Flat roofs: what to look out for

There are so many things that you need to look at, consider and manage when you're buying a home, from the finance to the legal side, from change of address notifications to the structural integrity of the property.

Surveyor Local occasionally looks at certain aspects of a property in more detail, and this week we look at flat roofs.

No doubt, there are horror stories that you might have heard about the fragility of flat roofs and the problems that they might be concealing, not the least because any excess rain might not run off into the gutters, and therefore cause damp problems.

The most immediate answer to that question is that installing a flat roof is usually the cheapest and quickest solution to making that part of the building watertight, which is why it is a popular choice.

Often the base of the roof (and therefore the ceiling in the room inside) is created using joists (often wood, occasionally metal or concrete - although before the ban, some construction might have used asbestos, particularly where the flat roof is over heat sources such as the kitchen). 

Inside and in between the joists will likely have suitable insulation filling the gaps before plasterboard is applied (allowing for any electrics for lights, extractors, fans and so on), and given a plaster skim. At this point, you'll be able to apply paint, wallpaper, lining paper or anything else to finish off the room.

Outside, the boards - often chipboard for its sturdiness and cheapness - are nailed over the joists and then covered in the choice of waterproof material. In the past few decades, the preferred material has been bituminous because of the simple reason that it is again a cheap option. 

The felt is supplied on a roll, and consists of layers of glass or organic fibre (and, for older properties, even asbestos), which is compressed into a sheet with a bitumen compound. Asphalt is also a common material, albeit more expensive than felt. Sometimes, gravel is spread across the laid surface.

Because of its relative cheapness, the lifespan of the material used is limited to about 20 to 40 years, which means it will deteriorate more quickly than other materials. Causal factors of its degradation are, of course, the weather (both hot and cold), along with poor maintenance, inadequate workmanship in laying the felt and slapdash attempts at making any repairs.

In addition, problems may occur with the pitch of the roof, which must be at a slant of at least 12 degrees towards the main edge of the roof to allow rainfall to channel towards the guttering.

If the rain is allowed to pool for extended periods of time, this will start the erosion process which will work away at the bituminous felt with holes appearing that will allows dampness to proliferate. This is usually identified by the telltale brown stains appearing inside on the ceiling and even down the walls.

Because of the structure of the roof felt and especially if there is gravel applied as a covering, there are several natural and tiny crevices, which allow seeds dropped by birds or borne on the wind to get a foothold. This in turn attracts further moisture, while the root systems will find the path of least resistance and begin to burrow down further, opening up further channels for water to pass through and then damage the roof structure.

Because of the space that is provided, it is also becoming increasingly common to see residents developing the flat roof into a terrace or seating area. While this is perfectly acceptable (assuming you don't contravene your neighbour's privacy and it is safe to do so without creating a danger of falling off), when it comes to felt coverings of the roof, care should be taken to ensure chair and table legs don't dig deeply into the surface, since this will create perfect places for water to pool, weeds to grow and damage to the structure to begin if not managed.

Also plant pots need to be considered, since sitting on the roof will be a potential source for water egress - and a source for unwanted plant growth elsewhere on the roof.

The first piece of advice is to appoint a chartered surveyor, who will have the knowledge and experience to know what to look for with flat roofs and the various materials that might have been adopted.

They will review, analyse and, where appropriate, test what is going on with a flat roof and put their findings in their report in an easy-to-understand manner, so that you know what the problems are, how serious any issue is, and an idea of how much any remedial work will cost.

If you already own a home that has a flat roof, you will need to ensure you have a suitable management plan in place, sweeping off pools of water that have collected after heavy storms, and keeping the surface free of weeds and other plants that have infiltrated. If you have furniture or plant pots on the roof, you need to regularly check that they aren't causing any unexpected damage.

Are you thinking of buying a property with a flat roof, but aren't sure whether that will result in big bills to sort out any problems?

You'll want to get a professional assessment of any existing problems and what the remedial effort and cost will be with a RICS Level 3 Survey. 

At Surveyor Local, we only work with chartered surveyors who are members of the Royal Institution of Chartered Surveyors, ensuring the survey you get is always professionally delivered.

They will work hard to identify all kinds of problems, including the impact of prior flooding on the structure of the property.

Call  to get your no-obligation survey quote started, or to discuss your concerns with the acquisition of your planned property.

Or you can get a quick quote, using Surveyor Local's easy-to-use quote generator. Simply input your name, postcode, email address, phone number and an approximate value of the property (usually the agreed price), and we'll give you an instant quote for the work (with an email copy). 

We'll do the rest once you confirm your acceptance of the quote.