Roof strengthening design
Design to strengthen an existing roof that is sagging, damaged or needs to carry new loads such as heavier coverings.
About this service
Many London roofs have been quietly overloaded or weakened over their lives. Victorian slate roofs were often re-covered with heavier concrete tiles in the twentieth century, purlin struts were removed to clear space for loft storage, and valley gutters in butterfly roofs have leaked into timbers for decades. The result is a roof that dips between rafters, sags along the ridge, or pushes the tops of the walls outward. Roof strengthening design deals with these problems methodically.
We work on pitched and flat roofs on houses, converted flats and small mixed-use buildings. Typical triggers are a visible dip noted by a surveyor, a roofer who finds rot or beetle damage once coverings come off, or a plan to switch to a heavier covering such as clay tiles. Where the new load is solar panels or plant, a roof load assessment is often the better starting point.
Common solutions include new or reinstated purlins and struts, sistered rafters, steel or timber ties to stop spread, replacement of decayed wall plates, and upgraded flat roof joists. Our three-step quality control process applies to every set of calculations, with final sign-off by our Head of Engineering. Fees start from £450 + VAT, fixed in writing before we start, and we liaise with Building Control where approval is needed.
When you need it
- The roof is dipping or spreading
- You're changing to a heavier roof covering
- Roof timbers have rot or insect damage
What's included
- Assessment of the existing roof
- Strengthening design
- Calculations and details
How it works
- Inspect inside and outWe look at the roof from the loft and externally, recording timber sizes, spans, support points, signs of spread at the eaves, and areas of rot or insect attack. Photos from a roofer help where coverings are already stripped.
- Work out what has changedWe identify why the roof has moved. Common causes are a heavier covering, removed struts or ties, decay at bearings, or ceiling joists cut during earlier loft or chimney works.
- Design the repairWe design new members, connections and bearings so loads find a proper route down to walls that can carry them, with details drawn to suit work from inside the loft wherever possible.
- Issue and support the rooferWe issue calculations and drawings, deal with Building Control where needed, and answer questions from the roofer or carpenter once they are on site and opening up.
What you receive
Roof spread, missing ties and why walls lean out at the top
A traditional pitched roof wants to flatten under load, pushing the rafter feet outward. Ceiling joists or collar ties normally hold the two slopes together. When those ties are cut, poorly connected or rotten at their ends, the roof spreads, the ridge drops, and the top courses of the front or rear wall may bow out. It often shows as a crack along the eaves line or a gap between ceiling and wall upstairs.
Spread is common after loft conversions carried out without engineering input and after chimney breast removal that took away support. Restoring the tie is usually central to the repair, alongside any wall restraint. If you are converting the loft at the same time, designing both together is more economical, as our loft conversion structural guide and loft conversion structural package explain.
Projects that used this service
Frequently asked questions
Is a sagging roof dangerous?
Not always. Many old roofs have a gentle historic sag that has been stable for decades. It becomes a concern when the sag is getting worse, timbers are cracked or rotten, the walls show signs of spread, or tiles are slipping. An engineer can tell whether the movement is active and whether strengthening is needed now or can be monitored.
Can I replace slate with concrete tiles on my roof?
Concrete tiles are considerably heavier than natural slate, and many Victorian roofs were sized only for slate. Changing to a significantly heavier covering usually needs Building Control involvement and a structural check of the rafters, purlins and supports. Sometimes the existing roof copes, but strengthening is often needed, so check before the roofer orders materials.
What causes roof timbers to rot?
Wet rot follows persistent moisture, usually from leaking valleys, blocked gutters, failed flashings or poor ventilation. Dry rot also needs damp conditions but can spread through masonry. Woodworm and death watch beetle are more common in older or damp timbers. Fixing the source of moisture comes first, then decayed timbers are replaced or strengthened to an engineer's design.
How much does roof strengthening design cost?
Our roof strengthening design starts from £450 + VAT. The fee depends on the size and complexity of the roof and how much opening-up is needed to see the timbers. We confirm a fixed fee in writing before any work starts. The cost of the building work itself depends on your contractor and is separate from our fee.