Stone Flag Roofs In Bolton And Westhoughton

Key takeaways

• The timber under a stone roof carries a load most modern roofs never see, and it has been doing so a long time.
• Old timber creeps slowly under sustained load, so a gentle even dip is often historic and stable.
• A sharp local dip, a kink in the ridge or recent movement is a different matter and needs a structural view.
• Bearing points at the eaves and where purlins enter the walls are where rot does the most damage.
• Strengthening a frame is usually possible and is far cheaper than the alternatives.

What is carrying the weight

Under a heavy stone roof there is generally a substantial cut frame: rafters running from a wall plate at the eaves up to the ridge, with one or more purlins running horizontally partway up to shorten the span, and those purlins carried either by the gable walls or by struts standing off an internal wall below. Ceiling joists tie the feet of the rafters so they cannot spread outward. Everything is in balance, and every element depends on the others. It was designed for the load it carries, which is why these frames are noticeably heavier than what you find under a slate or tile roof of similar size.

Creep, and why some sag is normal

Timber under continuous load deflects slowly over time, a behaviour engineers call creep. Over a century a rafter or purlin will bend gradually and then find a position where it is stable, and it will stay there. The result is the gentle undulating roof line visible on old buildings everywhere, and it is generally not a fault. This is worth saying plainly, because a dip in an old roof causes a great deal of unnecessary alarm and occasionally unnecessary expenditure. The distinction that matters is not whether the roof line is straight but whether it is still moving.

The shapes that are not normal

Certain patterns indicate something other than gradual settlement. A sharp localised dip, as opposed to a long smooth curve, suggests a specific member has failed rather than the whole assembly slowly deflecting. A step, where one section has dropped relative to the section beside it, suggests a support has gone. A kink in the ridge line suggests movement at a purlin or at a gable. And any dip accompanied by new cracking in walls or ceilings below, by doors that have started to bind, or by a gap opening between a ceiling and a wall, indicates the movement is current rather than historic.

Where rot does the damage

Timber in the middle of a span rarely rots, because it stays dry. Timber at its bearing points frequently does. The rafter feet sit at the eaves, which is exactly where water from a failed gutter or a blocked valley ends up, and the ends of purlins are often built into the gable walls, which is exactly where a failed verge or eroded pointing lets water in. Timber that stays damp loses strength and eventually crushes under load, and because the failure is at the bearing rather than mid span, the whole member drops. On a heavy roof that drop is more consequential than on a light one.

Alterations, and the support that quietly went missing

A great many older houses have had internal walls removed, chimney breasts taken out, or roof spaces converted. Where a purlin was strutted off an internal wall that has since gone, or where a chimney breast that was helping to carry something was removed without proper support being substituted, the roof finds a new position. Chronology is the diagnostic here. Work carried out shortly before a dip appeared is the most valuable thing you can tell an assessor, and it is the detail most often left out. Chimney breast removal is a particularly common culprit because the stack above frequently relied on the breast below.

Establishing whether it is moving

This is answered with time and a camera rather than with expertise. Set up one viewpoint you can return to, include something permanent such as a lamp post in the frame, and take the same picture every few months with the date recorded. Inside, if there is a crack, mark its ends in pencil with the date, or fit a tell tale across it, and watch whether it extends. Being able to demonstrate that nothing has shifted is valuable twice over: it settles what needs doing now, and it answers the question a surveyor would otherwise have to escalate when the house is sold.

Who assesses it

A roofer can report on the covering, the battens, the fixings and the visible condition of the timber, and that is a genuinely useful first step, particularly since they will be in the loft anyway. If the question becomes whether the structure is adequate and what has failed, that is a structural engineer's judgement, and their report is also what a competent builder will want before doing remedial work. Be cautious of any proposal to cure a sagging roof by recovering it, since a new covering adds weight to a frame that has already indicated it is struggling.

What remedial work usually looks like

Rarely demolition, and rarely a new roof. More often it is targeted: sistering a weakened rafter with a new timber alongside it, replacing a rotten rafter foot and the section of wall plate it sits on, renewing or supplementing a purlin, restoring a support that was removed, or introducing a steel where an original load path is genuinely gone. Work of this kind is disruptive while it happens and it is a fraction of the cost of replacing a roof. The important sequencing point is that any water getting in has to be stopped first, because putting sound timber into a wet structure simply starts the clock again.

Insurance, subsidence and what is actually covered

Movement in a building raises the question of insurance, and the answer is usually narrower than people hope. Policies commonly cover subsidence, heave and landslip as defined perils, subject to an excess that is typically substantially higher than for other claims. What they generally do not cover is gradual settlement, the ordinary deflection of old timber under load, or damage attributed to poor maintenance or previous alterations. Since most distortion in older buildings falls into those excluded categories, a claim frequently is not the route. It is still worth notifying an insurer where genuine structural movement is suspected, because they may commission an investigation that would otherwise cost you, and because failing to disclose known movement can affect cover later. Keep any correspondence, since a documented history of having reported and investigated movement is considerably more useful at sale than a recollection that somebody once looked at it.

Getting a useful engineer's report

If a structural opinion is being commissioned, it is worth being specific about what you want it to answer. A report that describes what is there without reaching a conclusion is of limited use. What you want is a view on whether the movement is historic or current, what caused it, whether the structure is adequate in its present state, and what if anything should be done. Ask for that explicitly. It is also worth providing whatever history you have, including the dated photographs, any knowledge of past alterations and the age of the building, because an engineer working without that context is reconstructing it from scratch at your expense.

Frequently Asked Questions

Is a dip in an old stone roof dangerous?

Often not. Timber under continuous load deflects slowly over decades and then stabilises, which produces the gentle undulating roof line common on old buildings. What warrants attention is a sharp or localised dip, a kink in the ridge, or movement accompanied by new cracks and sticking doors.

How do I know if the roof is still moving?

Return to one viewpoint every few months with something permanent in the frame, recording the date each time, and put a dated pencil mark at each end of any crack indoors. What you are testing is whether anything is still shifting, and that answer counts for far more than how pronounced the dip currently appears.

Why does rot happen at the ends of the timbers?

Because that is where water reaches. Rafter feet sit at the eaves where failed gutters discharge, and purlin ends are often built into gable walls where a failed verge lets water in. Timber that stays damp loses strength and crushes at the bearing, which drops the whole member rather than bending it.

Could my extension or loft conversion have caused this?

It is a common cause. Removing an internal wall that was carrying a strut, or taking out a chimney breast that was supporting the stack above, changes the load path and the roof settles into a new position. If the dip appeared or worsened after building work, that sequence is the most useful fact you can give an assessor.

Do I need a structural engineer or a roofer?

Start with a roofer, who can report on the covering, the fixings and the visible timber while they are in the loft. Escalate to a structural engineer if the question becomes whether the frame is adequate, since that is their judgement and their report is what a builder will want before doing remedial work.

Will the whole roof need replacing?

Usually not. Remedial work is typically targeted: strengthening a rafter alongside the existing one, replacing a rotten foot and wall plate, renewing a purlin, or restoring a support that was removed. Any water getting in has to be stopped first, or new timber simply begins the same process again.