The House Seems to Have Grown!
One of the easiest self build mistakes that can be made is for the groundworkers to dig to the wrong string line. When the property is set out on site, the positioning is marked out on timber profiles, which usually have two nails in them; one for the outside edge of the building and one – measured inwards at half the width of the proposed wall – for the centre dig.
In all the confusion and activity it’s all too easy to put the lines up and mark the ground to the wrong nail. If this occurs, and if the walling is to be properly kept in the centre of the foundation, the building will change size. At best you may have to go back to the planners or at worst, the property will not fit the site.
What you can’t do is build on the edge of the foundation and this may mean that you have to dig down beside the existing foundation and pour a new foundation that then ‘boots’ over the incorrect foundation by at least 225mm. That’s fiddly, and it still may not be enough and you might need to consult an engineer.
Diggers can slip. Dumpers can chew up the ground and obscure the lines. In all the rush the line of the trench can be lost. If this is happening, stop and take stock. If necessary, set up the string lines again and re-mark the ground. In any event, put the lines up again before concrete, just to make sure.
If all of this goes unnoticed, one of the dirtiest tricks can be by the bricklayers — when they find out that the wall is running off the foundation, they may simply smudge some mortar down on the mud and keep building. They may hide what they’ve done by copious mortar or the washouts from the mixer, slopped down beside the foundation blocks. Sometimes they’ll even backfill immediately.
This can also occur when there is a stub foundation that does not go all the way across to meet another foundation. Sometimes it’s measured short by taking the tape from the outside brickwork line rather than that of the inside brickwork. In many cases, such a stub foundation is there to support floor beams or even loads at superstructure level, which makes it doubly important that this doesn’t happen. Root around and check on your nightly visits.
Drains have to be laid to a proper fall. The plans should have the various cover and invert levels marked on them but a drain layer may lose the fall or allow sections to have a backfall.
Drains aren’t supposed to be backfilled until they’ve been inspected but, once again, the reality on site is often different. Check the falls before you allow this to happen and, to avoid the drains being disturbed by traffic, consider a road plate over the backfilled section. Check that the drains are haunched over with pea shingle to a depth of at least 150mm. This allows movement and prevents point loading.
Thought has to be given at foundation stage to those drains and services that have to enter the house. In foundation blockwork a hole will be left with a lintel over it. In trenchfill concrete a duct will have to be formed at the correct level.
Watch For Untidy Brickwork
It’s easy enough to spot brickwork that has been smudged with mortar and, although it’s possible to clean it afterwards, it is the sign of something not quite right.
Some bricks don’t particularly lend themselves to neat work, especially if they are of uneven sizes or contain a lot of creases or smiles in the face, but that makes it all the more important for the bricklayers to take extra care. Make a point of turning back the scaffold board nearest to the walling at night so that if it rains, mortar doesn’t get splashed onto the brickwork. In the past it was not unusual for designers to adjust dimensions to brick sizes.
That is no longer common and a conscientious bricklayer should, therefore, gauge the bricks in order to avoid odd sizes and cuts. Brickwork always looks better if the ‘perps’ or vertical joints line up. You also need to watch out for uneven beds or horizontal joints as this will not only spoil the look of the brickwork but will cause other problems with things like the cavity insulation.
Neat brickwork not only looks good, but it is more structurally sound. The cosmetic appeal is an indicator of the structural integrity and your choice of bricklayer should start with an assessment of how attractive the brickwork on previous projects of theirs is.
Check for Cavity Trays
Any moisture getting into and through the external leaf of the walling should run harmlessly to ground within the cavity. But if there is an interruption within the cavity, it may find its way into the inner leaf and/or the interior of the building.
Where a single storey extension juts out of a building, the external leaf of the main building above its roof line becomes the internal walling of the abutment. In order to prevent moisture (or in the case of some porous bricks or stonework, what can amount to a torrent of water) finding its way into the walling or expressing itself in the ceiling, a cavity tray or a series of stepped trays needs to be employed. These are built into the internal leaf, and cross the cavity at a lower level, to channel water either outside through weeper holes or beyond the abutment to the uninterrupted cavity.
Contractors can forget to put them in. Check they are there and set at the right level. If they’re too low the water will still be expressed in the building. They should run through the walling to tie in with and flash down and over the upright or stepped flashings between the roof of the abutment and the main building.
Maintain the Brickwork Cavities
The cavity, in both traditional and timber frame construction, is designed to create a barrier between the outside walling, which is open to the elements, and the inner shell. Its width is determined by the need for cavity fill insulation and/or the length of the wall ties that hold what are essentially two freestanding walls together. The cavity must be consistent and both leaves of the walling must be plumb upright.
From time to time it makes sense for the self-builder to check on these facts. It also behoves the diligent self-builder to see that the cavities are clear and that no mortar has been allowed to drop into them and lodge on either the wall ties or the insulation. Either of these occurrences will create gaps in the insulation and, thus, a cold bridge.
There are three bedding joints in the blockwork to every nine beds in the brickwork. If care is not taken the blockwork beds can slip below those of the brickwork and the ties will be sloping backwards, which could lead moisture onto and into the inner leaf.