coverings, asphalt is considered by many the best possible option for durability and longevity. However, that’s not to say that these roofs don’t need the occasional TLC like any other.


These can most often be caused by ageing or thermal movement and actually may not prove to be an immediate problem in themselves. However, during winter if any water that has penetrated these cracks freezes it can cause them to expand to the point where a repair will be needed.


Bumps can be easily recognised as small pockets of moisture trapped under the roof surface. As with cracks, they are not considered an issue in small numbers unless they burst, become hollow and begin letting in water. You should also be on the lookout in case they are sitting below laying or ponding water.

Both issues can be repaired in the same way. For a quick fix, you can simply use a paint-on waterproofing solution or sealant. Heat-applied Bitumen Flashing Tape can also work and may be easier as it’s self-adhesive. The permanent way to solve this issue is essentially the same method you would use on felt. Cover the affected areas with a small patch of torched on roofing felt (or asphalt) and in the case of a large area an entirely new layer over the top of the roof. Just be sure that all cracks and bumps have been sealed first and that the roof is dry and clean of all debris. If the appearance of felt is an issue here, most can be painted with a grey/silver reflective coating.

How to Repair a Concrete Roof

Less common these days because of difficulties with insulation, concrete nonetheless remains an incredibly durable type of roof and one that can be repaired without too much difficulty. You should be on the lookout for the following:

Cracks and Splits

Not necessarily the easiest to spot, cracks and splits are far more subtle on concrete roofs than on others. However, they can lead to leaks just the same, so it is important to scour your roof carefully when you think you may have one.

Surface Wear

Much easier to spot, surface wear essential amounts to the top layer of concrete being eroded away to reveal the sharp stones underneath. Whilst only an aesthetic problem initially, this can hinder other repairs and if not filled in has the potential to puncture any new layer laid on top of it.

Bitumen-based waterproof tinned paint/sealant is your go-to here as a temporary fix for both issues. You should take care to first apply a bitumen primer and allow it to fully dry as this will net you better long-term results. Before doing any of this, however, you will need to clear the area of any moss and dust using a scraping tool and a brush.

If the crack you are dealing with is significant enough to be very obvious to the eye, or if the surface wear is extensive you can look into applying a full torched-on layer of roofing felt to repair the whole area.


This is the big one to be on the watch for. A large area of dampness on either side of your roof can be an indication that the original mixture of concrete didn’t have enough cement. In cases like this whole roof can become rough or porous, in which case an extra layer on top will not be enough to prevent further damage.

To resolve this a roofer should be contacted who can then bond or screw a whole new timber deck onto the concrete which can house a more reliable roof covering.

How to Repair a GRP Fibreglass Roof

GRP fibreglass roofing has seen a rise in popularity not only because it is long-lasting but also visually appealing without any visible joints or seams. This also means that any potential problems can be spotted and repaired fairly quickly.


Just like many other types of roof, GRP can split over time in response to ageing, debris and changing temperatures, most commonly around upstands, trims and board joints. Not only does this ruin the look of your roof but can also lead to the ingress of water and thus mould and mildew.

The good news for DIYers, fibreglass is one of the easiest materials to repair, with kits be readily available at very affordable prices. Most should come with a set of instructions, but the process generally follows these steps: Clean the affected area, sand with coarse sandpaper, clean with acetone, cover with chopped strand matting and then apply a top coloured layer.

Leaking Flashing

In the case of a leak, splits may not be the only cause. You should always take the time to check on any GRP abutment flashing as there may be a gap in the sealant that is allowing the entry of water. This can be generally fixed by applying a small amount of acrylic sealant just a few millimetres on either side of the area in question. This can then be reinforced with a plasterer’s scrim if necessary. As always, the area should be thoroughly cleaned beforehand, ideally using warm soapy water.

If a permanent solution is needed, this will take the form of laminating down chopped strand matting after first cleaning and sanding the whole area with acetone and grit paper to a minimum of 75mm around the affected areas. Once this is cured, a matching colour topcoat can be applied.

GRP whilst simple to repair if the problem is common, can also be a bit tricky if you run into the issue of delamination, meaning that the fibreglass is splitting into its individual layers (this can sometimes look akin to peeling paint). If you spot this, you should definitely look into consulting a GRP roofer who will be able to provide further advice.

How to Repair an EPDM Roof

More commonly known as a ‘rubber roof’, EPDM membranes are similar to fibreglass in that they are very easy to repair by both DIYers and professionals. As it is not hot bonded either, EPDM is quickly becoming a very popular material to choose for your flat roof.

Holes, Rips and Tears

Caused by any kind of impact damage, it hardly needs explaining that these can lead to leaks and further damage to the structure of your roof.

Installation Issues

If not fitted correctly, EPDM can suffer from shrinkage and wrinkles across its surface. Obviously not nice to look at, these issues can also make the whole surface more susceptible to issues raised above.

To buy yourself some more time, any holes, rips and tears can be treated with sealant. Be sure when ordering you get yourself an approved rubber roof product rather than any old repair material. Following the manufactured instructions, a small amount applied to the problem area will generally be enough for a temporary solution.

You will more than likely want to come back to this at some point for a longer-lasting repair, at which point you will have to first remove the sealant initially used. After this, it is a straightforward patch job using self-adhesive flashing tape suitable for rubber roofs.

Clean the area, apply an EPDM primer and then, once this has been allowed to cure and is fully dry, firmly apply the tape from the centre outwards with a hard roller. You should aim for 75mm of coverage on all sides of the rip or hole. If the patch is anywhere near lying water, it is also worth using some sealant around the edge for added security.

These are ideal when the problems with your roof are localised to a single spot. If you find these issues occurring across your roof it is time to approach a professional so that they can replace it.

How to Maintain Your Flat Roof

If you want to get the most out of your flat roof and maximise its lifespan there are a few simple maintenance tips that can really help you out.

Bi-annual Checks

This means once every six months, not every two years. Checking your roof on a regular basis will allow to keep up to date with any seasonal changes that may have occurred as well as provide the opportunity to catch any small issues before they develop into much larger (and more expensive) ones.

These checks should not just be done on the external side of your roof either. Keeping an eye on the inside is just as important. Checking the ceiling for rising moisture and damp in addition to any water damage from the outside can save you a lot of hassle down the line.

Remove Debris

Regardless of the time of year and where you live, there is a very high likelihood that twigs, leaves and dirt will collect on your roof. If left to build up and block gutters you could find yourself having to deal with pooling water, which in conjunction with a crack and cause hefty damage. Gutters should be cleaned at least twice a year and any surrounding foliage should be cut back to decrease the amount of debris falling onto your roof.

If you are in the market for a roof sealant or are looking into building a new flat roof and need a covering, we have a range of options to choose from. Simply order online, talk to us via out online chat or contact our award-winning customer service team directly at 01295 565 565.

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