Roof structure repairs for sagging and spreading roofs

Most roof problems people notice from the street, a dipping ridge or a wavy line of tiles, come from the timbers underneath rather than the coverings. Rafters bend when purlins are undersized or struts have been taken out, and a roof without enough ties pushes its feet outward and the walls with them. Many of these roofs have moved a little and then stopped, while some keep going.

A common trigger is re-roofing. Swapping lightweight slates for heavy concrete tiles can add significant weight to a frame that was never designed for it, and older roofs often have little spare strength. Loft conversions and storage boarding that cut out struts or collars do the same. We look at the frame from the loft and the outside, and bring in an independent chartered structural engineer where strengthening needs designing.

What are the signs you need roof structure repairs?

• Ridge line dipping in the middle when seen from the street
• Roof slope looking hollow or wavy between the ridge and the gutter
• Top of an outside wall leaning out at eaves level
• Horizontal cracks in the wall just below the eaves or ceiling line
• Struts, collars or ties cut out or missing in the loft
• Daylight through the tiles, or timber that is damp, stained or crumbling

What causes it?

Roof spread

Rafters push outward at their feet as well as down. Ceiling joists or collars normally tie each pair together and resist that thrust. Where ties are missing, poorly fixed or cut away, the rafters spread, the ridge drops and the walls are pushed out at the top, often with a horizontal crack just below the eaves.

Heavier roof coverings

Re-roofing with concrete tiles in place of natural slate or lighter tiles can add a lot of weight to a frame sized for the original covering. The structure should be checked when the load increases significantly, but this has not always happened, and rafters and purlins then deflect, sometimes years later.

Undersized, cut or removed purlins and struts

Purlins support rafters mid-span, and struts carry the purlins down to a load-bearing wall. Older roofs often have slim purlins for their span. Others have lost struts to a loft room or water tank, or are propped on a ceiling rather than a wall, so the load has nowhere proper to go.

Rot, woodworm and failed wall plates

Leaks around chimneys, valleys and at the eaves keep timber wet, causing wet rot at rafter feet and along the wall plate the rafters sit on. Woodworm weakens timbers more slowly. Once the wall plate or rafter feet lose strength, the roof can settle unevenly and pull away from the wall head.

How is it repaired?

How much will roof structure repairs cost?

The price depends on how many members need work, whether access is from the loft or from scaffolding, the roof’s span and covering, and whether an engineer’s design and building control are involved. Re-covering with a lighter material, where it is chosen, is priced separately.

See regional prices on the prices page or work out a guide price for your home with the guide price tool.

What happens, step by step?

1. Send photos

   Photograph the roof from across the street, both slopes if you can, and the loft timbers with a torch. Include the eaves and any cracks just below them. From these we can usually tell sag from spread and whether an engineer is needed.

2. Loft and outside survey

   On site we check rafter sizes and spacing, purlins, struts, ties and wall plates, look for rot and past alterations, and measure any lean at the wall head. The covering is noted too, because its weight affects the repair.

3. Engineer’s design where needed

   Where purlins, struts or ties must be redesigned, or the roof carries a heavier covering, an independent chartered structural engineer is appointed to specify the work. We can arrange this, and their design sets the sizes and fixings.

4. Written quote

   You receive a no-obligation written quote listing each timber repair, any ties or straps, propping, scaffolding and making good. Once the repair is specified, the figure becomes a fixed price in writing.

5. Prop, strengthen and tie

   The roof is propped where members are being changed, then rafters, purlins, struts and ties are repaired or added as specified. Coverings disturbed by the work are relaid, and building control inspects where the work needs it.

How long will roof structure repairs take?

Strengthening rafters or purlins from the loft usually takes two to four days, and tying in a spreading roof with scaffolding in place often takes about a week.

Do you need building control or a party wall notice?

What if you have an insurance claim open?

Roof spread, overloading and slow timber decay are usually treated as wear, poor workmanship or design faults, and most buildings policies exclude them. Storm damage and sudden collapse can be different, and some policies cover damage caused by a sudden event even when the gradual cause is excluded. Check your policy wording, and if you think a sudden event is involved, tell your insurer before permanent repairs start.

Roof structure repairs: frequently asked questions