Be Your Own Building Inspector

Like everything else in life, to be forewarned is to be forearmed, so looking out for these common errors should help you gain a sense of control over your project and avoid problems later.

The use of washing up liquid as a plasticiser

The proverbial washing up liquid is all too frequently used as a plasticiser in mortar and render mixes. Unfortunately the phosphates and other constituents dont agree too well with the cement! Also, the wetting characteristics of washing up detergents remain active in the wall for too long. This results in the wall being more permeable to rain, causing problems long after the brickies have left. Plasticisers are useful and help to produce good, consistent brickwork, but it is essential that only proper mortar plasticisers are used. They are formulated with resins compatible with cement and work by entraining tiny air bubbles. These act like ballbearings, enabling the mortar to flow smoothly and not merely as wetting agents. They also have short life characteristics, when used in accordance with the manufacturers instructions, and degrade without causing damage. Household detergents are useful in the kitchen but they have no place on your site!

Uneven and mismatching mortar in brickwork

Mortar colour can vary from batch to batch depending on the proportion and quality of ingredients sand, cement, and lime that are used. In high quality work, mortar is specified to be batched by weight to ensure consistency. In normal site practice, it is mixed by the shovelful of sand, and by the bag (or even more variably, by the half bag) of cement. The result can be a very visible difference between mixes, giving the appearance of a cheap job, even if expensive bricks have been specified. The best ways to reduce this effect are to mix only by the whole bag of cement, rigorously control the use of additives and, provided the time between mixes is not too long, keep back some of each batch to blend in with the next.

Failure to properly fill the perp joints in brickwork

Bed-joints are not the only joints in brickwork. Dont forget the vertical (i.e. perpendicular) joints or perps to the brickies. A common practice is to kiss the front and rear face of the brick with mortar before placing it in position followed, if you're lucky, with a token dab in the middle. Again, this makes for a faster rate of laying but the result is a wall which can leak like a sieve! Perps must be fully filled if the wall is to satisfy its purpose. Rain penetrating through poorly built cavity walls is one of the most frequent causes of claims picked up by the NHBC. Don't forget it is your money and your house and such practices are not acceptable.

Bridging the wall cavities

Faulty cavity walls, usually manifesting as damp batches on the inside wall surfaces, cause most warranty claims. This problem has become more noticeable since cavity walls are required to be insulated and are no longer ventilated. The problem lies not with the insulation but is almost always the fault of the brickwork. Before cavity insulation, any damp penetration could dry out quickly because of ventilation through the cavity: this cannot now occur. There are two golden rules for cavity construction: the first is that the inside faces of both leaves must be kept clean. This means no mortar snots and no accumulation of mortar droppings onto the top edge of the insulation batts. These snots make it easier for rain penetration to bridge across the cavity, creating a damp patch. The second rule is to never allow the wall ties to slope down toward to the inner skin. This too will facilitate water tracking across the cavity. It is true that wall ties are twisted or have other drip devices to shed water but these are fall back, fail-safe features and should not be relied upon as your main defence.

Failure to allow for expansion

Apart from bed and vertical joints, expansion joints are sometimes necessary and, when they are, are often overlooked by the self-builder. Expansion joints are required in external walls to cater for expansion caused by sunshine. Most people will have seen these on garden walls or more likely the spectacular effects resulting from their omission! What is not so well known is that movement joints are also necessary on long internal skins and partition walls built from aerated blocks. Self-builders should get hold of the technical information sheets published by their selected blockwork supplier, and ensure that movement joints are built in as recommended by the manufacturer.

Colour banding in brickwork

Bricks are made in batches, and just like loaves of bread, batches will vary in shades and texture. Many an otherwise good house has been spoiled because the bricklayers have used up each pallet of bricks before starting the next one. This results in bands or areas which stand out differently from the whole. To avoid this, make sure that bricks are taken by the gang from three or four pallets to ensure a good mixing. This will even out any extremes of colour and texture.

Flues installed upside down

If your home is to feature an open fire, your bricklayers will probably be putting in clay flue liners, unless you are going in for ox-roasting, in which case a generously sized rendered brick flue will be required. The point is that clay flue liners, like clay drain pipes, have spigot and socket joints. A common mistake is to think of the flow of the flue gases as being like water in a drain, and lay the spigot upstream relative to the flow. Flue liners must be laid with the sockets uppermost. The reason is that it is not the combustion gases flowing upwards which is important, but any condensation draining down. This condensate is acidic and may contain tars or hygroscopic compounds. If it escapes out of the flue lining and into the masonry it will cause damage, damp patches and staining on internal walls. The fired clay flue liners will keep this condensate safely contained provided the spigot and sockets are correctly laid so it can't leak out!

Poor handling and storage of roof trusses

Roof trusses cause lots of problems, but it is their storage and handling before fixing that can be the common error, not the actual fixing. Trusses are engineered to be very efficient in using the minimum amount of timber to carry the roof load, i.e. when they are standing vertically with the ridge uppermost. They are not designed to be strong when horizontal; the connectors and timbers can distort, weakening the truss. NHBC inspectors will reject any trusses where the connectors have become loosened for whatever reason. Trusses should be handled in the vertical position both when offloading from delivery and when being hoisted onto the wall plates. Trusses should be stored vertically, supported under the rafter feet where the wall plate will fit, and propped to prevent them toppling. This method of storage also has the benefit of taking up less space on site than stacking flat, and is easier to sheet over to protect from rain.

Failure to install tanks in attic before fixing roof trusses

Trying to get a hot water cylinder through an attic hatch or between manufactured roof trusses is impossible, so make sure you put any tanks in the attic there before the trusses are all fixed and ceiling boards nailed up. When your plumber arrives on site he will expect his tanks to be awaiting his attentions in the correct place, and not in your materials store.

Failure to stengthen floors for cylinders and tanks

Roof trusses are designed so that the timbers are stressed in tension or compression and this is achieved by ensuring that loads are transmitted only at the connection points. Bending of the timbers is to be avoided wherever possible and, tempting though it may be to use the bottom boom of the truss as though it were a floor joist, this must be avoided. Water storage tanks are of course fixed in place whilst empty, weighing a few kilos. When full of water, a typical tank will weigh 300kg or more, so supporting it by packing it up directly from the bottom truss member is not a good idea. Tanks must be supported on a platform, which itself is supported by bearers located at truss node points and spanning over three or four trusses ask the truss supplier for tank support details and make sure that site work complies.

Leaving the putlogs open

After the scaffolding has fulfilled its purpose, it will be put off-hire. When it is dismantled, gaps will be left in the vertical brickwork joints where the scaffold putlogs were fixed. These gaps need to be filled in with mortar, mixed to the same proportions as the main brickwork. If the mix is varied, they will show as a visible reminder for evermore and will detract from the appearance of the brickwork.

Mixing incompatible plasters

Whether you are going to use wet plaster or dry lining, it is good practice to ensure that all the materials finishing and base coat plasters, boards and adhesives all come from the same manufacturer. Some spectacular problems have arisen because of incompatibility between materials from different manufacturers. Plaster finish can de-laminate from the base coat, base coat can come away from the board material, dot and dab adhesives don't bond to the boards and so on. If all these materials are from the same stable, the fault is easily shown. But if they are not, no one manufacturer will accept liability and you will end up footing the bill, so dont run the risk.

Poor quality brickwork

Apart from their visual appeal, the function of the external skin of brickwork is to act as a rainshield to the internal structure. The quality of bricks is usually reliable and generally consistent the potential weak spot is the mortar, or rather the workmanship and how it is used.

Frogged bricks the v-shaped indent on the top of the brick should be laid with frogs uppermost. However, this requires more mortar to fill them than laying frogs down. Extra mortar means the hod carrier holds more mortar and fewer bricks and as the gang is paid by the number of bricks laid, there is a temptation to speed things up! The problem is even more acute if the bricklayers are being paid for labour and materials frogs down uses far less mortar than frogs up. Frogs up ensures the frog is filled with mortar, and this means the wall is more resistant to rain crossing the bed-joint, reducing the likelihood of damp problems.

Untreated sawn timber ends

Modern houses have a design life of at least 60 years and to help achieve this, structural timber is generally treated before delivery to site against pest attack and rot. Treatments vary, but the protection penetrates only the outer layers and not all the way through. When timber is cut to length, or reduced in thickness after treatment, the cut areas lose their protection. The NHBC requires these cut areas to be re-treated on site. This can be clearly shown to have been done if the preservative applied on site is coloured. Don't use clear treatments or you may end up having to do it twice!

Failed double glazing units

All sealed double glazed units, whether fitted into PVCu, aluminium or timber windows must be fitted into the frames or sashes so that any water can drain away, as prolonged contact with water can cause premature failure of the edge seals. Sealed units housed in timber windows face an additional hazard in that all too often the site carpenter will bed them solidly into linseed oil putty (or mastic) as though they were single sheets of glass. If this is done, the seals around the double glazing will be attacked by the solvents in the putty or mastic within months and will fail. The glazing supplier will of course disclaim liability. The best practice is to arrange for a supply and fit contract, normal with PVCu and aluminium systems. This is not so easy to arrange with timber windows, so in this case be sure first of all that the windows are manufactured with rebates and glazing beads, have proper drainage provisions, all suitable for sealed units, and that the manufacturer supplies comprehensive instructions for fitting the sealed units.

Movement in wooden flooring

Wooden flooring is very fashionable but can provide problems. Solid timber will always be susceptible to moisture changes in the atmosphere. It will always swell during periods when the air contains a lot of moisture and shrink when the moisture content drops. This movement has little or nothing to do with whether the heating is on or not.

The lifting and warping of wooden flooring is usually caused by two common errors. The first is not allowing the concrete slab and screed to dry before laying the floor allow at least one month per inch of thickness after the building is weathertight. The second is not allowing for seasonal expansion leave an adequate expansion gap around the edges and all fixed points as with radiator pipes, door linings and so on.

Poorly laid drains

Drain runs must pass two tests. The first is that they should be laid to a uniform gradient and in as straight a line as possible. Curves should be very gentle and any unavoidable bends located immediately adjacent to a manhole or rodding eye. Self-builders often take on what looks like a simple job and lay the drains beautifully to line and a uniform fall. To achieve this, the drains are often propped up from the trench bottom on any handy brick batts or large stones to get the correct level, then the trench is backfilled. If the brick batts are not removed during backfilling, they will form pressure points on the pipes. Flexible pipes will sag around these points, creating backfalls and ruining the uniform gradient, whilst clay pipes can crack and leak. Either way, an expensive problem results. Any such temporary supports must be removed as backfilling progresses.

Blocked drains

Drains must be free from internal obstructions. These can cause blockages, often self-builders don't discover there are obstructions until the drains are tested. Panic stations ensue to find where the obstruction is and then to clear it. Professionals avoid this problem by threading through a strong cord as they lay the drains and then periodically pulling to and fro a block of wood to keep the run clear.

Damaged vapour membrane on timber frame sections

Timber frames need a vapour barrier between the warm room and the insulation. The barrier is usually located underneath the plasterboard lining and over the internal face of the frame members and is installed at the same time that the plasterboard is fixed. This is after the first fix of services, i.e. electric cables, central heating and so on, which may have been located behind the boarding. These first fixes of services will leave the tail ends of cables, pipes and so on protruding into the room, to be drawn through holes cut in the boards ready to be incorporated within the electrical sockets and radiators. Before they can be drawn through the boards, they also have to be drawn through the (easily torn) plastic sheeting vapour barrier. The openings in the vapour barrier must be kept as small as possible and closed up around the cabling with adhesive tape to maintain the integrity of the barrier. It is a good idea to be around when this is being done, as you need to see that the dry-liners have plenty of adhesive tape to do the job properly. Failure to do so can result in some nasty condensation problems within the insulation element, potentially storing up problems for the future.

Poorly sized floor joists

Floor joists are usually specified and bought as sawn timber, although engineered timber or steel compound beams are increasingly being used. Sawn timber is sold in nominal sizes, and for good reason the actual sizes do vary noticeably. This variability in the depth of joists can cause problems. The undersides of joists are normally bedded level, either on wall plates or joist hangers, so ceiling boards have some chance of being installed flat. This simply means that the upper floor decking has to accept the variations, resulting in a wavy floor if floorboards are used. If sheet materials are used, they bridge over the depressions, causing squeaks and movement underfoot and very possibly peaks and troughs. The real solution is to spend a little more money and pay for the joists to be regularised they will then all be planed to a uniform depth, enabling both ceilings and floors to be fully supported and laid flat. Note that regularising results in a loss of some timber, so make absolutely sure that the resultant depth is still adequate to carry the floor loads over the spans.

Further Reading:

• 20 Most Common Renovation Mistakes, and How To Avoid Them
• 20 Most Common Mistakes Made By Self-Builders, and How to Avoid Them
• The Homebuilding & Renovating Beginners' Guide