FLAT ROOFS

Architects may have a soft spot for flat roofs but surveyors tend to regard them with suspicion – because so many are poorly constructed and prone to developing leaks.

In many older properties flat roofs commonly play a minor role on small bays, porches and dormers, often clad with good quality metal sheet.  But today a lot of homes have large flat-roofed extensions which can often be problematic because cheap mineral felt coverings have very short lifespans – typically lasting only 10 to 15 years or so (depending on workmanship).  So when it comes to re-covering them it makes sense to specify longer-lasting materials such as artificial rubber (EDPM), fibreglass, or ideally traditional lead sheet.

A lot of older flat roofs also have little or no insulation making them cold and prone to damp. So before re-covering a flat roof it’s important to note that Part L1B of the Building Regulations stipulates that when half or more the surface is renovated or replaced the insulation must also be upgraded.

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Things to check

1/    Defective felt coverings

Felt flat roof coverings are one of the cheapest and most common types of flat roof covering.   Signs that replacement of felt coverings is overdue include profuse plant growth or severe ponding of water into puddles on the surface. However water staining on ceilings (usually a brown colour) is often due to minor localised leaks at joints – (see point 4 below).

 Fixing roof leaks is always a matter of urgency. However, it’s usually a false economy to patch a felt roof except where replacing leaking felt upstands (see point 3). Ponding of water may be due to insufficient fall (angle of slope) – flat roofs need a minimum 1:80 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. And if any insulation within the structure becomes wet it can hasten decay to adjoining roof timbers. Some older decks are made of chipboard which is prone to disintegrate when damp and is best replaced with marine plywood. So as a safety precaution it’s advisable to use scaffold boards when walking on old decks.

Premature failure of covering materials can also be caused by over-exposure to UV sunlight. Damage can also be caused by people walking on the roof, puncturing the surface material.

Remedial work:  Once the old felt has been stripped away the condition of the deck can be checked. If a replacement deck is needed (for example in 18mm exterior grade WPB plywood) it must be laid to a suitable fall (ideally 1:40) 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. Where decks are replaced, you need to allow for possible associated works such as renewing damaged fascias and re-fixing guttering. The insulation (if there is any!) will need repalcing by fixing insulation boards on top of the deck over  polythene vapour control sheeting (see point 4 below). Before the new surface membrane is applied (e.g. artificial rubber EPDM)  the insulation can be boarded over in 12mm plywood (or 9mm OSB3).

2/  Defective sheet metal coverings

Sheet metal coverings are the traditional method of cladding flat roofs and are comparatively dureable and long-lasting. Lead sheet has a typical lifespan in excess of  100 years, whereas zinc typically lasts about half as long.

If you can see old patch repairs, splits, holes, surface ripples, impressions of the boarding below, or past attempts at sealing with bitumen paint, then it’s likely to be on its way out.

Over time corrosion can develop due to exposure to acidic conditions, such as rainwater from roofs covered in algae, lichen or moss growth, eventually becoming brittle and crusted with carbonate.  Alkaline conditions can be similarly detrimental, typically caused by eroding cement mortars, or chemical reaction with the wrong sort of fixing nails. Metal sheet is also vulnerable to corrosion caused by dampness from condensation forming on the underside (which can also rot timber decks) – hence the importance of insulating. If sheet metal coverings are inadequately secured they can come loose, or there may be insufficient space for thermal expansion. If the problem is extensive, original roofs 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 re-cycling.

Remedial work:  Small defects can be repaired with soldered dots or patches of the same metal as a short-term measure. Where complete replacement is necessary, lead sheet needs to be laid to a minimum fall of 1:80. Old defective zinc is best replaced with lead which is more durable, but the two materials mustn’t be used together as they can react and corrode. Lead sheet should be fixed using copper fixing nails, having first checked the condition of the deck (which may need to be replaced with marine plywood and insulated). Metal sheet is prone to expansion in hot weather so it is laid in sections with movement joints in between.

Any large areas without expansion joints will eventually split and buckle.  Detailing can be checked with the diagrams on the Lead Sheet Association website.  At joints with walls, the sheets need to be turned up and lapped over by a separate lead flashing.

3/    Leaks

An otherwise sound roof can suffer from leaks at badly fitted junctions, exacerbated by long term thermal movement, e.g. between roofs and walls. The source of leakage 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 are potential weak points, such as with roof lights or projecting pipes. Damp may have penetrated down to the ceiling below or soaked through adjoining walls, leaving damp patches and tell-tale brown staining. However dampness may also be due to condensation – see below.

Leaks obviously need to be repaired urgently, but this shouldn’t be too expensive as long as there’s reasonable access to do the job. However many Victorian bay windows to lower storeys have small flat ‘balcony’ roofs. Over time damp due to blocked rainwater pipes can cause serious decay to the large ‘bressumer’ lintels that span the main window opening.

Remedial work:   Where rainwater on the roof surface can’t drain away, check and clear any blocked gutters or downpipes. With flat roofed bays, where there are signs of damp penetration and movement to adjoining walls, the beam will need to be exposed by hacking of surrounding plaster internally. Depending on the extent of any decay, complete replacement could be necessary. Masonry to adjoining parapet walls may also have deteriorated requiring localised repair (or in extreme cases rebuilding) such as re-pointing eroded mortar joints and re-fixing loose coping stones.

Where flat roofs abut an adjoining wall, at the corner junction the covering will need to be dressed over triangular angle fillets to lessen the angle. 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).

4/    Damp to rooms below (not due to leaks)

Condensation and dampness to ceilings and walls in the rooms below, usually accompanied by unsightly black mould.

Because flat roofs are often very poorly insulated, moist air from the rooms below (especially bathrooms and kitchens) will condense into water when it hits cold surfaces. 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 below to extract water vapour, and also improve ventilation to the roof structure. It also helps to fit extractor fans to expel humid air from bathrooms etc.

Remedial work:  There are 3 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. However this will raise the surface height 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 loft-style ‘cold roof’ has the insulation placed internally above the ceiling between the joists, but 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 air vents added to the fascia.

* 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 3 methods).

N.B.  This is by no means an exhaustive list, and much depends on the age and type of property you’re surveying

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