Back-to-back houses: a structural guide

Leeds holds the largest surviving stock of back-to-back housing in the country, and went on building them later than anywhere else. Bradford has a substantial stock too. They are a genuinely distinctive building type, and almost everything written about terraced houses assumes something that is not true of them.

What a back-to-back actually is

A through terrace has two external walls - front and back - and party walls on either side. A back-to-back has one external elevation. It shares party walls with the house on either side and with the house directly behind it, which faces the opposite street.

So each house is enclosed on three sides by other people's houses. That single fact produces most of what makes them different, structurally and practically.

The structural consequences

• Three of your four walls are shared. Almost any structural alteration to them engages the Party Wall etc. Act 1996, and often with more than one neighbour at once.
• The block is one structure. A row of back-to-backs is a deep, mutually restraining mass of masonry, which is part of why they have stood so well. It also means movement, and alterations, propagate.
• There is one elevation to work with. Extensions, new openings, light and ventilation all have to come from a single wall, or from the roof.
• Chimney breasts and flues are frequently in party walls, shared or adjacent to a neighbour's. Removing one is not a private decision.
• Most have cellars, and the cellar walls are retaining ground on at least part of their perimeter as well as carrying the house.
• Roof structures often run continuously across several properties, so a roof problem is rarely confined to one address.
• Many were later converted - two knocked into one, or a back-to-back joined to the house behind to make a through house. Those conversions are where the interesting structural questions usually are.

What is normal

Shallow foundations, lime mortar, solid walls, sloping floors, out-of-square door frames, old decorated-over cracking and a damp cellar are all ordinary in a building of this type and age. So is a degree of settlement across a whole block.

These houses have a good structural record. Deep, well-restrained masonry blocks are stable things, and the population that has survived to today is a self-selected one - the poorly built stock was largely cleared decades ago.

What is worth looking at

• Previous alterations, and whether they were designed. Knock-throughs between two back-to-backs, or into the house behind, remove masonry from a structure that works by mass and restraint.
• Chimney breast removals in party walls, and whether the stack above is supported - this is the most common defect we find in the type
• Cellar walls: bulging, water ingress, and whether anyone has lowered the floor to gain headroom without underpinning
• The single external elevation, which takes all the weather the house gets and is the one wall you can inspect properly from outside
• Roof structure and its condition across the block, including whether stone slates have been replaced with something lighter or heavier
• Joist ends bearing into the external wall, where damp concentrates
• Damp generally - not a structural defect in itself, but it drives timber decay, which is

What you can and cannot do to one

You can do a great deal. Back-to-backs are altered constantly - opened up internally, converted, extended into a yard, and given loft rooms. What you cannot do is treat any of the three shared walls as if they were yours alone.

Practically, that means: expect the Party Wall etc. Act to apply, expect to serve notice on more than one neighbour, build the notice periods into your programme, and get the structural design done before you talk to a builder. It also means an engineer will want to understand the whole block, not just your house.

Cellar conversion deserves a specific warning. Lowering a cellar floor to gain headroom means underpinning walls that are already retaining ground and carrying a house that is restrained by two or three neighbours. It is the highest-risk domestic alteration available in this building type, and it is not fitting-out work.

When to call an engineer

Get it inspected if you are buying one and want the alterations checked, if a chimney breast has been removed with nothing evident supporting the stack, if there is fresh cracking or bulging, if the cellar walls are moving or you are thinking about lowering the floor, or before any work touching a shared wall.

Sloping floors, old cracking and a damp cellar on their own are the building being 130 years old.

How we would investigate it

By reading the block as well as the house: the external elevation and the neighbours' elevations from outside, the roofline across the row, cracking classified against the BRE Digest 251 categories, the cellar, the roof space, and every alteration we can identify with a view on whether it was ever designed.

The commonest conclusion is that the house is doing exactly what it has done for a century and the alterations are adequate - and where a chimney breast has been left unsupported, that it has been that way for decades and can be put right calmly. We do not carry out the work and take no commission from anyone who does. A structural survey and report is a fixed £580 + VAT, report inside 10 working days of the inspection, guaranteed.