This page is part of our blog and contains a troubleshooting guide for flat roofs that are leaking and are in need of repair.
Our guide is intended for anyone who is competent with roof safety such as tradespeople and some experienced DIYers.
We will cover the following:
- quick fix repairs you can use to keep the flat roof watertight until a longer-term solution can be applied.
- long term fixes without replacing the entire roof.
- examples of when you really need to bite the bullet and replace the flat roof.
As we are sure you can appreciate, there are many different types of flat roof materials and therefore different solutions will apply.
We will try to cover the most common types of materials that are in use on flat roofs in the United Kingdom.
[Tip: if repairing a flat roof is outside of your skill level, check out our price guide for replacement flat roofs here]
The General Rule of Thumb
As a general guide, you can repair most flat roofs if:
- the leak is caused by an impact such as a slipped tile, a tree branch or the like
- the leak is caused by a one-off hole, tear or localised issue that is isolated and not indicative of a more widespread problem
You are probably wasting your time and money trying to patch repair a flat roof that is:
- very old and has leaks in multiple places
- that is bowing, dipping, bending or buckling between the joists
- where the entire surface has deteriorated due to age or UV damage
Of course, many people do carry out patch repairs on such roofs, but they are doing so at great risk.
Because these roofs are flat they often hold water, especially during spells of heavy rain, even a small tear or hole could cause a significant amount of water to enter the property.
Common Issues and What to Look For
Regardless of the type of flat roof material that is used, leaks are often:
- located near a join or seam
- where the felt has pulled free from the vertical wall
- caused by the failure of the supporting timber decking underneath
- made worse by water discharging onto the roof from a rainwater pipe
- made worse by water pooling on the roof when it should ideally discharge off the roof
The photo below may help with troubleshooting:
- check where the felt has been inserted into the wall, thermal movement may pull the felt out of the brickwork.
- check all seams/joins near any leaks, if the felt hasn’t adhered sufficiently it may allow water to pass through.
- can the rainwater pipe be relocated or an additional pipe laid across the flat roof to minimise the amount of water (and thus wear and tear) on the roof?
Below are two photos showing damage caused by squirrels that had migrated from woodland just across the road.
This type of damage is very common, it’s also frequently seen around chimney lead flashings on houses in rural areas.
If left unchecked, this damage could lead to rotten timbers and a leak onto the ceiling.
Below are our suggestions to repair this type of damage:
Temporary repair – cover the damaged area with a specialist lead sealant such as this to keep out any rainwater until a more permanent solution can be applied.
Longer term fix – a roofer with lead welding skills could easily repair this damage or the damaged section can be cut away and a new section fitted with folds. This would negate the need for costly welding.
Permanent solution – as the lead roof is showing signs of damage to several areas and the leadwork around the nearby chimney has also been chewed. A permanent solution would be to replace the roof with something less appealing to the squirrels such as a bitumen-based flat roof material.
For the chimney flashing, it could be replaced with a lead alternative such as Ubiflex – www.ubbink.co.uk/products/Roofing/Non-lead-flashing
The reason why squirrels chew lead isn’t entirely clear. Some say it’s because they are attracted to the sweet taste while others suggest it’s because they need to sharpen their teeth.
Whatever the reason, the use of lead should be minimised if the property is located near woodland.
Further reading and photos can be found on this university website.
For leadwork to large areas that have split, the best bet is to cut in an expansion joint. This work should be done by someone with experience in leadwork.
Bitumen Flat Roof Repairs
Bitumen based flat roofs come in two different flavours:
- old fashioned “pour and roll”
- a more modern “torchon” system
The two photos below explain the difference:
Both types of felt will last between 15-25 years and there are some situations when they can be repaired:
Splits and Small Holes in Felt
For splits and small holes in the flat roof consider the following types of repair:
Temporary repair – for a quick fix that is cheap and can be done by anyone with DIY skills, first dry and thoroughly clean the area and then apply two or three layers of Thompsons Roof Seal.
Medium term repair – if you want a longer term repair then dry/clean the area and apply one layer of cold bitumen followed by a layer of scrim (aka webbing or mesh) for reinforcement and another final layer of cold bitumen. See cold bitumen here and over here you can see scrim/mesh. There is an instructional video on Youtube that explains how to repair a flat roof with scrim and sealants.
Long term repair – to maximise the life of the roof without replacing it, torch a new top layer (cap sheet) onto the roof. Because this work involves a gas-powered torch, it will need to be done by a professional.
Dips and Bowing Felted Roofs
If the felt is showing signs of bowing downwards between the timber supports then in our opinion, repairs are a waste of time and money.
As the roof continues to dip/bow, any repairs will tear apart and the leaks will resume.
Because you have roof timbers, decking and potentially ceiling boards and plaster under the roof, it’s just not worth trying to save money by repairing a roof that is in this state. By missing an opportunity to replace the roof, you could end up with significant and costly damage to the timbers or plaster should the roof suffer a major leak.
A better solution would be to remove the felt layer, inspect the decking underneath and replace any boards as required before laying a new felt roof.
(Update May 2021: We’ve heard reports of insurance companies refusing to pay for damage caused by old leaky flat roofs that should have been replaced years ago so if your flat roof looks anything like the one in the photo above, you should really get it replaced)
Fibreglass Flat Roof Repairs
Fibreglass roofs are generally very long-lasting and also look appealing as they don’t have any joints or seams.
One of the most common issues with this material is that it can suffer from splits.
This rarely happens on small roofs such as those found on porches but is often seen on very large areas such as double garages.
The good news is that fibreglass can be easily repaired.
All in one fibreglass repair kits cost less than £30 and it takes less than an hour to seal over a hole, split or crack.
There are plenty of instructional videos on Youtube but in a nutshell, the following steps should be taken:
- clean area thoroughly
- sand with a coarse sandpaper
- clean with acetone
- chopped strand matting is then laminated into place
- after its dried, a top coloured layer is applied
Mastic Asphalt Roofs
This type of flat roof is expensive and should last for decades.
It’s considered by many in the trade to be the “Rolls Royce” of flat roof materials.
Unfortunately, because it’s not very flexible and is very heavy, any movement of the roof below may result in splits and cracks.
There are several repair options available to you but the following two are best:
- add an extra layer of mastic asphalt – this should be done after reinforcing the area around the crack to prevent it from transferring into the new layer.
- torch on a layer of bitumen felt, either as a patch repair or to the whole roof. If the appearance of this felt is an issue, it, along with the rest of the roof, can be painted with a grey/silver reflective coating.
This type of material, known colloquially as a “rubber roof”, is quick to install and doesn’t require a gas-powered heat torch.
While the material itself is long-lasting, weak points are usually located at the joints and upstands.
Small holes, rips and tears, whether at the joints or as a result of impact damage, can be easily repaired by applying one of the following:
- by applying a dedicated rubber sealant such as this Firestone sealer. This is a short term repair but for very small holes, it may last for decades.
- by using a sealing tape such as this Firestone flashing tape. This is a long term fix and should last as long as the roof.
The expected repair time is around one hour and that includes locating the leak, cleaning/drying the area and apply the sealant or patch.
The FixMyRoof website has some excellent videos on YouTube showing you how to repair flat roofs, they also have a helpful guide on their website:
The National Federation of Roofing Contractors has a very helpful guide to flat roof construction:
Get a Quote
If you think your roof needs the attention of a professional roofer, hit the link below to see how we can help you get a quote from a local tradesperson: