Flat Roofing FAQ’s

Got a question? We’ve got the answers.

Q: How does rain run off a flat roof?
A: Flat roofs are generally built with a slight incline so that rain water can drain away into the gutters. Basically flat roofs aren’t always absolutely flat.

Q: How do I maintain my flat roof and what should I inspect?
A: Inspect the roof twice a year in March and November – ideally soon after rain or snow so you can check that the water is draining away. Before you attempt any inspection ensure that access is safe. If you’re unsure of the condition of the deck do not step on the roof. If you do need to step on the roof use a sheet of OSB3 or Plywood to distribute your weight evenly.

Q: My flat roof is leaking. Can it be repaired, or do I need a whole new roof?
A: If it’s obvious what’s caused the leak, and the rest of the roof is okay, then a minor repair is probably okay. This sort of random patching up isn’t guaranteed though. That said, if the whole roof is in poor condition, or you need a warranty, then a full replacement (or possibly an ‘overlay’ – see below) is the best thing to do.

Q: How do I find a reputable contractor?
A: Asking around for local roofers with a good reputation in the area is a good place to start. Ensuring they are members of the appropriate trade association or the Competent Roofer scheme is also advisable.

Q: What do I need to consider when replacing or repairing my roof?


  • Protection from rain and snow – Your finished roof should have a slope of at least 1 in 80 and should drain to one or two edges. It’s best to use conventional eaves gutters rather than internal outlets. If you do use internal outlets make sure they are big enough to deal with storm conditions and are fitted with leaf and gravel guards. The waterproofing should extend up adjacent walls at least 150mm (6ins) from the roof surface in all situations.
  • Protection from sun and frost – It pays to insulate. Your heating bills are lower in winter and your rooms are cooler in the summer. Don’t forget any insulation of a new flat roof must satisfy Part L of the Building Regulations.
  • Protection from condensation If you have a cold roof it’s essential you include adequate through-ventilation.
  • Protection from the wind – Your roof should be able to resist wind forces and an experienced contractor should be able to advise you. Make sure you ask for advice if your roof is particularly exposed.
  • Protection from people – The materials you choose should suit the roof usage. If the use is changed, you might have to strengthen the structure.
  • Protection from fire -Don’t forget to consult your local authority if you are planning a new flat roof.

Q: I have several quotes, some mentioning ‘Pour-and-Roll’, others suggesting ‘Torching-on’. What’s the difference?
A: Pour-and Roll is the traditional method of laying bitumen membranes. It uses hot bitumen from a boiler poured onto the layer below as an adhesive for the membrane. This is then rolled onto it. Products designed for torching-on have special bitumen on the lower surface of the membrane, which is heat-activated and melted using a large & powerful gas torch. The membrane is then rolled onto the softened bitumen. Note: It isn’t a good idea to apply a torch flame to any flammable surface (e.g. plywood, timber edgings, below the edge of a pitched tiled roof, etc.) The torch flame can reach 1000 degrees C. Please be aware that the contractor’s insurance will not cover fires caused in this way – and neither will your own home insurance.

Q: The contractor wants me to put insulation onto my roof – do I need to? If so, where does it go?
A: Thermal Insulation reduces heat loss through the roof, and saves energy during the winter. It also keeps your house cooler and more comfortable in the summer. This is now a requirement of Building Regulations (Part L:2006). If you are putting up a new building or a new extension, or you’re repairing or replacing more than 25% of an existing flat roof area, then YES – you do need to upgrade by including a layer of thermal insulation.

Q: I’ve got ponding on my existing roof. Do I need to replace the roof?
A: Not unless it is leaking, or showing other signs of damage. If and when you do have it replaced, you should tell the contractor so they can check the cause of the ponding, and itemise the cost of dealing with it in their estimate.

Q: I have just had my roof replaced, and it is still ponding where it used to. Why?
A: Unless the contractor was told about the ponding they are unlikely to have quoted for changing the falls. This would have meant removing the decking, placing firring pieces (wedge-shaped timber strips) on the joists, and re-fixing the old deck or a brand new one, before installing the new waterproofing system. This could have doubled the cost of the job. The decking might have deteriorated and sagged between the joists, in which case it should ideally have been replaced.

Q: I have had a new roof put on and I am not happy with it. What should I do?
A: Your first point of contact should be with the contractor, who should be able to explain what he has done, and why. If not, then you need to talk to an independent expert who will be able to advise you.

Q: My quotation contains a reference to re-decking. Can you tell me what this means?
A: The deck of your roof is the layer of plywood or OSB3, which goes over the joists to support the insulation and membranes. Where this is damaged or is a part of the waterproofing system, you’ll need to replace it. This is known as re-decking. The contractor should be able to give you a price for the work.

Q: I am told the contractor can ‘overlay’ my roof. What does this entail, and is it advisable?
A: In many cases the existing roof is sound, and the membrane is simply getting old. If so, the contractor might advise you not to expose the building structure to the elements while he strips off the old membranes. Instead you leave them in place to act as a base layer for a new system. This is known as overlaying the roof. As long as there are no other problems such as condensation or damaged decking, then overlaying is perfectly acceptable. There are a number of products available on the market you can use.

Q: What guarantee/warranty should I expect?
A: Material warranties usually come from the waterproofing layer manufacturers, and are handed over by the contractor when you pay. Depending on the materials used, this could be for 5, 10, 15 or 20 years. If the contractor is on the manufacturer’s select list, this could also cover the workmanship. Always read the small print to see what’s included.

Q: What are the main methods of waterproofing on a domestic residence?
A: Pour and roll: hot bitumen is poured in front of the felt as it is unrolled, and acts as the adhesive and seals the laps. Torching: in this method, specially designed felt is heated with a gas torch and no separate bonding bitumen is needed. This is best suited to repair work, small areas, or where access to the roof is difficult. The contractor must take precautions against fire. Torching is not suitable over or adjacent to flammable materials. Self-adhesive: the undersides of these membranes include a high-tack adhesive and release paper. Once positioned, the release paper is peeled off, and the membrane stuck down to the substrate. They are not suitable for laying over uneven surfaces or in cold conditions. They require rolling, or a soft broom applied over the whole area to help stick them down fully. Liquid Roofing and GRP Roofing : this is a cold applied system so you don’t need hot bitumen or gas torches. The waterproofing layer is built up by brushing or spraying liquid in a number of layers to provide a waterproof layer once the coatings have cured. Often a mesh or fabric is used as part of the build up to add strength and flexibility.

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