How are Flat Roofs Constructed?

On a basic level, flat roof construction comes in the form of roofing timbers, or joists as they are more commonly known or steels that are laid across two uprights (walls in most cases). The steels, or RSJ’s (Rolled Steel Joists) are often built into one or both walls to give added stability to the roof structure. The joists are then either laid on top of the steels or cut into them. These joists are spaced at a specified distance apart, usually 400mm depending on the width or span they are covering and the surface that is to be applied. In the majority of cases the covering will be plywood or OSB timber which is more commonly called Stirling board.

When timbers are used in flat roofing they obviously need to be waterproofed so boards are normally laid across the joists and then a felt or rubber is used to cover the boards to make them waterproof. Where two boards meet and the joint is not on a joist, tongue and grooved decking boards make the joint strong enough to support the flat roof load.

When is a Flat Roof Needed?

Sometimes town planners will not allow the height required for a pitched roof. This is because it may obscure a neighbours view or light. Sometimes a pitched roof is not appropriate for an extension because it would mean obscuring light to the home owners own windows. Usually however, Top contractors NYC when an extension butts up to an existing building. To use a pitched roof in these circumstances would mean adding roof valleys or box gutters and make draining the roof difficult and expensive. Using a lean-to flat roof allows water to run easily away from the building into new gutters which can be connected to existing drainage systems or even new soakaways. Occasionally a new flat roof will be required to replace an old roof, for example asbestos or corrugated tin roofs which are either not allowed under building regulations or are in a complete state of disrepair.

Sizes of Joists Needed for Flat Roof Construction

As you may imagine there is a fair amount of weight involved here, in terms of boards, felt etc…. so any timber joists that are used need to be strong enough to support this weight. To ensure that these timbers are strong enough for the span they have to cover a formula is used to calculate the correct width and height that the joists should be. Additionally as part of the Building Regulations Approved Document A for structure further calculations are required to take into account additional loads such as snow that some roof may be subjected to. In most cases this depends on the area of the country the property is in and also on the ordinance datum (vertical height) of the property above sea level. Due to these factors it is very important that before construction begins that you gain building regulations approval.

How to Construct a Flat Roof

Never confuse the word flat, with the word level. Flat roofing should always be flat, but never level…. not in just one direction anyway. Water must be allowed to flow off flat roofs as puddles of water are very heavy and they soon force a dip into the timber roof decking which then becomes bigger allowing more water to puddle until the roof can bear the weight no longer and leaks occur.

Types of Flat Roof Construction – Warm and Cold Flat Roofs

Flat roof construction can come in the form of a warm roof or cold roof depending on the requirement of the owner and architect as they relate to the building regulations regarding heat loss and insulation. Put simply a warm flat roof has insulation of some kind packed into every available void and a cold flat roof allows air to circulate between the joists. This air is vented at either end of the roof to allow a continuous air flow. In terms of rules on ventilation, BS 5250: 2011 (Control of Condensation in Buildings) is the document to view. It states that there should be at least a 50mm gap between the underside of the roof decking and the insulation below it.

The design of the roof, either warm or cold, must be precise. If a warm roof is not fully packed with insulation, yet a constant air flow is not achieved, condensation will form in any unvented void. Condensation forms as warm air (in insulated voids) hits cooler surfaces (underside of roof decking) and can rot the roof from the inside out. You will need to look at Approved Document C and also Approved Document F of the building regulations (for Scotland see Section 3 in the Technical Handbook) as these documents will help you calculate the amount of ventilation that will be required for your particular project.

What is a Cold Flat Roof?

having laid out the rules in simplistic terms above, the most common type of flat roof these days is one which combines a warm insulating layer, with an air flow to protect the joists from rot, more commonly known as a cold flat roof (see diagram below). This is a very simplistic diagram designed to show the user the principles of the cold flat roof. If you are designing or building a flat roof at home you need to do a little more research to show you how and where to incorporate a vapor barrier into your roof and whether or not you need (in the case of an externally insulated roof) to add a further layer of roof decking timber to the top of the insulation.

As mentioned above, you will also need to ensure that you have an adequate amount of air change or ventilation to ensure that no condensation is allowed to build. It is impossible in a generic best roofing contractors NYC project such as this to “cover all bases” so please ensure you investigate your own project thoroughly. One point to note with cold flat roofs is the they are not a recommended solution in areas of high humidity and due to this are not generally recommended by Scottish Building regulations.

What is a Warm Flat Roof?

There are effectively 2 different formats for a warm flat roof – one where any void is fully packed with insulation leaving no air gaps at all to avoid any chance of condensation buildup and the other where there is an air gap below the insulation. In this latter case, warm air rising from below will travel up through any voids as you would expect, but because the roof deck itself is insulated it will be warm and not cold and therefore, any rising warm air won’t condense. The air, however, will still need to be ventilated! By fully packing a warm flat roof with insulation there can be no air to condense. However this type of warm roof relies on there being absolutely no voids whatsoever, otherwise interstitial condensation will occur and this can go unnoticed for many years.