Sagging Roof: Rafter Failure, Purlin Problems & When to Call a Structural Engineer

Quick Answer: A sagging roofline can indicate rafter deflection (individual rafters bending under load), purlin failure (the horizontal beam supporting rafters at mid-span), ridge board problems, or inadequate triangulation of the roof structure. The cause determines the remedy: minor rafter sag may be addressed by sistering new rafters; purlin failure requires beam replacement; missing or rotted collar ties or ceiling ties are a serious structural concern. Any visible drop or bow in the ridge line, or widespread outward movement of the eaves, requires a structural engineer's assessment before any remediation work.

Summary

Roof sagging is one of the more serious building defects a tradesperson is likely to encounter. Unlike a bouncy floor or a crack in plasterwork, a failing roof structure can cause rapid progressive failure — the weight of tiles, battens, felt, and any snow loading is significant, and once a key structural element fails, the loads redistribute to adjacent members which may also be at their limit.

For roofers, builders, and surveyors, the key skill is distinguishing between aesthetic sag (old roofs that have always settled into a slight bow but are stable) and active or progressive structural failure (movement that is ongoing and increasing). This requires understanding how traditional and modern roof structures work and where the failure points typically occur.

Key Facts

• Traditional cut roof — rafters cut and fixed on site; ridge board provides fixing point at apex but is not a structural beam; ceiling joists act as ties preventing the feet of opposite rafters from spreading; collar ties prevent mid-span rafter spread; purlins support rafters at mid-span and are carried by gable walls or internal structure
• Trussed rafter roof — factory-made; triangulated; all members work together; generally very robust; failures typically caused by modification (cutting members) or connection plate failure
• Purlin — horizontal beam running parallel to the ridge at mid-rafter span; typically set into masonry gable or supported on struts from internal wall; if the purlin drops, the rafters lose mid-span support and bow downward
• Ridge board — the horizontal board at the apex; rafters are nailed to it from both sides; it is not a structural beam (it has no bearing on the wall below); its purpose is to provide a fixing point; a failing ridge board causes spreading at the apex but not typically downward sagging
• Collar tie — horizontal timber connecting two opposite rafters at upper third of the rafter; prevents the apex spreading and the ridge dropping; if collar ties are absent or cut, the ridge will drop and the eaves will spread
• Ceiling joist / tie — at the wall plate level, connects the feet of opposite rafters; prevents the feet spreading outward and pushing the tops of the walls outward; critical in traditional roof structures; cutting ceiling joists to create a room-in-roof without adding alternative triangulation causes eave spread and wall damage
• Outward eave movement — the eaves walls of a building move outward when the rafter thrust is not contained; evidenced by a horizontal crack in the masonry at or below roof level, and by the eaves overhang increasing (the tiles move outward as the wall pushes out)
• Rafter rot — particularly at the foot (at the wall plate and near the fascia board) where moisture from blocked or failed gutters saturates the timber; end-grain timber at the foot is most vulnerable
• Snow loading — UK roof design typically assumes 0.6 kN/m² snow loading; a heavy snowfall adds significant temporary load; rafters that are borderline adequate may fail under this load
• Building Regulations (Part A) — any structural repair to a roof must comply; significant work requires Building Control notification; loft conversion (including creating head height by raising ridge) is notifiable

Quick Reference Table

Sagging Roof Fault Finder

Detailed Guidance

Inspection Procedure

Exterior assessment:

1. Visually check the ridge line — is it straight or bowing upward/downward?
2. Check the roofline slopes — should be straight from eave to ridge; a bow in the middle indicates purlin deflection; a sag at the eave indicates foot rot or spread
3. Check the eave line — should be level and consistent; a dip in the eave line indicates rafter foot failure or wall movement below
4. Look along the tile surface — undulation in the tile line reflects the rafter condition below; wavy tiles indicate wavy rafters

Interior assessment (from loft):

1. Carry a torch and wear a hard hat (hanging purlin/rafter nails are a hazard)
2. Look along the rafter undersides — sighting from one end to the other shows bowing clearly
3. Check the purlins: look along the length; probe with a bradawl at the bearing points (purlin sitting on masonry or struts); test the struts for rot and connection
4. Check collar ties and ceiling joists: are they all present? Are any cut, cracked, or notched beyond limits?
5. Check the wall plate: the timber at the top of the wall; probe for rot; look for gaps between plate and masonry (indicates movement)
6. At rafter feet: probe for rot; check the connection to the wall plate (should be skew-nailed or rafter clipped)

Assessing Purlin Condition

Purlins are the most common cause of significant roof sagging. A deflecting purlin allows all the rafters it supports to sag simultaneously — the characteristic "wavy slope" pattern.

Types of purlin support:

• Gable support: purlin rests in a pocket in the masonry gable end; check the pocket condition and whether the end of the purlin is decayed
• Internal strut support: purlin is carried by diagonal timber struts rising from an internal load-bearing wall; check the strut connection at both ends and the condition of the wall plate below
• Hanging post: purlin is carried by a vertical post from the ridge purlin in a king post truss arrangement (more complex structures)

Signs of purlin failure:

• Purlin has visibly bowed downward (sight along its length)
• The purlin has twisted or rolled, so the rafters are no longer sitting squarely on it
• Decay at the bearing point (purlin end in masonry, or strut junction)
• Connection between purlin and gable has opened (gap visible)

Remediation:

• If purlin decay is limited to the end: cut back to sound timber; install a flitch plate (steel plate bolted to the side of the purlin and extended to a new bearing); or install a new purlin alongside the old one (not always possible in a low loft)
• If purlin is deflected but structurally intact: add a mid-span support (timber post or steel prop to a structural wall below); a structural engineer should confirm load path to the foundation
• If purlin is failed: replace; this requires temporary propping of all rafters, removal and replacement of the purlin, then removal of temporary props; scaffolding access and a structural engineer's design

Rafter Sistering

For individual failed or decayed rafters:

1. Remove the relevant tile area (tile lifter + roofer or scaffolding)
2. Expose the rafter
3. For rot at the foot: cut back to sound timber; fit a new rafter section using a bolt or connector plate joint; the new section bears on the wall plate or on a new wall plate piece
4. For a full-length undersized rafter: sister a new full-length rafter alongside (same size or larger); connect to ridge board and wall plate with appropriate fixings; fix the two together with regular nailing or ring shank nails
5. Reinstate tiles; refix battens to new rafter if needed

Emergency — Active Structural Movement

If the inspection reveals active movement (crack monitoring shows progression; walls are visibly moving; the ridge is dropping month on month), emergency propping may be required:

1. Prop the ridge (or purlin) from below using Acrow props on a spreader board
2. The spreader board distributes the load; the Acrow transfers it to the floor below; ensure the floor below can carry the load (check if it is a suspended timber floor itself)
3. Call a structural engineer to design permanent repairs
4. Inform Building Control if the props are a temporary measure pending notifiable work

Frequently Asked Questions

The loft has been converted and the collar ties were removed to create head height. How serious is this?

Very serious if alternative triangulation has not been provided. The collar ties prevent the roof from spreading (the apex dropping, the eaves spreading). Removing them without adding alternative measures (such as a substantial ridge beam, or a new structural frame to carry the rafters at the level where the collars were) is a structural modification that changes the roof from self-supporting to requiring a new load path. A structural engineer must assess the condition and design any remediation. This situation also means the loft conversion was probably carried out without Building Regulations approval — a potentially serious issue for the property's sale and insurance.

I can see some wavy tiles but the loft looks fine from inside. Could the tiles just be laid unevenly?

Yes — tiles can be wavy due to uneven battens, a wavy sarking board/felt, or tiles that have slipped slightly. Access the loft and sight along the rafter undersides directly. If the rafters are straight but the tiles are wavy, the issue is in the battens or tile laying. If the rafters themselves are bowing, the structural investigation begins.

Does a sagging roof affect buildings insurance or mortgage valuations?

Yes, potentially both. A mortgage valuer who flags a sagging roofline in their report may require structural investigation as a condition of the mortgage offer or a retention against the purchase price. Insurers may also decline or limit cover if a known structural defect has not been addressed. Having a structural engineer's inspection report confirming the cause and a remediation plan in place — even if the work is not yet done — is usually sufficient to satisfy both. Do not ignore a sagging roof when selling or remortgaging; disclose known defects to avoid later claims of misrepresentation.

Regulations & Standards

• Building Regulations Part A (Structure) — requirements for structural elements including roof structure

• BS 5268-3:2006 — Structural use of timber; trussed rafter roofs

• BS 8103-3:2009 — Structural design of low-rise buildings; floor, roof and ceiling

• NHBC Standards Chapter 7.2 — Pitched roofs; timber specification and structural design

• TRADA: Roof Design Guidance — Timber roofing structural guidance

• Historic England: Roof Structure Repairs — Repair guidance for traditional roofs

• BRE Good Repair Guides — Roof repair and assessment guides

• cracked walls — Wall cracking associated with roof spread

• bouncy floor — Similar structural assessment for floor members

• roof tile types — Tile removal and refixing during repairs

• leadwork — Valley and flashings around roof structure