How Much Does a Fit a Flat Roof??

Job Costs

job	Description	labour
1	To re roof (using either of the felting methods described below) a garage which is attached to the house. This will take 2 men 2.5 days.	£700
	Materials, tipping etc. relevant to the above	£400
2	Should it be necessary to replace all the above newly exposed timber decking as work proceeds with 18 mm weather proofed plywood sheets. This will take 2 men 1 day	£300
	Materials, tipping etc. relevant to the above	£175
3	Supply and fit a new lead flashing where the garage roof meets the house. 1 man half a day + materials	£170

“Labour” at £175 a day (tradesman) £100 (labourer), includes incidental fixings etc. and tipping charges. “Materials” if mentioned, are larger things (a boiler) and stuff only you can choose (tiles etc). Also VAT must be added all round.

Information Sheet on Flat Roofing

Don’t ask if a flat felted roof will leak but when will it leak. 15 years is doing well!

Flat Roofs

Standard flat roofs are made up of three layers of felt on top of a slightly sloping wooden deck. They should never actually be flat. The top layer should have protection from the sun’s ultra violet rays and from sparks from bonfires etc. This top layer is usually in the form of an integral layer of mineral flakes (usually green) or 50mm (2 inches) of small stone chippings.

The mineral flakes do the job perfectly well. The chippings add considerable weight to the roof BUT they are very good at hiding standing water from prying eyes if a builder hasn’t put a proper fall on the roof.

The standard requirement is a fall of only 1:80, which is pretty flat but most builders go for 1:40.

There are basically two methods of sticking the felt to the roof and to itself.

The oldest way and one which is still extensively used is by heating blocks of bitumen in a gas fired boiler placed close to the job, hauling buckets of the boiling black stuff up onto the roof and spreading it around with brushes before it goes off.

The newer way is to use torchon felt which is pre-bituminised. This is heated in situ. (on the roof) with a gas fired burner.

Both methods are effective provided you know what you’re doing!

Mastic Asphalt Roofs

These are the “Rolls Royce” of flat roofing.

Asphalt is also known as bitumen and is a by product of petroleum. Under average temperatures it is a very dense and solid material, but when it is heated to extreme temperatures it forms a thick, sticky (mastic) substance which can be spread over a surface. As it cools, it becomes waterproof. In its liquid form it can be mixed with aggregates such as sand or gravel.

A good thick layer of very hot asphalt is hand spread over the entire roof with no seams at all. It even continues up adjoining walls etc. for 150mm (6 inches) and is forced into a suitable mortar bed in the brickwork. It is expensive, specialist work, but if you want a flat roof to last 50 years this is the one!

All roofs, flat and pitched have the greatest likelihood of leaking where they meet other things, such as house walls and chimneys, vent pipes, roof windows, parapet walls etc.

There are two reasons for this. The obvious one is the abrupt change of angle, which requires a joint to be made. The other, which is relevant to felted roofs, is called thermal difference. Basically, different materials expand and contract at different speeds and bricks and felt are no exception. When they are stuck together, to form a joint between a garage roof and the house wall, for example they will eventually crack apart. Rain does love a crack! Anywhere a felted roof meets an obstacle it can eventually leak. Upstands (where the roof covering runs up a wall) have to be given special attention. Very often these are made using the same felt as the roof but they don’t last very long. By far the best material to use is lead which is good for 100 years if it’s fixed properly.

If you are lucky and the roof hasn’t leaked after twenty years, it is likely that it will instead have suffered from ultra violet action. This will have caused blisters to form which will eventually burst and let the rain in anyway!

Replacing a Flat Roof

If your flat roof leaks somewhere in the middle, it’s time to replace it. This is done by completely removing the existing felt to expose the timber decking (sometimes called sarking). You will be surprised to discover this is done with spades. Hopefully the decking will be sound and dry but if you have had a leak it’s likely it won’t be!

So, what state is it in? There are many types of decking that can be revealed. If you are lucky and it’s wooden planks (just like floorboards), then sections of them can be easily replaced. If the deck comprises marine/roofing grade plywood sheets, then it may be possible to replace a small section or even one complete sheet.
However if the decking is chipboard, it is very likely that the top layer of it will lift off with the last layer of felt and it will all have to be replaced.

The decking maybe of a composite, straw-like insulation material (from the 70s) about 75mm (3 inches) thick. In this case the roofer could fix it by covering it with new plywood and new roofing on top. However, as this will raise the overall height of the roof, he may have a problem with the existing height of fascia boards etc.

It is also possible that the roof is concrete, but in this case the roofer should have already worked this out and stated in his quote that he will need to apply primer before felting.

Essentially no roofer with any sense wants to find any problem when he removes the felt. He has just one day to get the old roof off and the new one on, at least to a watertight situation, before he leaves for the night as it will rain and no amount of covering with tarpaulins will stop it getting in!

Questions to ask the Roofer during his Quotation Visit.

Why hasn’t he got on the roof to have a look? He must at least get up and look at the roof. If he says it’s not necessary, he’s going to charge you so much, that no matter what he finds when he does the job, he still can’t lose money! You probably won’t want to employ him.

Will you be removing all the felt? This is essential, don’t allow any roofer to felt on top of old stuff.

Will you put a tarpaulin down to protect my new “paviour” driveway and white house walls? He will be chucking the old felt off the roof because time is of the essence and it will mark everything it smashes into.

What sort of decking do you expect to discover and how will you be including the cost of any probable repair/renewal work in your quotation? He probably won’t know what decking it is until he gets the felt off, so he can’t give an accurate price but he must tell you in writing how much per hour, per man, he will charge for both repair and replacement work. There may also be extra materials to buy. He will charge you for these but you have a right to see the invoices.

Note He will have paid VAT on these materials and so will you when you pay him for them. Be careful he doesn’t include them at the gross price in his invoice, because if he is charging you VAT as well, then you will be paying this bit twice!

What he can quote for, relevant to unforeseen work, is to remove all the newly exposed decking and replace it with new roofing grade plywood (or a special timber roofing sheet called “Sterling Board”), plus any allied works associated with complete deck replacement such as replacing damaged fascias, guttering etc. Then you will know the maximum price you can possibly be charged.

What type of solar protection will you use? Mineral felt or chippings?
If he leaves the top surface as black felt, he doesn’t know his job. Even a black top painted with solar reflective paint will be a second rate job because it will weather off in 2 years.

What type of upstands will you be creating? As mentioned, lead is by far the best material. If he uses lead flashing then he should use code 4 lead which he must cut into lengths not exceeding 1500mm because longer lengths will buckle over time as they expand and contract. The lead must be fixed into a thoroughly raked out mortar joint between two courses of brickwork with lead wedges and then pointed with mortar to finish.

He may however have to use his roofing felt around any roof windows, as there may be no possible fixing point for lead.