Inspiration and advice for your building project
The subject of flat roofs is one that still divides opinion. Architects have a soft spot for them, while surveyors tend to regard existing flat roofs rather like old shower trays — almost certain to spring a leak in the not-so-distant future.
Here is what to consider if you are looking to buy – or already own – a home with a flat roof.
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In Victorian houses, flat roofs played a fairly minor role, adorning small bays, porches and dormers. Their construction was equivalent to a timber floor made of joists and softwood boards, and clad with good-quality metal sheet — generally performing without undue drama.
In the 20th century, flat roofs made from robust, reinforced concrete made an appearance on the occasional block of flats, but were rarely used in housing other than for small subsidiary structures like stores and porches. Asphalt was sometimes applied as a hard-wearing covering — particularly on surfaces subject to foot traffic, like roof terraces.
Fast forward to the present day and it’s fair to say that while flat roofs have grown in size, this has not always been matched by improvements in quality. Many homes have substantial flat-roofed extensions covered in felt. Felt coverings have notoriously short lifespans, typically lasting only 10 to 15 years (depending on the workmanship). While refelting can be a cost-effective solution for low-level, single storey roofs, on hard-to-access dormers, for example, using cheap felt can be a major false economy. Each time re-covering is required, it will entail substantial labour and scaffolding costs.
So, despite the additional expense, it makes sense to specify longer-lasting materials such as:
Sheet metal coverings are the traditional method of cladding flat roofs, and are relatively durable and long-lasting. Lead sheet has a lifespan of at least 80 to 100 years, whereas zinc typically lasts about half as long. This means that in some older properties, these coverings may now be nearing the end of their life.
Exposure to acid pollution can cause sheet metal to become brittle and crusted with carbonate. Alkaline conditions – caused by eroding cement mortars or chemical reactions where the wrong type of fixing nails were used – can be similarly detrimental. As if that wasn’t enough to contend with, metal sheet is also vulnerable to corrosion caused by dampness from condensation forming on the underside (which can also rot timber decks). This makes insulating a flat roof even more important.
Other issues can include inadequately secured sheets which come loose, or those laid with insufficient space for thermal expansion.
Look out for these signs that the roof is in need of repair:
If the problem is extensive, the original roof may require complete replacement with new sheets of lead. Although the material itself is relatively expensive, the cost of new lead sheet can be offset by selling the old coverings for recycling.
Before undertaking the task of replacement, the condition of the deck below needs to be checked. If damaged or substandard, it may need to be replaced with marine plywood and insulation applied.
If a flat, felt-covered roof leaks somewhere in the middle, it is time to replace it. However, water stains on ceilings (usually brown in colour) can sometimes be due to minor localised leaks at joints.
Other signs that replacement is overdue include profuse plant growth or severe pooling of water into puddles on the surface — sometimes made worse by the accumulation of debris. Damage can also be caused by people walking on the roof — puncturing the surface material.
Fixing roof leaks is obviously going to be a matter of some urgency. However, it can be a false economy to patch a felt roof, except where replacing leaking felt upstands.
Ponding of water may be due to an insufficient fall — flat roofs need a minimum 1:40 slope to disperse rainwater. If rainwater ponds, then subsequent freezing and expansion can cause the felt to split, allowing water to seep into open joints and down through the deck to the rooms below. Timber decks can also start to sag under the weight of accumulated water.
If any insulation within the structure becomes wet, it can hasten decay to the adjoining roof timbers. Some older decks are made of chipboard which is prone to disintegrate when damp, so, as a safety precaution, it’s advisable to use scaffold boards when walking on old decks. It’s best replaced with marine plywood or 18mm exterior grade WPB plywood.
Premature failure of the covering can also be caused by overexposure to UV sunlight, so the surface layer should have protection — usually in the form of an integral layer of mineral flakes (usually green) or small 50mm stone chippings.
Strip old felt away to check the condition of the deck. If a replacement deck is needed, it must be laid to a fall of 6° to 10°, so that rainwater can discharge into the guttering. This is normally achieved by placing timber wedges known as ‘firrings’ on top of the joists before fitting the deck.
When replacing a deck, you may need to budget for possible associated works such as renewing damaged fascias and refixing guttering. In most cases, insulation will need adding or upgrading, and the best way to do this is by fixing insulation boards on top of the deck. Before the new felt membrane is applied, the insulation can be boarded over in 12mm plywood (or 9mm OSB3).
It’s possible to use clout nails and brush-on adhesives to secure the polyester underlay and felt membrane layers, but professional installers generally use more robust three-layer ‘torch-on’ felt which contains integral bitumen. It is applied as follows:
1. A base layer of fibreglass bonding felt is nailed in place with galvanised clout nails at 200mm centres. Then, a layer of pre-bituminised black base felt is heat-bonded to the base layer using a special burner (shown). A surface layer of heavy-duty, pre-bituminised green mineral felt is, heat-bonded on top. Each layer should have a minimum 75mm overlapping between sheets.
2. Strips of 75mm timber ‘angle fillet’ are fitted under the felt to make a gentle curve where the membrane is folded up the wall, rather than a sharp right angle which could cause a split. To help direct water, the sides of the roof (parallel to the direction of flow) are raised up slightly using a timber fillet or fascia board. Detailing is important with both layers of felt dressed over drip battens and into the guttering, with the edges neatly finished at the fascias. Most important of all, junctions at the walls of the house must be watertight, with the surface membrane taken up the wall into a mortar joint to form the upstand, with a lead flashing dressed down over it.
3. Finally, some form of UV protection finish will be needed to help prevent blistering and splitting of the felt. One solution is to simply use green mineral felt for the surface covering as it already has a built-in protective layer. As an alternative, solar-reflective chippings can be applied. These tend to perform better than reflective paint, which can also be specified.
nb. Over time, chippings have a tendency to wash away and block up gutters. Plus their additional weight – which can be as much as a quarter of a tonne on some larger roofs – will obviously need to be factored in.
Allow about £925 for three-layer refelting on a roof around 15m². This includes:
Where the timber decking needs complete replacement, add at least £460, with a similar sum to cover the deck with new insulation, creating a ‘warm roof’, prior to felting.
Hiring access equipment, where required, will add to the cost too.
An otherwise sound roof can suffer from leaks at badly fitted junctions. This can be exacerbated by long-term thermal movement, i.e. between the roof and walls.
The source of a leak can often be spotted in the form of cracks and gaps at upstands, loose flashings, or old botched repairs with paint or tape. In fact, any junctions – such as rooflights and projecting pipes – are potential weak points.
The biggest tell-tale sign is that damp may have penetrated down to the ceiling below or soaked through the adjoining walls, leaving damp patches and brown staining. However, dampness may also be due to condensation (see far right).
Leaks obviously need to be repaired urgently, but this shouldn’t be too expensive so long as there’s reasonable access to do the job. However, there is one situation where, if you’re unlucky, major remedial work might be required.
Many Victorian bay windows to lower storeys have small, flat, ‘balcony’ roofs. These rely on rainwater discharge pipes to prevent a build up of water, but the pipes are usually narrow and prone to blockage. Over time, this can allow damp to seep through the adjoining main wall of the house, resulting in serious hidden decay to the large ‘bressumer’ lintels that span the main window opening. Decay to these heavy, hidden timber beams is likely to be evident in the form of associated cracking to adjoining plasterwork and to masonry above the lintel.
Many Victorian homes have bay windows with flat roofs
Where flat roofs abut an adjoining wall and are subject to leaking, the covering will need to be dressed over triangular angle fillets to lessen the angle at the corner junction. The covering is then turned up the wall (forming an upstand) and tucked into a mortar joint. Upstands are often made using the same felt as the roof, but these don’t last very long.
Best practice is for the turned-up felt to be lapped over by a separate strip of lead flashing, also tucked into a raked out mortar joint approximately 150mm above the roof surface, secured with lead wedges and pointed up. Code 4 thickness lead should be used, and the flashing cut into maximum lengths of 1.5m and overlapped (to allow for thermal expansion).
The cost of fitting a new Code 4 lead flashing over felt upstands – where the flat roof meets the walls of the house – is about £160 for around a four to five metre length. However, where there’s no fixing point for lead – such as around any roof windows – roofing felt may simply have to suffice.
Because many flat roofs are often poorly insulated, moist air from the rooms below will condense into water when it hits a cold surface, and can lead to condensation. The main danger is where hidden condensation has occurred within the roof void, with the risk of decay to the timber structure.
The solution is to insulate the roof and ventilate the rooms, and also improve ventilation to the roof structure. It also helps to fit extractor fans to expel humid air from bathrooms and kitchens.
There are three ways a flat roof can be retro-insulated:
Above the decking: The optimum method is to create a ‘warm roof’ with sheets of rigid foam insulation boards placed on top of the deck. This, however, will raise the height by about 100-150mm, with consequent detailing issues at junctions with walls, windows and fascias. If the existing deck is sound, it can be retained with the insulation laid on top before applying the new roof covering.
Above the ceiling: A conventional ‘cold roof’ sees insulation placed internally above the ceiling between the joists. It’s important to allow at least 50mm ventilation space above the insulation. Ventilation often needs to be improved to older cold roofs with additional vents added to the fascia, too.
Below the ceiling: Where the existing roof covering is sound, a flat roof can instead be insulated from below to form a ‘cold roof’. This could either take the form of a new suspended ceiling packed with mineral wool, or sheets of foil-backed rigid insulation applied directly over the old ceiling, fixed with dry wall screws into the joists. Before plasterboarding, a polythene sheet vapour-barrier should be fitted on the room side of the insulation to prevent steamy air entering the roof (in fact, this is advisable with all three methods).