Be Your Own Surveyor, External Walls – Outside.

Be Your Own Surveyor, External Walls – Outside.

Most UK homes have walls of masonry, e.g. brick, block, stone or concrete, either with or without a cavity. The cavity separates the damp, cold outer layer from the dry, warm inner layer. There are exceptions using modern timber framing methods, and Elizabethan houses are often timber-framed with a cavity, so it’s nothing new! Cavity wall insulation really helps keep the inner skin warm and condensation-free. Most homes built before about the second decade of the last century have solid walls.  If masonry walls are about 9 inches thick or less they are solid (a brick is 4 inches thick, so two bricks plus plaster = about 9 inches). Thicker walls doesn’t necessarily indicate a cavity as earlier homes were often built with very thick solid walls.

The first check should be whether the walls are plumb. If there is significant lean it’s usually perceptible to the eye. You can visually line up the edges of the house with other buildings as a rough gauge to see when walls are out of plumb. Are the two edges parallel? To check if it’s your building that’s leaning, dangle a plum bob (a bit of string with a weight attached) out of an upstairs window. If it’s parallel to the wall, all well and good. If the variation between top and bottom is more than a third of the thickness of the wall, it’s potentially unstable. I wouldn’t necessarily leave a house because the walls were leaning slightly, particularly if they were very thick, but it could be due to subsidence. If unsure get a structural engineer’s report. Underpinning if needed will cost several hundred pounds per linear meter, depending on depth, access, soil type etc

Another cause is that roofs can push them out. The rafters are diagonal and press out sideways on the walls, unless they are tied in at the bottom by ceiling joists which are continuous throughout the whole width of the house. Tying them in is not complicated, but re-righting leaning walls is generally impractical.

A little surface cracking of render is usually due to thermal movement as the building expands and contracts over the seasons and at different times of day. They are nothing to worry about, just a reminder of where someone should have put an expansion joint but failed to do so! Cracks more than three or four millimetres wide can indicate ground movement of the foundations. This is also indicated by the courses (layers) of bricks becoming uneven and out of level. Go to the edge of the building and look along the mortar joints. Are they straight and level? Or do they dip at piers and rise at windows? The latter indicates that the heavier parts of the building are sinking on it’s foundations. If the building has been rendered, you can look along the window sills, or along any coping. Again, a little movement isn’t necessarily a concern, especially if the building is over 100 years old and most of the movement may have occurred early in its life.

Render (a wall-coating of sand-and-cement) should be checked for adhesion to the wall by tapping it with a screwdriver handle or similar. If it sounds hollow, it may be partly detached and need replacement. This can let water in. Try scratching it with an old screwdriver, if you can easily it’s too soft. In a misguided attempt to save money, some buildings are rendered with as little as 1 part cement to 12 parts sand, which is far too weak, porous and soft.  It’ll need chipping back and replacing with a minimum 4:1 mix and a waterproofing additive.

If the masonry is unrendered, check the pointing (the outer layer of harder cement in the joints). Is it loose or missing? Are the bricks or stones themselves crumbly from frost or wind damage? Soft, cool-fired bricks are particularly prone to this, as are Yorkshire gritstone and schist. This can allow moisture penetration. Rendering is generally cheaper and more reliable than re-pointing. A bonding chemical called SBR is used to prime the masonry. If walls are very loose, sheets of perforated metal (expanded metal lath or EML) should be bolted to the walls to provide an adhesive base. With scaffolding this can be quite expensive.

I’ll write about inspecting external walls from the inside next week.

Rich

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