How to Build a Flat Roof

HOW TO BUILD A FLAT ROOF – Building a flat roof is much easier compared to constructing a pitched roof. You often see flat roofs over porches, garages and home extensions. Build a flat roof does require careful attention as they, like any structural work, require you to follow strict building codes and regulations.

In this guide we’ll show you how to build a flat roof with easy to follow instructions, advice and tips.

In this video you learned how a ‘Warm Flat Roof‘ works, its advantages and disadvantages.  In this next video you can learn how a ‘Cold Flat Roof‘ works, its advantages and disadvantages.

Step 1

Make sure you have all the necessary safety equipment. Remember you are going to be working at heights, which will involve a ladder. Make sure you use the appropriate type of ladder and that someone is at the bottom to hold it when you are ascending and descending.

Step 2

Once your structure is complete the first thing you will need to do is the fit the roof joists. The size of and the spacing required will depend on the size of the flat roof you are building.

You will need to consult with your local planning authority who will be able to tell you exactly how big the joists need to be and how far apart they are to be spaced. Once you have this information you can order the joists and materials you need to complete the job by taking a trip to your local builder’s merchant.

You should be able to give them the specifications of the roof and they will then advise you on all the building materials you will need. There are variations but you will need Joists, Joist Fixings, Plywood, Under Felt, Top Felt and all the Fixings.

Again with the measurements in hand the Builder’s Merchant should be able to supply everything you need to your specifications and to the regulations.

Step 3

It is a good idea if you secure 1″ x 4″ timber batons to the top of the wall. These need to be level and secured to the brickwork. This will allow you to then secure your joist to these batons.

Place the ladder to access the roof – just remember if your roof is particularly big it is a good idea to place a ladder either side and have a friend help you to position the joists.

Lay out each joists as per the regulations required for the size of roof you have. The joist can be secured in place with the joist fixings to the batons you fitted. Once all the joists are in place you will need to fit noggins.

These are pieces of timber, of the same dimensions as the joists and are fitted in between the joist spaces to add strength. You should fit these every 2m along the spaces. This will add strength to the joists and prevent them from twisting.

Step 4

Now that your joists and noggins are in place you need to fit the ‘Firing’ timbers. These are lengths of timber that fit the lengths of each joist.

These are cut to a wedge shape with the thickest end where the roof meets the wall and then tapers off to the front of the roof. This allows you to create a slope to allow for rainwater to run off.

Again, there are strict building codes and regulations you will need to adhere to and all this information can be provided by your local planning office.

Cut the firing timbers and secure each one along the length of each joist using appropriate fixings, such as galvanized screws.

NOTE: If you buy the materials from a Builders Merchant or Timber Merchant then they will be able to supply the Firing Timbers already cut to the specifications required and within the respective building regulations.

Step 5

You joists are now ready to be clad with boarding. The boarding needs to comply with exterior standards in either Plywood or OSB Timber Sheeting.

If you are unfamiliar with this don’t worry, the builders merchant will supply all the materials to the required regulations. When you fit these each board needs to finish at the half-way mark on each joist.

The boards can then be secured to the joist using galvanized screws. You should secure the boards to the joist every 4″ to 6″ along the joist. Repeat this until you have covered the entire roof.

Step 6

Once all the decking boards are in place you can fit the facial boards around the roof line using screws that are screwed into each of the joists.

Step 7

You are now ready to install the insulation that is required for a WARM ROOF. A Warm Roof is less prone to condensation problems.

If you are intending to construct a COLD ROOF then the only place for the insulation is in between the joist and you will be required to leave a air gap of a least 50mm for the air to circulate. However, in this particular guide we will continue the instructions for the installation of a WARM ROOF.

The Builders Merchant would have supplied you with all the necessary materials and the first step in insulating the roof is to lay down a waterproof membrane.

You need to cover the boards and ensure that you overlap each strip by approximately 6″. The waterproof membrane can be secured into place with a staple gun along the edges at intervals of 4″ to 6″.

Step 8

Once the waterproof membrane is complete you can now install the rigid insulation broads – these are generally a foam board and again there are strict building regulations and codes for the thickness that is required.

Again, from your specifications the Builders Merchant will have supplied you with all the materials that meet the regulations and codes. The insulation boards can be glued into place, which is easier considering they are mostly made of foam.

Step 9

Once the insulation board is in place you can now lay the ‘Overlay Board’. Again this needs to be exterior quality Plywood or OSB and this can be fitted by gluing it to the insulation board and then using screws to help secure it firmly.

Step 10

The final layer is the Roofing Felt. There are a number of products on the market and each has its own way of insulation, from hot tar to self adhesive rubberized felt, which seals the flat roof.

You will need to fit one that complies with the building regulations and codes. When fitting the felt ensure that you leave an overhang of 8″ over the fascia boards.

The edges can then be folded in and upwards and then secured to the fascia boards. When laying the felt also ensure that you overlap each strip by at least 6″ and seal well along the seams to ensure no water can penetrate the underneath.

You must lay the felt, starting at the lowest point of the Fire Timbers, so that the overlap is on top of each, which will help prevent water from penetrating.

Step 11

When your flat roof is complete the only thing to do is fit the guttering and down pipes and then do a final inspection of all the seals to make sure everything is water tight.

Check out what people are talking about in our DIY Roofing & Gutting Group.

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how to build a flat roof

  • Son Lakhani

    Besides being cheaper than other roof types, flat roofs can also help improve the ventilation inside the house because they can absorb heat better and transfer  inside the house during winter. Plus, you can use the extra space to build your own roof top garden or swimming pool.

  • Allyson Ripple

    One obvious advantage of a flat roof is that it’s easier to climb and inspect. It’s also easy to put up, and maintenance is very cheap compared to other roof types. If you want to install a flat roof, make sure the foundations of your house can handle it. Also, make sure that the area to be covered is not too big because the bigger the area it covers, the less stable it becomes.