Repairs: roofs and rainwater goods

If the purlin is at its most efficient position (just under the halfway point of the roof slope) then the purlin props may not conveniently secure onto a central load-bearing wall within the property at the right angle. It all depends on the size and angle of the roof.

Solution 1

Within limits, a departure from the ideal right angle of prop to roof pitch line may be acceptable, but if the angle is too severe you may need to consider some other options. Using 2 (or more) purlins per slope to avoid the need for any (or many) props is one possibility.

Solution 2

Another possibility is to oppose the ends of the purlin props onto a stiff beam (probably timber), which is in turn secured onto a load-bearing wall accessible from within the roof void.

This is feasible provided the loads on each side of the beam would be fairly equal and the beam can be secured and there is no risk of it seesawing or tipping over. It will probably be necessary to secure the bottom of the props onto the beam to prevent this.

Solution 3

The use of galvanised perforated mild steel strap or perhaps of plywood webs is useful.

Approximately equal purlin loads are taken by props onto, e.g. a substantial timber beam supported in turn on load-bearing walls. Typically enclosing walls around the staircase in a terraced house. The joints between the props and bearer could be reinforced with a ply gusset screw-fixed over.

Plywood web fixings or plates – typically using a reasonable quality of 12mm (minimum) plywood to secure together timber joints – is a very sound way of locking together timber by creating a sandwich, providing the elements are of uniform thickness. (Any new timbers inserted as the 'jam' in the sandwich need to match the existing ones to prevent distortion or weaknesses being created).

Secure any critical fixing points using a good quality shear resistant screw – preferably one with a known rated shear strength, particularly where any calculations are required. This emulates the style of gang nail plate fixings found in trussed rafters or joists and provides a very solid fix, while spreading the fixing stresses across a wider area than a simple nailed or screwed joint.

Solution 4

Using plywood webs (see above) can assist with supporting a failing roof onto a new load-bearing ridge beam. For example, if the rafters are not properly opposed and an old ridge board has started to buckle and fail, a series of new ply roof apex webs can be formed to hang the ridge onto a new timber (or steel) load-bearing ridge beam, which in turn can be supported onto props onto the load-bearing walls.

Checks:

• Have the load capacities of the web, screw fixings and supporting posts checked before proceeding, and ensure that Building Control is comfortable with this solution.
• Sometimes the load path and foundations need to be checked and the footing reinforced – perhaps by simple and localised concrete underpinning of the load-bearing wall to pick up the new load transfer. Establish this before specifying and getting the works quoted; or explain the risks in terms of cost and delay (engineer and Building Control) if the client wants to chance it. The risk is theirs, but you need to make it very clear that this may be required as the works progress.

COST SEQUENCE - Plywood web repair to top of rafter/bending ridge board

1. Cut triangular ply webs to fit apex/roof pitch. Cut slot through apex of web to accommodate the ridge beam/ridge board. Use minimum 25mm plywood.
2. Fix web to one face of rafters slotted over the existing ridge board. Allow for minimum 5 screw fixings per rafter slope in premium quality screws rated for shear resistance or use, say, M10 engineering stud bolt.
3. Fix ply gusset/web to each rafter set.
4. Locate new ridge beam beneath the plywood bearings formed in the apex. Allow for folding wedges to finally tension once fixed.
5. Timber ridge beam assumed of nominal section 200mm by 50mm to be located into timber prop's minimum section size 100mm by 50mm taken to load bearing walls internally.
6. Allow for 20 no. engineering stud bolts to be resin grout socketed to adjoining walls with PVC damp-proof course sleeved beneath to isolate any timber from potentially exposed wall surfaces.
7. Allow for stripping back the roof covering over 2m2 to enable the new ridge beam to be inserted into the roof void and reinstatement of roof covering on completion of works.

Plywood web repair to top rafters/beinding ridge board - COST £1,080

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 upon which they are based.