Repairs: roofs and rainwater goods

Problem 6: Inadequate purlin end bearings

The size and fixing of the original purlins can be a hit and miss affair, so check the position and section sizes of the purlin (for example, against TRADA loading tables: Span tables for solid timber members in floors, ceilings and roofs for dwellings).

Ideally, when installing new purlins and props, the props should be angled at about 90 degrees to the line of the roof surface to obtain maximum efficiency from the prop, and reduce the likelihood of it bending or being pushed by a lateral load.

Frequently they cling onto the gable or party wall by the equivalent of their fingernails. Often any old soft red brick providing support may have now compressed and crumbled away (wartime bomb damage may have taken its toll too). Cheap bricks were often used on the hidden parts of older buildings (and not always just older ones) and the brick stock may be badly under-fired - consequently the brick could be quite soft.

  • Don’t assume that because the front elevation looks all right that everything else is. Facing bricks were often reserved only for the front elevation, especially in cheaper housing stock.
  • Establish that the props or struts supporting the purlin locate onto something useful (such as a load-bearing wall). Frequently there was no wall within reach of the convenient bit of timber the builder had to hand, and the prop might be found to cantilever onto a ceiling joist.
  • Ensure that the wall is continuous and offers full support down to foundation level. Just because the bit at the top under load looks solid doesn’t mean that it really is!

The sagging first floor

A common problem in an area where the author worked for many years was the 'improvement' of small flat-fronted mid-terraced houses, which had serious consequences for the continuity of load support. For example, the separating wall between the lounge and dining room were removed, often leaving minimal (if any) piers, sometimes with only perfunctory support on the first floor spine wall - which in turn picked up the roof load from the purlin props and sometimes part of the first floor too!

The author was often called in to advise a perplexed householder who had purchased a lovely 'builder modernised' project, where they couldn't account for a sagging first floor that had developed, or perhaps the tears through the woodchip wallpaper lining.

Conveyancing lawyers are now much more aware of the risks of previous alterations and the pre-contract enquiries aim to address this.

The introduction of a universal Home Inspection Pack (HIPs) - a pre sale condition survey proposal which has now been dropped - might have helped highlight this sort of problem caused by ignorance or the gross negligence of the cowboy builder sort, as featured on TV. It is now the rather more specialist sort of topic to highlight in a Homebuyer or Building Survey.

The separating wall between the lounge and the dining room was often removed ...

Load from purlin props, chimney and floor may all impact on the 'missing' wall

Solution 1

If the purlin sits upright in section then a conventional joist hanger may be used to pick up the end and provide an adequate fixing to the wall.

Alternatively, a steel angle 'shelf' fixed to the wall may also provide a good bearing, with the angle fixed to the wall using lengths of stud bolt glued into sockets in the brick using a polyester resin to provide a very strong bond (provided the bricks are not too fragile). (See diagram below).

  • Timber blocks may be required to ‘locate’ the purlin and prevent it sliding on the shelf.
  • Assess the masonry. Are the bricks sufficiently robust? Is a longer shelf or more bolts needed to spread the load?

Otherwise it may be appropriate to carefully have part of the brick course chased out and a reinforced concrete lintel let in to provide a suitably dense bearing (shown below).

Note: If the wall is a party structure you will need to serve a Party Wall Act Notice if the fabric or loading is being modified in any way.

Also ensure that the purlin has adequate fire protection on a party wall.

COST SEQUENCE - Improve purling end bearings

Improve fixity to purlin end bearings and installation of single purlin prop on each of 2 roof slope elevations (traditional cut roof).

  1. Fix steel angle shelf beneath each purlin bearing. Allow for minimum angle shelf with nominal 100mm faces. Allow for minimum 3 no. M15 stud bolt fixings resin grouted into drilled sockets in masonry to inner leaf of gable wall. Use folding wedges to tighten end bearing to purlin as required.
  2. Locate position close to centre of purlin span and cut purlin prop, in minimum 100mm by 50mm section soft wood to locate on internal load bearing wall.
  3. Fix timber to head of internal wall to receive props.
  4. Oppose props onto timber plate and secure with timber gusset/web with screw fixings to side faces or purlin props. Allow for 2 gussets - one each side of the props.
  5. Allow for folding wedges to tighten fixing/bearing points of prop to underside of purlin.

Note: No allowance made for any Party Wall Act service, which may be required for party walls receiving purling end bearings.

Improve purling end bearings and install purlin props - COST £380

Small works pricing

Since the original price estimating in 2008 the economic turmoil of the last few years has caused some disparities in pricing - if you are reviewing these figures in the London area they may look laughable - if you are located in Hull they might still look a bit expensive. Please treat them with caution and of course always consider obtaining more than one quote against which to test prices. Reference to the BCIS or a small works pricing book such as Spons or Laxton's will help assess costs and the hourly labour rates on which they are based.