Changing an older slate roof to interlocking concrete tiles can alter the load carried by a roof structure. The actual effect depends on the products selected, the existing roof build-up, span and support arrangement, and the condition of the timber. A visual look from the street cannot establish whether an existing roof is suitable for a heavier covering.
Why the covering matters
Roof coverings have different unit weights, and the total permanent load also includes battens, underlay, insulation, ceilings and other fixed components. Concrete tiles are not all the same weight, and slate products vary too. Compare the specific manufacturer’s declared product information and the full proposed build-up rather than relying on a generic rule of thumb.
What should be checked?
A competent assessment may consider the rafters or trusses, purlins, struts, wall plates, connections and supporting walls. Relevant details include member sizes, spacing, spans, bracing, signs of deflection, decay, previous repairs and any alterations such as removed or cut members. The condition of the roof structure and the way loads are transferred to the rest of the building are as important as the tile weight.
Do not overlook wind and weatherproofing
A roof replacement is not only a dead-load question. The fixing specification, exposure, roof geometry and manufacturer’s installation instructions matter for resistance to wind actions and weather ingress. Underlay, ventilation, eaves details, flashings and junctions should be designed and installed as a coordinated system.
Building Regulations and records
In England, roof-covering replacement may fall within the Building Regulations, depending on the work and circumstances. Requirements can include demonstrating that the roof can safely support the new covering and meeting relevant energy-efficiency provisions where applicable. Check the current guidance with the local authority building control team or an approved inspector before work starts; requirements can differ across the UK and depend on the scope. Keep product specifications, structural calculations where required, approvals and completion documentation with the property records.
When to seek structural advice
Arrange an assessment before committing to a heavier covering if the existing roof is old, has visible sagging or damaged timbers, has been altered, or its capacity is uncertain. A structural engineer can review the available information and advise whether calculations, opening-up or other investigation is needed. Avoid assuming that a roof which has carried its existing covering for decades will automatically be adequate for a different one.
A practical checklist
Before work: identify the existing and proposed covering products; establish the roof framing arrangement; check for visible defects and past alterations; confirm the applicable Building Regulations route; and obtain any necessary professional design advice. During work: ensure the installation follows the approved design and product instructions, and document any unexpected defects or changes for review.
This article is general information, not a design or certification for a particular roof. Suitability must be assessed for the individual building and proposed materials.
Want an engineer to check your roof before you change the covering? See our structural inspection service in London. Inspections start from £420* with a written quotation before we visit, or call 020 3983 5057.