What to consider when costing a roof

The cost of tiling or slating a roof is dependent on the complexity of the roof to be covered and the final choice of tile. If you watch roofers at work, you’ll notice that the tiling to the main roof plains are laid very quickly, often in just a few days, and that the edge and joint details follow on at a more sedate pace.

Costing a roof Therefore the higher the complexity of roof shape will result in a significant increase in labour charges. That said, it is of course possible to advise on how different roof tiling systems compare. To that end, the Build It Estimating Service has modelled the effect of choosing 6 different tile types for tiling a moderately complex ‘test’ roof.

This ‘test’ roof contains a number of features. Information on the necessary batten size and spacing and tile usage quantities for plain areas of tiling are readily available from merchants and manufacturers, but the estimator needs to think of the extras required for details such as the hip, ridge and verge fixing and ventilation systems; extra tile nailing and clipping, additional battens, undercloaks and eaves tilting fillets; purpose made tiles or slates for certain systems; the choice of valley detail and lead and mortar work where required.

Furthermore, when ordering tiles, and calculating your roofing costs you need to make allowance for breakages, again a more complex roof shape will necessitate more tile cutting and therefore risk more breakages. Here the tile choice also has an impact. For example fibre cement slates are much easier and quicker to cut. Compared to natural slates so the breakage rate will be much reduced.

Costing a roof

Here we have concentrated just on the cost of the roof finishes, but do remember that your choice of tile system may also affect the cost of the roof structure beneath. Plain tiles, which require tiles to be double lapped to prevent water penetrating the gaps between adjoining tiles. It can increase the tile load on the structure by as much as 50% over interlocking tiles which only need to be single lapped.

Prices Ex VAT inc. OHP
Tiling Type Labour Material Total VAT @20% Total inc. VAT
Small format plain interlocking concrete tiles £3,258.50 £4,309.41 £7,567.91 £1,513.58 £9,081.49
Plain clay tiles £4,482.69 £7,057.74 £11,540.43 £2,308.09 £13,848.51
Natural welsh slates (400 x 200) £4,982.15 £13,324.38 £18,306.52 £3,661.30 £7,060.67
Large format roman interlocking cocrete tiles £2,080.12 £3,803.77 £5,883.89 £1,176.78 £7,060.67
Fibre Cement Slates £4,196.56 £6,002.45 £10,199.01 £2,039.80 £12,238.81
Clay Pantiles £3,004.66 £5,428.09 £8,432.75 £1,686.55 £10,119.30

In response to an increasing number of insurance claims for wind induced roof damage, 2015 saw the introduction of the new roof tiling and slating standard BS5534. There were two key elements to the new code. Firstly, an increase in the number of mechanical fixings required between tile and substrate. Secondly, a tightening of the standards governing the testing of roof underlays (commonly referred to as felt).  Exact mechanical fixing requirements calculates on a roof-by-roof basis. Taking into account geographical location, orientation and wind exposure.

Tile manufacturers will be able to provide tailored fixing specifications for your site and tile choice. For some tile types, particularly plain tiles the extra nailing requirements can make a significant difference to labour costs. Plain tiles usually have a coverage of about 60 tiles/m2 depending on roof pitch. Previously, perhaps 1 in 3 of these tiles would have nailed down. So the nailing workload has increased from 20 to 60 nails per m2. It’s also worth keeping an eye on the hip and ridge tiles. Attaching each with mechanical fixings, not considering mortar as sufficient.

Likewise, look out for the choice of roofing nails. Galvanised nails are fine for fixing down battens but can not be used to attach the tiles. Movement of the tiles under wind loads will wear of the zinc coating and lead to premature failure through rusting. Applying either aluminium, stainless steel or copper fixings instead.

This article was written by Adrian Wild, Managing Director of HBXL Building Software and the HBXL Estimating Service. To find out how we can help ensure you provide professional, accurate quotations, give Ian a call on 0117 9167894, or drop us an email.