Renovations: Penetrating Damp
Renovations: Penetrating Damp
Renovations: Penetrating Damp
Pointing pointers
Check the mortar pointing between bricks. If this has crumbled or fallen out, it is fairly easy for rain to enter the wall. Begin by knocking out old unsound pointing with a little cold chisel and club hammer, aiming to cut back to a depth of about 10mm. Brush out any loose debris, then splash in clean water to aid the new mortar to stick.
Use a 1:4 mix of cement and sharp sand: for little areas purchase this dry ready-mixed in bags. Make a 'hawk' from a square of chipboard with a dowel deal nailed inside the centre and use this to hold a modest amount of mortar close to the wall. Make the mortar into a pudding shape then cut off a sausage-shape on the back of a little pointing trowel. Press this firmly into place between bricks, then continue until you have covered about one square metre then go back and apply a pointing profile to match the surrounding wall.
You can find three principal pointing profiles created to reflect rainwater from the surface of the wall:
Flush pointing - Smooth the mortar level with the face of the wall, making use of a wad of sacking or coarse hessian. Use for sheltered walls only, or where the surface would be too big to be painted.
Rounded pointing - Indent the mortar making use of a length of 10mm diameter dowel or plastic tube to give a delicate half-round profile.
Weather-struck pointing - For exposed walls use the trowel to shape the mortar into a neat sloping bevel, use a lengthy straight timber batten held against the wall to steady your hand as you work.
With either pointing profile, tackle the verticals just before the horizontals.
Wall faults
With the pointing completed, check the masonry. Moisture penetrating a cracked brick may perhaps freeze in winter, then expand and literally 'blow' the face off the brick: this "spalling" allows moisture to soak in to the extent that rain often appears to pour from the wall. Cracked or soiled bricks ought to be hacked out with a cold chisel and club hammer and replaced. Chop around the brick to totally free it or drill into it numerous times with a big masonry bit, then attack the brick and remove the fragments.
If bricks aren't badly spalled, they can be fixed with a bit of car body filler and brick dust sprinkled on to match the brickwork.
Replacing a brick
To replace a brick attempt to find a second-hand one to match the rest of the wall. Clean up the inside of the hole, dampen it, and then line the bottom and one end with a stiff mortar mix.
Butter the top and one end of the replacement brick by scraping the mortar off the trowel. Form a wedge-shape and furrow the mortar to aid suction. Insert the brick in the hole, buttered end to clean end, tap into place and point the joints.
Filling
Where the damage is much less extensive, rake out cracks and modest holes, undercutting the surrounding sound material to form a ledge for the filler material to grip to. Fill with a 1:5 mix of cement and sharp sand or a proprietary exterior filler.
Blown render
'Blown' or blistered areas of render that have come away from the brickwork will need to be cut away with a hammer and chisel and filled with mortar to within about 25mm of the surface. Enable this to stiffen, then scratch the surface with the edge of a steel float or trowel to present a key for the finishing coat. This can consist of plain mortar or mortar mixed with fine aggregate - depending on the texture of the original - to attempt to match the surrounding surface. Once applied, saw it off level with the surrounding wall by drawing a straight edged timber batten across the surface
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