Repairs: roofs and rainwater goods

Typically this type of roof is found on a single-storey addition for, say, a kitchen or outhouse in an old terraced house.

Many of the roof spans were probably at the span and load limit when originally constructed, so the addition of any new heavier roof covering (concrete tiles perhaps) is likely to rapidly exacerbate the problem. Many such roofs are badly ‘dished’ and sagged as a consequence. Even when covered in lighter slate the roof will be prone to sag.

The sagging may not look too drastic until checked against a long straight edge such as a 1.5-m spirit level; the divergence is likely to assume more dramatic proportions when it can be measured rather than simply checked by eye.

Often these roofs have 'skilling' ceilings – that is to say, the ceiling finish is applied directly to the underside of the rafters and there are no separate ceiling joists to restrain the rafter framework. A common result is that the roof starts to slide away – opening up flashings at the external top edge or possibly along the sides of any enclosing walls, and possibly causing the outer wall to buckle as well.

On older properties these roofs tend to be built off half-brick-thick single-storey back addition walls, so consider:

• Is the repair economic, or is it worth reconstructing the whole part of the building to a higher standard?
• Is a local authority grant available for improvement works as part of the project?
• Are there any other ways of improving the weather resistance of the wall? Examples include:
• applying an external damp membrane, insulation batts and metal lathing and render; or
• an internal membrane, insulation batts and a plasterboard finish (care usually needs to be taken however to ensure the floor is also isolated by a damp-proof membrane (DPM) or similar, and also to insulate it in many instances, to prevent damp ingress or cold bridging since the old floors are often solid and have no original DPM).

Solution 1

If the wall plate at the head of the wall is in sound condition it can be fixed or refixed to the wall and a mild steel strap used to lock the rafters to it.

• Check that the tops of the rafters have not been affected by years of moisture trickling through cracked flashings, and that they have adequate bearing and integrity onto the wall plate.

Solution 2

If the wall plate at the head of the wall is decayed or compromised it should be fairly straightforward to 'plate up' the rafters with new timbers screwed up alongside for, say, a metre or so along the length of each affected rafter, and fixed back onto the wall plate.

Sometimes the rafters are found to be only resting or leaning onto the wall; sometimes set into sockets in the brickwork.

Solution 3

A new wall plate can be pushed up from underneath and fixed with resin anchors or studding set into polyester resin glue and then bolted in place.

If there is sufficient headroom, a ceiling can be formed on new ceiling joists and the joists can be fixed at each end to restrain the roof – if the span is short enough. If not, then noggings can be used to brace the ceiling joists and binders run over them to tie the rafter feet in.

Solution 4

Where a new ceiling cannot be created, a purlin can be run across the roof in the usual way; if needs be, making a feature of the timber by cleaning it up and finishing it if it has to be exposed through the skilling ceiling.